ASTRONOMY

LIBRARY LIBRARY

UNIVERSITY OF CALIFORNIA,

Class

CELESTIAL OBJECTS

H>

HE

JNlVERSfTY

ASTRONOMY LIBRARY

C/W". Smart & Son. Leamington. Spa.Pholo.

7.

CELESTIAL OBj^

FOR COMMON TELESCOPES

BY

THE REV. T. W; WEBB, M.A., F.R.A.a

VICAB Of tl U. . >; J01£, l.KKEiX>RD3li!KR

NEW IMPRESSION

BEING A REPRINT «.» iFTH EDITION, REVISED AND

GRi LARGED (IN 1

BY M]\ IN, M.A., F.K.A.S.

•^ rA,5

LONG . G LIE EN, AND CO.

39 ! HOW, LONDON

CELESTIAL OBJECTS

FOR COMMON TELESCOPES

BY

THE REV. T. W. WEBB, M.A., F.K.A.S.

VICAB OF HABDWICK, HEBEFOBDSHIBB

NEW IMPRESSION

BEING A REPRINT OF THE FIFTH EDITION, REVISED AND GREATLY ENLARGED (IN 1893)

BY itEV. T. E. ESP1N, M.A., F.K.A.S. /R?^V

THE \

IN TWO VOLUMES

VOL. I.

LONGMANS, GKEEN, AND CO.

39 PATERNOSTER ROW, LONDON

NEW YORK AND BOMBAY

1904

All right* reserved

Xv\

ASTRONOMY LIBRARY

Many things, deemed invisible' tu secondary instruments, are plain enough to one who ' knows bow to see tbeni ' SMYTH

\\ ben an object is once discovered by a superior power, an inferior one will suffice to see it afterwards SIK W. HKKSCHBL

Inertia mors est j bilosopbiae — vivamus nos et exerceamur

KEPLER

Pulchra sunt omnia faciente Te, et ecoe Tu inenarrabiliter pulchrior, qni fecisti omnia S. AUGUSTINE

Sic enim magnalia sapientise sute decoravit Is, qui est ante speculum ct usque in sseculuui ; nihil redundat, nibil deficit, nee locus est censura? cujusquaiu. Quam desiderabilia opera ejua !***** et quis saturabitur videns gloriam eoruui? - KEFLER.

INTRODUCTION.

THE intention of the following treatise is to furnish the possessors of ordinary telescopes with plain directions for their use, and a list of objects for their advantageous employment.

None but an eye-witness of the wonder and glory of the heavens can thoroughly understand how much they lose by description, or how inadequate an idea of them can be gathered in the usual mode, from books and lectures. It is but the narrative of the traveller instead of the direct impression of the scene. To do justice to this noble science, — to appreciate as we ought the magnificent testimony which it bears to the eternal Power and Godhead of Him ' Who by His excellent wisdom made the heavens/ we must study it, as much as may be, not with the eyes of others, but with our own.

This, however, is no easy matter: nor is the want of a telescope the only difficulty. Instruments quite sufficient for the student's purpose are far less expen- sive than formerly ; a trifling outlay will often procure VOL. L k

551)25

vi INTRODUCTION.

them, of excellent quality, at second-hand ; and many are only waiting to be called into action. But a serious obstacle remains to the inexperienced possessor. How is he to use his telescope in a really improving way ? What is he to look for ? And how is he to look for it? For want of an answer many a good instrument is employed in a desultory and unin- structive manner, or consigned to dust and inactivity.

Materials for his guidance exist, indeed, in pro- fusion, but some of them are difficult of access ; some3 not easy of interpretation; some, fragmentary and incomplete: and the student would find it a dis- couraging task to reduce them into a serviceable form. This, then, is what has been attempted for him in the following pages, by one who, during many years, would have rejoiced to avail himself of some such assistance, if he had known where to meet with it, and who does not know where it is to be met with, in a convenient shape, to the present day.1

For the more advanced observer, the ' Cycle of Celestial Objects/ published in 1844 by Captain, now Vice-Admiral Smyth, will be found a treasury of varied information, and of the highest value as the companion of a first-rate telescope : but its very superiority, to say nothing of its bulk and cost, renders it more suitable for his purpose, than for those humble beginnings which are now in view. It

1 The original edition was published in 1859.

INTRODUCTION, vii

has, however, been of the most essential service in the preparation of the present undertaking, which without it would, in all probability, never have seen the light, and which, as far as the sidereal portion is concerned, is based upon it as the standard authority.

Nothing would have been easier than, on so fertile a subject, to have expanded this treatise to a much larger bulk : but it would thus, in some measure, have defeated its own object. In order therefore to reduce the size of the volume, without omitting such details as may seem to be required by the present state of Astronomy, the reader will have to excuse a condensed mode of expression, the result of necessity rather than of choice ; and, as considerable pains have been taken in the verification of facts, a general list of authorities will supersede references at the foot of the page.1

Limited in extent, imperfect in execution, and in parts only suggestive in character, this little book may perhaps serve as a foundation on which students of astronomy may raise the superstructure of their own experience ; and in that case the author's inten- tion will be fulfilled. He will be especially gratified, if his endeavour to remove some difficulties may tend to increase the number of those who ' consider the heavens/ For he is convinced that in such a personal examination of their wonders will be found an

1 Omitted in the present edition, as its bulk would have boon con- siderably increased, with little correspondent advantage.

vill INTRODUCTION.

interesting and delightful pursuit, diversifying agree- ably and instructively the leisure hour, and capable of a truly valuable application, as leading to the most impressive thoughts of the littleness of man, and of the unspeakable greatness and glory of the CREATOR. To such a study, the impressive words of the late Sir K. H. Inglis may be most suitably applied : ' Every advance in our knowledge of the natural world will, if rightly directed by the spirit of. true humility, and with a prayer for GOD'S blessing, advance us in our knowledge of Himself, and will prepare us to receive His revelation of His Will with profounder reverence.'1

1 Report of British Association, 1847.

ADVERTISEMENT to the FIFTH EDITION.

BEFOKE commencing the work of editing a new edition of ' Celestial Objects/ by the courtesy of the editor of the English Mechanic, a request for suggestions from amateurs was made in the columns of that paper. The Astronomical and Physical Society of Toronto appointed a Special Committee to draw up a note of any improvements that might be made. From these sources much valuable information was derived, and in the following pages the suggestions have been adopted as far as possible. The enormous increase in the number of telescopes and observers has led to the publication, in the last twelve years, of innumerable observations in every branch of Practical Astronomy, and has made it impossible for any observer to follow up all the various branches of Planetary and Stellar Astronomy. It seemed best, therefore, to seek the help of specialists, and in the following pages the additional matter has been contributed by Miss Brown for the Sun, Mr. T. G. Elger for the Moon, Mr. A. Stanley Williams for

X ADVERTISEMENT TO THE FIFTH EDITION.

Mercury, Venus, Mars, Rev. W. R Waugh for Jupiter, Rev. A. Freeman for Saturn, Mr. W. F. Denning for Comets. Mr. Denning has also kindly contributed a chapter on Meteorites. For this kind help and assist- ance my sincere thanks are due.

The original text has been left unaltered as far as possible, and the new matter added in foot- notes. Since Parts I. and II. are used mainly for reference, they have been placed in a volume by themselves, while Part III., which would naturally be in constant use in the Observatory, forms a volume by itself. For Yolume II. I am entirely responsible, and it has been re-written. The catalogue of Struve has been used as a basis, instead of the 'Celestial Cycle/ as the latter work has been republished by Mr. Chambers, and has thus become generally available. In selecting new objects it was thought best to go down to a certain limiting magnitude. After careful consideration, and consultation with those eminently fitted to give an opinion, it was determined to insert all double stars where the primary was above mag. 6*5, and the distances less than 20". The double stars in the old edition have been retained, with the exception of one or two very wide pairs from Struve's Appendix. It is thus hoped that the requirements of telescopes of all sizes will have been met. '

The objects have been arranged in order of R.A. in each constellation, saving thereby the fifty pages

ADVERTISEMENT TO THE FIFTH EDITION. xi

of Appendix II. in the fourth edition. The KA. has been given to a decimal of a minute at the earnest request of various observers, but with no idea of attempting greater accuracy than in former editions. Professors Burnham, Hall, Perrotin, Schiaparelli, Messrs. Maw and Tarrant have kindly furnished me with their double star measures, and where any motion was mentioned in the last edition the later measures from their lists have been added. While, however, of some stars there are many measures, others have rarely been observed since Dembouski's remeasurement of the double stars of Struve. Mr. Tarrant is at present engaged in the systematic work of remeasuring all Struve's stars, and this will be a valuable addition to our knowledge of Celestial Motions. Some measures are my own, but want of experience and suitable apparatus have caused them to be few and incomplete. The latest orbits of Binaries have been furnished me by Mr. Gore.

Under the heading of 'Stars with Remarkable Spectra' will be found the most interesting stars of the III, IV, and Y types, and all the variable stars at present known. These have been taken from the new edition of Birmingham's 'Red Stars/ and from material kindly placed at my disposal by Professor Pickering. This list has been made as complete as possible, since so many of III and IV type stars are variable, and work on stellar variation offers' one of

xii ADVERTISEMENT TO THE FIFTH EDITION.

the most promising fields to the amateur. The periods, etc., of the variable stars are taken from Chandler's or Gore's catalogue.

Some additional matter will be found under Nebulae and Clusters ; but, as a rule, it was felt that the fourth edition was fairly complete in this respect, and that the former objects are more suited for the photographic plate than the eye. Some slight additions have been made to the Southern Objects, but the completion of this part must be left till a large telescope has been erected in the south. Besides the help already acknowledged, my thanks are due to Mr. Mee, Mr. Knott, Captain Noble, Mr. Wesley, for valuable assistance, and to Mr. Ranyard for the engravings of "The Hercules Cluster/ etc. Mr. Sadler has added various notes throughout the work.

In response to a generally expressed wish, two notes have been inserted at the end of Part I. on Celestial Photography and on the Spectroscope as applied to the Telescope. The only omission of any importance is the list of transits of strange bodies over the Sun's disc, the general consensus of opinion being against their retention.

T. W. WEBB.—A REMINISCENCE.

THOMAS WILLIAM WEBB was born Dec. 14, 1807, and was the only son of a clergyman, the Rev. John Webb. His mother died when he was still a child, and he was educated by his father. From very early years he showed a remarkable taste for experimental science. In 1826 he entered at Magdalen Hall, Oxford, and in 1829 he took a second class in Mathematics. In the same year he was ordained in Hereford Cathedral, and in 1 843 he married Henrietta Montague, daughter of Mr. Arthur Wyatt of Troy House, Monmouth. In 1852 he was appointed to the living of Hardwick. In 1882 he was made one of the Prebendaries of Hereford Cathedral. On Sept. 7, 1884, Mrs. Webb died from apoplexy, a terrible blow to him, but borne with that patient resignation and perfect faith which was one of the marked features in his character. From this time his health gradually failed, and he died May 19, I885.1

1 These particulars are taken from M.N. of the E.A.S., vol. xlvi.; eee also a memoir in Mee's ' Observational Astronomy.'

xiv T. W. WEBB. — A REMINISCENCE.

His first telescope was a fluid achromatic, which was replaced by a refractor of 3^ in., of 5 -ft. focal length, by Tully ; in 1859 he obtained a 5^-in., of /-ft. focal length, by Alvan Clark. In 1864 he was using an 8-in. silver on glass reflector of 6-ft. focus by With, and was engaged in trying this side by side with the Alvan Clark Refractor. From 1866 to his last observation (1885, March 19) he used a 9j-in. silver on glass reflector, by With. This was mounted equatorially on a Berthon stand with rough circles, and in spite of several attempted improvements, the positions obtained with it were never satisfactory. The telescope was placed in a little observatory made of wood and canvas, and stood a few yards SE. of the vicarage front door. Mr. Webb left behind him four large notebooks filled with observations, solar, lunar, planetary, and stellar. The observations are interspread with numerous exquisite drawings. The manuscript observations are a model of neatness, patience, and care. The stellar and nebula observa- tions amount to 3463. Each page has a red line ruled for a margin, and each observation is prefixed with a number, entered in the margin. The rough notes were made at the telescope, prefaced by the state of the air, the powers used, etc., and written out in full next day. The earliest recorded manuscript observation is one on Jupiter, Dec. 2, 1834. From 1847 to 1856 Mr. Webb was engaged in observing the

T. W. WEBB. — A REMINISCENCE. XV

objects in the Bedford Cycle with the 3170-, and in 1859 he published the first edition of 'Celestial Objects.' In later years, besides work in connection with the four editions of his book, he gave considerable time to the observation of Red Stars for the first edition of Birmingham's Red Star Catalogue. Mr. Webb discovered a considerable number of new ones, and amongst them he detected, on Christmas Day, 1869, the variable S Orionis, which was also the last star he observed, 1885, March 19. His sight, although latterly slightly astigmatic, was remarkably good, especially for planetary detail. A curious instance of difference of vision was well illustrated one superb evening, when Mr. Webb and the writer were observing Saturn with the 9^ -in. reflector at Hard wick. Mr. Webb saw distinctly the division in the outer ring which the writer could not see a trace of, while the writer picked up a faint point of light, which after- wards turned out to be Enceladus, which Mr. Webb could not see.

Mr. Webb was a father to all amateur astronomers, and the post brought an appalling amount of corre- spondence from them to Hardwick. All were care- fully, kindly, and encouragingly answered in letters charming alike for their elegant writing, and the extraordinary amount of learning, and originality, and witticism. Mr. Webb's versatility was one of the features that struck every one who knew him

XVI T. W. WEBB. — A KEMINISCENCB.

intimately. Not only did he conduct researches into each branch of Astronomy with untiring patience, but he painted and sketched admirably, as did also Mrs. Webb. Like Mr. Birmingham, he was fond of music, and the talents he displayed in completing his father's work, ' The Civil War in Herefordshire/ showed an antiquarian of no mean order. Every- thing, too, in Nature had a charm for him, and the amount of knowledge and powers of observation in this respect were well known to any one who had the privilege of a country walk with him. Setting out early in the afternoon, with a knapsack laden with all kinds of little comforts for the sick, he would walk with vigorous stride up the hills to see some distant parishioner, and converse all the way, handling the most difficult topics with keen logical ability, at the same time with most unassuming humility, and deference to the thoughts of others. And then, when the cottage was reached, there was no mistaking the warmth of welcome and the smile of pleasure with which he was received. And there he would sit, the children gathering round him, and talk to his people of their everyday life, and local matters, making himself one of themselves, and imparting the sunshine of his life to theirs.

So passed his life at Hardwick — a life of continuous work, seeking neither offices in societies or Church preferment, but working for work's sake, and that

T. W, WEBB. — A REMINISCENCE. xvii

from the highest motives, as is well shown by an extract from his observation-book, with which these few lines may well conclude. It well summarizes his motives, and shows his guiding principle in life. It runs : —

' Gratias D. 0. M. refero, qui servo suo indignissimo per tot annos benignissime concesserit, ut opera Ejus mirifica cum tanta voluptate contemplatus essem. Quamobrem verba illustrissimi Kepleri mihimet assumo, dicendo, " Quis saturabitur videns gloriam eorum ? 0 Qui lumine naturse desiderium in nobis promoves luminis gratise, ut per id transferas nos in lumen glorise, gratias ago Tibi, Creator, Domine, quia delectasti me in factura Tua, et in operibus manuum Tuarum exultavi " Amen.'

CONTENTS.

INTRODUCTION ••••••

ADVERTISEMENT TO THE FIFTH EDITION T. W. WEBB — A REMINISCENCE .

f 4OE V

ix xiii

PART I. THE INSTRUMENT AND THE OBSERVER.

THE TELESCOPE . . . I THE MODE OF OBSERVATION 10 NOTE I. ON CELESTIAL PHO- TOGRAPHY « . . 19

NOTE II. ON THE SPECTRO- SCOPE APPLIED TO THE

TELESCOPE ... 22

PART II. THE SOLAR SYSTEM.

THE SUN . . .

MERCURY. . . . VENUS .... THE MOON

INDEX TO THE MAP OF THE MOON ....

APPENDIX I. ADDITIONAL ! II. BOOKS OF HE

25	MARS . .	. 150
47	JUPITER .	, 163
5i	SATURN . . .	. 188
66	URANUS AND NEPTUNE	. 204
	COMETS . . .	. 205
40	METEORS . . ,	. 222
)TES REN<		. 23I

PLATES.

PHOTOGRAPH OF KEV. T. W. WEBB . MAP OF MOON .... CHART OF MARS ...»

To face Title-page 66 158

4

OF THF

( UNIVERSITY 1 V or /

XC4MrOR*\>^

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PART L

THE INSTRUMENT A»D THE OBSERVE K.

0 multiscium et quovis sceptro pretiosius Perspicillum ! an, qui te dextra tenet, ille non rex, non dominus constituatur operum Dei? Vere tu

Quod supra caput est, magnos cum motibus orbes Subjicis ingenio. — KKPLEK.

THE TELESCOPE.

ALTHOUGH the professed design of this volume is to provide a list of objects for common telescopes, it may not be out of place to premise a few remarks upon the instruments so designated.

By * common telescopes ' are here intended such as are most frequently met with in private hands; achromatics with apertures l of 3 to 5 inches ; or reflectors of somewhat larger diameter, but, in consequence of the loss of light in reflection, not greater brightness.2 The original observations

1 'Aperture' always means the clear space which receives the light of the object; the diameter of the object-glass in acbroniatics, or the large speculum in reflectors, exclusive of its setting.

2 Maskelyne estimated the apertures of metallic reflectors and achromatics of equal brightness as 8 to 5. Dawes gives this value for Gregorians, but like Herschel II. rates Newtonians as 7 to 5. Arago strangely asserted that no light was lost in achromatics; but

VOL. I. B

2 THE INSTRUMENT AND THE OBSERVER.

In the following pages were chiefly made with such an instrument — an achromatic by the younger Tulley, 5^ feet in focal length,1 with an aperture of 3^ inches, and of fair defining quality ; smaller instruments of course will do less, especially with faint objects, but are often very perfect and distinct : and even diminutive glasses, if good, are not to be despised ; they will show something never seen without them. I have a little hand telescope, 22^ inches long when fully drawn out, with an object-glass of about 14 inches focus, and i|~inch aperture : this, with an astronomical eye-piece, will show the existence of the solar spots, the mountains in the Moon, Jupiter's satellites, and Saturn's ring. Achromatics of larger dimensions have become much less expensive than formerly, and silvered specula of very considerable size are now comparatively common ; even for these it is hoped that this treatise, embodying some of the results of the finest instruments, may not be found an inadequate companion as far as it goes.

In judging of a telescope, we must not be led by appear- ances. Inferior articles may be showily got up, and the outside must go for nothing. Nor is the clearness of the glass, or the polish of the mirror, any sign of excellence : these

the effects of absorption and reflection are so considerable, that with very large apertures the advantage of ike achromatic disappears. The silver- on-glass specula, invented by Foucault and Steinheil, but perfected in England, take their place betAveeu the metal Newtonian and the achromatic, approaching more nearly to the latter, especially when the plane mirror is replaced by a prism (which, however, does not always conduce to critical definition). Buffham assigns equal light to silvered Newtonians of 9, 6|, and 4^, and achromatic^ of 8, 5f, and 4 inches respectively. Sa finds 6|-in. silv. refl. equal to 6-in. achr.

1 The focal length is measured from the object glass, or speculum, to the spot where the rays cross and form a picture of the sun or any celestial body.

THE TELESCOPE. 3

may exist with bad * figure ' (i.e. irregular curvature), or bad combination of curves, and the inevitable consequence, bad performance. We need not regard bubbles, sand-holes, scratches, in object-glass or speculum ; they merely obstruct a very little light. Actual performance is the only adequate test. The image should be neat and well-defined with the highest power, and should come in and out of focus sharply ; that is, become indistinct by a very slight motion on either side of it. A proper test-object must be chosen ; the Moon is too easy : Venus too severe except for first-rate glasses ; large stars have too much glare ; Jupiter or Saturn are far better ; a close double star is best of all for an experienced eye ; but for general purposes a moderate-sized star will suffice ; its image, in focus, with the highest power, should be a very small disc, almost a point, accurately round, with- out ' wings,' or rays, or mistiness, or false images, or append- ages, except one or two narrow rings of light, regularly circular, and concentric with the image ; l and in an uniformly dark field ; a slight displacement of the focus either way should enlarge the disc into a luminous circle. If this circle is irregular in outline, or much brighter or fainter towards the centre,2 or much better defined on one side of the focus

1 The real diameter of a star in the telescope would be incon- ceivably small. The apparent or 'spurious' disc, and rings, result from the undulatory nature of light. They seem, however, to be somewhat affected by atmospheric causes. Herschel II. speaks of nights of extraordinary distinctness, in which 'the rings are but traces of rings, all their light being absorbed into the discs.' I have entered 1852, March 23, as 'a very fine night, though the rings and appendages around the brighter stars were rather troublesome ; ' 1852, April i, 'an exceedingly fine night at first, with scarcely a trace of rings or appendages.' See also the star 70 Ophiuchi, in the following catalogue.

2 The small mirror in a reflector causes a central darkness out of the focus, which should be nearly the same on either side of it.

4 THE INSTRUMENT AND THE OBSERVER.

than the other, the telescope may be serviceable, but is not of high excellence. The chances are many, however, against any given night being fine enough for such a purpose, and a fair judgment may be made by day from the figures on a watch-face, or a minute white circle 011 a black ground, or the image of the sun on a thermometer bulb, placed as far off as possible. An achromatic, notwithstanding the deriva- tion of its name, will show colour under high powers where there is much contrast of light and darkness. This ' out- standing ' or uncorrected colour results from the want of a perfect balance between the optical properties of the two kinds of glass of which the object-glass is constructed : it cannot be entirely remedied, but it ought not to be obtrusive. la the best instruments it forms a fringe of violet or blue round luminous objects in focus under high powers, especially Venus in a dark sky. A red or yellow border would be bad; but before condemning an instrument from such a cause, several eye-pieces should be tried, as the fault might lie there, and be easily and cheaply remedied. Reflectors are delightfully exempt from this defect ; and as now made with specula of silvered glass, well deserve, from their com- parative cheapness, combined with admirable defining power, to regain much of the preference which has of late years been accorded to achromatics. The horizontal view of objects at all altitudes in a Newtonian reflector with rotating tube is ex- tremely pleasant, when a little experience has been gained in finding and following : the same advantage, however, attends the use of a diagonal eye-piece with the achromatic, but with loss of light. The chief disadvantage of reflectors is the greater aperture, and consequently greater atmospheric disturbance, corresponding with the same amount of light : and the occa- sional renewal of the film causes a little expense or trouble. The eye-piece, or ocular, is only a kind of microscope,

THE TELESCOPE. 5

magnifying the image formed in the focus of tho object-glass or speculum. The s;ze of this image being in proportion to its distance from the glass or mirror which forms it, the power of the same eye-pieco in different telescopes varies as the focal length. Hence the disadvantage of a short tele- scope; to get high powers, we must employ minute and deeply-curved lenses, which are much less pleasant in use ; with a telescope twice as long, half the curvature in the eye- piece produces an equal power. The magnified focal image, as in the camera, is always inverted, and so in the astro- nomical eye-piece it remains.1 For terrestrial purposes it is erected by two additional lenses ; but a loss of light is thus incurred, and as the inversion of celestial objects is unimpor- tant, erecting eye-pieces (always the longest of a set) should never be employed for astronomy ; the eye soon becomes accustomed to the inverted picture, and the hand to the reversed motion in following the object. The lateral vision in the Newtonian reflector interposes another difficulty, easily mastered, however, by practice, and by attention to the direction of motion through the field. A multitude of eye-pieces is needless, but three at least are desirable ; ono with low power and large field, for extended groups of stars, nebulm, and cornets, supplying also, if necessary, the place of a ' finder ' for deeper magnifiers ; a stronger one for general purposes, especially the moon and planets ; and a third, as powerful as the telescope will bear, for minuter objects, especially double stars. A greater number of eye-pieces admits, however, of what is often important — an adaptation of the power to the brightness of the object. Ordinary

1 It is erect in the Galilean eye-pieco and the Gregorian reflector. But the use of the former is almost confined to opera-classes, as its field \\ith high powers is exceedingly small; and the latter, an inferior construction, is now little employed.

6 THE INSTRUMENT AND THE OBSERVER.

astronomical eye-pieces are shorter in proportion to their power. It is a better plan to change them by means of a short tube, or * adapter,' than by a screw ; in which case they are more liable to be dropped and injured. The power may bo much increased by unscrewing and taking away the 'field-lens' — that farthest from the eye; but the centre of the field only will be distinct. The highest powers of large telescopes are sometimes made thus, with single lenses for the advantage of light; but the lens is then turned the other way, convex towards the eye, as it gives sharper vision. Sir W. Herschel used the double convex form, as having shallower curves. The common kind, with two lenses, having the flat side of each next the eye, is called the Huygenian or negative eye-piece : the positive or Ramsden eye-piece has a flatter field, but is not, like the other, achromatic. The interposition of a combination called a Barlow lens raises the power with little loss of light ; and as one may be made to suit all the eye-pieces, it doubles the set at a small expense. Browning's achromatic eye-piece, and Home and Thornthwaite's aplanatic, and the Kellner con- struction (for large fields) are all excellent in their way. The brightness of the field varies inversely as the square of the power: and hence minute stars are commonly more visible with deep eye-pieces ; the reverse, however, for some unknown reason, sometimes occurs.

If the power of our oculars has not been engraved upon them,1 we may get a fair approximation to it by viewing an equally divided scale at a distance (for low powers, a brick

1 These figures are not, however, always to be depended upon, and must be wrong if the eye-piece was made for an instrument of a different focal length. The celebrated Short exaggerated the powers of his reflectors : and those of the great achromatics of Dorput and Berlin were found by Struve and Encke to be overrated.

THE TELESCOPE. 7

wall will answer) with one eye through the telescope, and with the other alongside of it, and noting how many unmagnified divisions are covered by a single magnified image. Or, better still, we may have recourse to the Berthou Power-gauge, a little apparatus, the simple, efficient, and inexpensive character of which entitles it to very warm commendation.1

The test of excellence in separating power has been fixed by Dawes at the quotient, expressed in seconds, of 4-56 divided by aperture in inches. Thus a lo-inch object-glass or speculum ought to separate double stars at o"*456 of distance between their centres. This value practically concurs with those given by Dallmeyer and Alvan Clark. Reflectors somewhat surpass achromatics in this respect, as theoretically they ought to do : but they are apt to be more troubled by rings and flare, and scattered light. The best telescopes of either kind will bear a power of 100 per inch of aperture on stars: for planets, or the moon, half that power will usually more than suffice.

An object-glass of inferior definition may sometimes be improved by stopping out defects, or contracting the aper- ture. Streaks or specks of unequal density are very in- jurious : they may be detected by turning the telescope to a bright light, taking out the eye-piece, and placing the eye in the focus ; every irregularity will then be visible in the illumination which overspreads the object-glass ; and, if of small extent, may be stopped out by a bit of sticking-plaster. If the performance is not thus improved, try a contracted aperture : make a cap of pasteboard fitting over the object- glass like the usual brass cap, but with a circular opening a little less than the clear aperture ; — if the indistinctness is

1 See English Mechanic March 5, 19, April 9, 1880, respecting au excellent method of employing it.

THE INSTRUMENT AND THE OBSERVER.

thus diminished, but not removed, try several discs of paste- board placed successively within this cap, with progressively contracted openings, till distinct vision is obtained ; there we must stop, or valuable light will be lost. An excentric opening in the pasteboard disc may sometimes be serviceable, being turned round the axis so as to expose different parts of the glass or mirror, till the best effect is produced : in other cases, a central pasteboard disc, supported by narrow arms from the sides, and leaving an open ring of light all round, may be tried. But for comets or nebulae, it will be best to restore the original aperture, as with faint and ill- defined objects light is more essential than distinctness. The centring of a reflector is more liable to derangement than that of an achromatic ; it is, however, easily rectified by the cautious use of the screws which are provided for the purpose. When in correct adjustment, the eye-piece being removed, a dark spot will be seen in the centre of the small mirror, which is the image of that mirror reflected by the large speculum: in proportion as it deviates from a central position, the adjustment is incomplete, and the performance defective.

The definition of reflectors may often be greatly improved by the use of a tube perforated with large and numerous openings ; it is also desirable, where practicable, to interpose between the observer and the instrument, in cold weather, a moveable screen of felt or some non-conducting material.

A good stand is essential : if unsteady, it will spoil the most distinct performance ; if awkward, it will annoy the observer ; if limited in range, it may disappoint him at some interesting juncture. It may be well left to a respectable optician ; but where expense is a serious consideration, a little mechanical ingenuity and knowledge of such contri- vances will devise one which will answer sufficiently. The old arrangement, with a vertical and horizontal, or ' altitude

THE TELESCOPE. 9

and azimuth ' motion, is simple and manageable : the equa- torial form, which makes the telescope revolve on an axis parallel to that of the earth, has great advantages, in follow- ing the object by a single motion, and where the expense of divided circles and spirit-levels is admissible, in finding planets and bright stars by day, and identifying minute objects by night: but, to do its work, it must be placed accurately in the meridian, and out of that position has little superiority. The reflector, too, must rotate in a cradle, or the ocular will assume very awkward inclinations. In any case, if the stand is to be movable, let it be strong enough for steadiness without being too heavy for portability.1

A sidereal clock is often considered a necessary adjunct to an equatorial mounting, in order to find objects invisible to the naked eye. But it may be dispensed with by the following method of ' differentiation ' in all cases, excepting during the brief season of twilight, when neither sun nor stars can be employed. Write down the difference of Eight Ascension (taking particular notice whether additive or sub- tractive) between the required and some known object — the sun by day, a neighbouring bright star by night. Seek the known object by the finder, and place it in the centre of your largest field : clamp the R. A. circle : set the telescope to the declination of the object sought, and clamp it there : unclamp in R. A. and move the telescope E. or W. as the case requires, to the value of the ascertained difference in

1 A very cheap equatorial stand is described in Astron. Register, No. 14, vol. ii. Franks observes that a common equatorial mounting may be made very efficient at a trifling expense by the addition of plain metal circles, on which slips of paper graduated with pen and ink are fastened by glue dissolved in strong acetic acid, and after- wards sized and varnished. A good pillar-stand may be made by letting a 4-inch iron pipe deep into the ground, in which a small tiible with a long foot revolves.

10 THE INSTRUMENT AND THE OBSERVER.

R. A. and the object will be found in the field, somewhat "W. of the centre, by a distance dependent on the duration of the process.

An observatory is by no means essential, but it would be difficult to over-estimate its advantage in point of comfort as well as economy of time. It used to be an expensive luxury; but a very simple and cheap 'telescope-house,' combining shelter with open-air freedom, to the great merit of which I can bear full testimony, has been devised by the Rev. E. L. Berthon, and is described in the English Mechanic, Oct. 13 and 20, 1871. The Eev. W. Conybeare Bruce has also handled the subject very ably in the same publication, Feb. 6 and 27, and April 2, 1880.

We will close this section with the encouraging words of the Council of the Koyal Astronomical Society, in their Report for 1828: 'Every one who possesses an instrument, whose claims rise even not above a humble mediocrity, has it in his power to chalk out for himself a useful and honour- able line of occupation for leisure hours, in which his labour shall be really valuable, if duly registered ; . . . those who possess good instruments, have a field absolutely boundless for their exertions.'

THE MODE OF OBSERVATION.

AN ordinary telescope may be easily prepared for use : to fix it on its stand ; to point it by means of the finder ; to adjust the focus to the eye (remembering that different eyes require different adjustments), are processes scarcely requir- ing instruction. But many mistakes may be made in detail ; aiid in this, as in everything else, there are various methods

THE MODE OF OBSERVATION. II

of doing the thing the wrong way. The present section will therefore consist of negative rather than positive directions, pointing out rather what should be avoided than what should be done.

1. Do not begin by fixing the telescope in a warm room and opening the window. A boarded floor is bad, as every movement of the observer is liable to produce a tremor ; but the mixture of warm and cool currents at the window is worse; it is an artificial production of the fluttering and wavering which, as naturally existing in the atmosphere, are such an annoyance to astronomers. If a window must be used, let it be opened as long beforehand as may be, and let the object-glass be pushed as far as possible outside ; there should be no fire in the room ; and any other windows, as well as the door, should be shut before beginning to observe : the nuisance may thus be sometimes abated ; but the right place is unquestionably out of doors.

2. Do not wipe an object-glass or metallic speculum more than can possibly be helped. Hard as the materials o.re, scratching is a very easy process ; and the ultimate result of ordinary wiping may be seen in an old spectacle-glass held in the sunshine. The most valuable part of a good telescope deserves much more careful treatment; and, if protected from dust and damp, it will very seldom require to bo touched. Nothing but great carelessness would expose it to dust : and the dewing of the surface may be almost always avoided. The object-glass or speculum, if kept in a cold place, should not be uncovered, if possible, in a warmer air till it has gained something of its temperature ; and it must be invariably closed up in the air in which it has been used before it is removed in-doors ; or, in either case, it may be dewed like a glass of cold water brought into a heated room. The object-glass, however, being much exposed to radiation,

12 THE INSTRUMENT AND THE OBSERVER.

requires additional protection ; and this may be easily con- trived. A tube of tin, pasteboard, or very thin wood, such as is used for hat-boxes, or, best of all, calico stiffened with shell-lac and varnished, fitting on to the place whence the brass cap has been removed, and three or four times longer than wide, will, in general, keep the object-glass bright. This ' dew-cap ' must fit tight enough to stand firm, or it will bend down and intercept the light ; but not so tight as to cause trouble in removing it to put on the brass cap in the open air. It is better to blacken its interior — indeed, neces- sary, if of tin ; this may be done with lamp-black mixed with size or varnish, so as neither to show a gloss nor rub off; or a piece of black cloth or velvet may be glued or pasted inside it. A lining of blotting-paper is serviceable in heavy dew. A dew-cap on the finder will often save much trouble. Should it be necessary to leave the telescope for some time in the cold, a clean handkerchief thrown over the end of the dew-cap will be a complete safeguard. Should an object- glass or speculum become damped after all, do not close it up in that state ; if the cloud of dew is very slight, it may quite disappear in a warm room, especially if exposed to a fire ; if dense, however, it may leave a stain which ought to be quickly removed, as well as any little specks of dirt or dulness which will form, one knows not how. To do this, dust the dried surface first with a soft camel's hair pencil or varnishing brush, which will remove loose particles ; then use, very cautiously, a very soft and even piece of chamois leather, which has not been employed for any other purpose, and must be always kept in a wide-mouthed stoppered bottle or wrapped up from dust ; or a very soft silk handkerchief (which Lassell uses for glass) preserved with similar care. But the wiping must be as gentle as possible ; rubbing is inadmissible in any case. Proctor advises sweeping from *

THE MODE OF OBSERVATION. 13

small space near the edge as a centre. Any refractory stains may be breathed upon, or touched with pure alcohol, and wiped till dry : but if the glass has become discoloured, we must put up with the defect ; and care should be taken not to mistake specks in the substance of the glass for foreign matters lodged on its surface. A slight tarnish may fre- quently be removed from a metallic speculum by lemon-juice, or a solution of citric acid, or spirit of hartshorn, carefully wiped off in a short time : if this does not restore its bright- ness, it is better to leave it alone : a slight loss of light is not so great an injury as would result from strong friction. The taking out or replacing of an object-glass or mirror is a deli- cate operation, and hurry or carelessness may easily make it a very dangerous one; speculum metal is nearly as brittle as glass : but this material is rapidly going out of use, from the superiority of the silver-on-glass mirrors, which are now becoming appreciated as they deserve. The management of these need not be described here, as special instructions should always accompany them, such as will be found in Browning's * Plea for Reflectors,' or Calver's * Hints for Reflecting Telescopes.'

Dimness of vision often results from damp on the eye- lens. This will rapidly disappear, without wiping, in a warmer temperature. If the finder does not act well, this may be suspected to be the cause. For these and many other reasons, a small lamp, the light of which can be con- cealed at pleasure, is a convenient adjunct to the telescope : any glass surface held at a safe height over it will speedily be cleared of moisture. A ground or papered glass front to a lamp is advantageous for reading.

Eye-piece lenses lequire occasional wiping; the leather may be pressed to their edges with a bit of soft wood. A piece of blotting-paper rolled to a point, and aided by

14 THE INSTRUMENT AND THE OBSERVER.

breathing, answers perfectly. Their flat faces are easily scratched if laid downwards on a table. The screws demand very gentle usage : a previous turn backwards, before screw- ing in, causes the thread to fall with a snap into its place.

Brass-work should not be rubbed with polishing powder, which might injure the lacquering.

3. If the telescope does not seem altogether right, not- withstanding all the pains you can take in bringing it to focus, do not meddle with screws or adjustments, unless you thoroughly understand the construction, or can obtain good directions. In most cases a screw-driver is a dangerous tool in inexperienced hands.

4. Do not use any part of a telescope or stand roughly, or expose it to any blow or strain. It is a delicate instru- ment, and well deserves careful preservation.

5. Do not spare trouble in adjusting tho focus. It is well known that different eyes require a change, sometimes a great one : and the same observer's focus is not invariable, being affected by the temperature of the tube and the state of the eye, the adjustment of which, as Dawes has pointed out, shortens with intense gazing, and is apt to vary with the relative brightnesses of objects, besides being, to a certain extent, under the observer's control.

6. Do not over-press magnifying power. Schroter long ago warned observers against this natural practice, which is likely to lead beginners into mistakes. A certain proportion of light to size in the image is essential to distinctness ; and though by using a deeper eye-piece we can readily enlarge the size, we cannot increase the light so long as the aperture is unchanged; while by higher magnifying we make the inevitable imperfection of the telescope and the atmosphere more visible. Hence the picture becomes dim and indistinct beyond a certain amount of power, varying with the bright-

THE MODE OF OBSERVATION. 15

ness of the object, the goodness of the telescope, and the steadiness of the air. Comets and nebulas, generally speak- ing, will bear but little magnifying. For the moon and planets, the power should be high enough (if the weather is suitable) to take off the glare, low enough to preserve suffi- cient brightness and sharpness : the latter condition being preserved, minute details are likely to come out better with an increase of power. Stars bear much more magnifying, from their intrinsic brilliancy ; and they are enlarged very slightly in proportion : their images ought never, with any power, to exceed the dimensions of minute discs, — spurious discs, as they are termed, arising from the undulatory nature of light, and usually smallest in the best telescopes. A very high power has, however, so many disadvantages, in the difficulty of finding and keeping the object, the contraction of the field, the rapid motion of the image (in reality, the magnified motion of the earth), and the exaggeration of every defect in the telescope, the stand, and the atmosphere, that the student will soon learn to reserve it for special objects and the finest weather, when it will sometimes tell admirably. A very low power is apt to surround bright objects with irradiation, or glare. Experience in all these matters is the surest guide.

It may be very useful to know the diameter of the field of each of our eye-pieces. This may be obtained from the time which an object in or very ne*ir the equator takes in passing centrally through it: any star having but little declination will answer (y Virginia and 8 Orionis may be especially mentioned), or the moon or a planet in a corre- sponding position. Several trials may be made, and the mean result in minutes and seconds of time multiplied by 1 5 will give the diameter of the field in minutes and seconds of arc, or space, at the equator.

1 6 THE INSTRUMENT AND THE OBSERVES.

7. Do not be dissatisfied with first impressions. When people have been told that a telescope magnifies 200 or 300 times, they are often disappointed at not seeing the object apparently larger. In viewing Jupiter in opposition with a power of only 100, they will not believe that he appears between two and three times as large as the moon to the naked eye ; yet such is demonstrably the case. There may be various causes for this illusion ; — want of practice, — of sky-room, so to speak, — of a standard of comparison. A similar disappointment is frequently felt in the first impres- sion of very large buildings ; St. Peter's at Rome is a well- known instance. If au obstinate doubt remains, it may be dissipated for ever when a large planet is near enough to the moon to admit of both being viewed at once — the planet through the telescope, the moon with the naked eye.

8. Do not lose time in looking for objects under un- favourable circumstances. A very brilliant night is often worthless for planets or double stars, from its blurred or tremulous definition ; it will serve, however, for grand general views of bright groups or rich fields, or for irresolv- able nebulae, which have no outlines to be deranged : a hazy or foggy night will blot out nebula and minute stars, but sometimes defines bright objects admirably ; never condemn such a night untried. Twilight and moonlight 1 are often advantageous, from the diminution of irradiation. Look for nothing near the horizon; unless, indeed, it never rises much above it ; nor over, or to the leeward of a chimney in use, unless you wish to study the effect of a current of heated air. If you catch a really favourable night, with sharp

1 Secclii has found the detail of the Great Nebula in Orion much more visible in moonlight, which is also known not to obliterate even such objects as the satellites of Mars and Uranus, or some of the minuter comites of double stars.

THE MODE OF OBSERVATION. 1 7

and steady vision, make the most of it ; you will not find too many of them. Smyth, who thinks our climate has been unfairly depreciated, says : * Where a person will look out for opportunities in the mornings as well as evenings, and especially between midnight and daybreak, he will find that nearly half the nights in the year may be observed in, and of these sixty or seventy may be expected to be splen- did/ But ordinary students must of course take their chance, with their fewer opportunities. With due precau- tions as to dress, nothing need be feared from ' night-air : ' that prejudice is fully confuted by the well-known longevity of astronomers, even of such as have habitually protracted their watchings

" Till the dappled dawn doth rise."

9. In examining faint objects, do not prepare the eye for seeing nothing, by dazzling it immediately beforehand with a lamp, or white paper. Give it a little previous rest in the dark, if yon wish it to do its best.1

i o. When a very minute star or faint nebula is not to be seen at once, do not give it up without trying oblique or averted vision, turning the eye towards the edge of the field, but keeping the attention fixed on the centre, where the object ought to appear ; this device, with which astronomers are familiar, is often successful ; its principle depends pro- bably on the greater sensitiveness of the sides of the retina.

ii. Do not avoid the trouble of recording regularly all you see, under the impression that it is of no use. If it has no other good effect, it tends to form a valuable habit of accuracy ; and you might find it of unexpected importance.

1 Herscliel II., when about to verify his father's observations on the satellites of Uranus, prepaied his eye with excellent effect, by keeping it in utter darkness for a quarter of an hour.

VOL. !, 0

1 8 THE INSTKUMENT AND THE OBSERVER.

And, like old Schroter, trust nothing to memory. If there has been haste — and sometimes if there has not — it is sur- prising what unforeseen doubts may arise the next day: make at least rough notes at the time, and reduce them speedily into form, before you forget their meaning.

1 2. Do not be discouraged, by ignorance of drawing, from attempting to represent what you see. Everybody ought to be able to draw ; it is the education of the eye, and greatly increases its capacity and correctness; but even a rough sketch may have its use ; taken on the spot, and compared with the original, it will not be all untrue ; it may secure something worth preserving, and lead to further improve- ment.

In conclusion, may I be permitted to remind the young observer not to lose sight of the immediate relation between the wonderful and beautiful scenes which will be opened to his gaze, and the great Author of their existence ? In look- ing upon a splendid painting, we naturally refer its excel- lence to the talent of the artist ; in admiring an ingenious piece of mechanism, we cannot think of it as separate from the resources and skill of its designer; still less should we disconnect these magnificent and perfect creations, so far transcending every imaginable work of art, from the remem- brance of the Wisdom which devised them, and the Power which called them into being. Such is eminently the right use of the Telescope — as an instrument, not of mere amuse- ment or curiosity, but of a more extensive knowledge of the works of the Almighty. So new an aspect as has thus been given to the material universe — so amazing a disclosure as has thus been permitted to man, of the vastness of his Maker's dominion — can hardly be ascribed to blind accident or human contrivance : in thus employing Galileo's invention, we may well feel his grateful acknowledgment, that it was

THE MODE OF OBSERVATION. 19

the result of the ' previous illumination of the Divine favour ' l to have been not only beautiful, but true.

NOTK ON CELESTIAL PHOTOGRAPHY.

Celestial photography has been so largely undertaken since the last edition that a few hints may be useful to the amateur. The moon may be photographed in the focus of any telescope. It must be remembered, however, that in the refractor the visual ami photo- graphic images are not identical. In large telescopes a correcting lens is applied, but the actinic focus of a lens may be readily obtained by focussing the image through a violet screen of ghiss or gelatine which alone transmits the chemical rays. For photographing the stars, commercial lenses have been used with success by Gill, Pickering, Es, Wolf, etc. It will be obvious that the space-penetrating power will depend upon the aperture of the lens, its focal length, the sensitiveness of the plate, the duration of exposure. For this work lenses with large angular apertures are preferable, and short focal length, as the shorter the focus the smaller the images ; the two largest at the present time are the Bache telescope, at Harvard, and the Compton telescope,2 at the Wolsingham Observatory, each having an aperture of 8 inches, and focal lengths of 44 and 42 inches respectively. The Bache telescope is a doublet which allows of a larger field and better definition. With these telescopes, stars of magnitude 14, Argelander's pcale, are photographed with 60 minutes exposure. At the present moment, single long-focussed lenses of 1 3-inch aperture are in use at several observatories, and are employed in making a chart of the heavens. From the great focal length of the lens, however, the field is naturally small, and the whole sky has been covered meanwhile by the energetic Director of the Harvard Observatory, who, after com- pleting, the photo-survey of the northern heavens, sent the Bache telescope to Arequipa. The focal lengths of the Bache and Compton telescopes give plates on the same scale as Argelander's charts of the Northern heavens, and hence the stars are readily identified. A more powerful instrument, of 24-inch aperture, is at present in course of erection at Harvard. The Bache telescope is driven by an

1 Divina prius llluminante gratia.

8 Made by Messrs. Hinton & Co., Bedford Street, Strand.

2O THE IXSTRUMKNT AND THE OBSERVER.

ingenious electrically controlled clock, but the more usual methorl is to fix the photo- telescope to a guide telescope. Admirable work has thus been done by Wolf with a lens of eornewhat over 5-inch aper- ture, and by Barnard and others, the camera being strapped to an ordinary equatorial telescope. These photos have greatly increased our knowledge of the heavens, for besides detecting new nebulae, and remarkable groupings and configurations, they have shown that tho milky way consists of stars intermingled with nebulous matter, and suggest the possibility that the background of the milky way, and perhaps of the whole heavens is one vast nebula. Photography gives us a ready way of detecting variation in the light of the stars. Should any star vary, plates taken at intervals will show it, and though tho actual magnitude on either plate will be affected by the colour of the star, red stars decreasing, bluish stars increasing in size, yet the plates, when compared together, will at once show if there is variation, both alike having the same colour equation. Probably the best way of determining differences of magnitude, is by allowing the stars to trail for a minute or more at the end of the exposure, thus leaving for the brighter stars lines instead of round images, which are frequently over exposed. As regards developers, there are so many that it is difficult to choose. It must be remembered that the object is to obtain the faintest detail. Therefore, the plates should be of the most rapid kind, and the developer of the strongest. Most photo- graphers have in stock a one-fluid developer, made of hydrokinone and caustic potash. This is excellent, but care should be taken to see that the plate is completely flooded by the developer. During development the solution should be kept in motion. This may be done by a clockwork rocker. The plate should be carefully shielded from the light and left in the solution for fifteen or twenty minutes at least, as the faintest detail must be got out. As soon as the surface begins to turn dark development must be stopped. It should then be washed, and immersed in the usual fixing solution of hyposulphite of soda. Plates exposed to the action of caustic potash are liable to frill. Should there be any signs of frilling the plate should be immersed in a strong bath of alum and water. Finally, it should be well washed in running water, and then placed aside to dry. As in celestial photography, the exposure must be long compared with daylight work, every precaution should be taken to insure success. The dishes should be clearly marked, so that even in a dim light there should be no confusion. Both dishes and bottles and hands should be kept scrupulously clean. The lens should be well pro-

THE MODE OP OBSERVATION. 21

tected so as not to become dewed over, and an examination of it should be made between the exposures to see that no dew has been deposited. A slight film of dew will ruin a whole night's work. Plates should be invariably handled by the edges, and the film should never be touched with the finger. Plates may frequently be inten- sified with advantage. If the angular aperture of the lens is large, moonlight will rapidly fog the plate, and no lengthy exposure should be tried when there is any moon. Even on a dark night the plates exposed for an hour in the Compton Telescope show a distinct boundary between the part exposed to the sky and that shielded by the edge of the carrier. If the focal length of the photo and visual telescope are nearly the same, the star must be followed with a high power, as a small error in the driving-clock will causo the stars to shift on the plate. When the plate is examined under a magnifier the stars should appear as round dots, the size varying with the magnitude. With nebulae, the shorter the focal length the smaller and brighter the image, consequently a small lens of short focus will show greater extension than a large one of long focus. In photo- graphing comets, the nucleus should be carefully followed with the guide telescope, as comets generally have a rapid motion of their own. Wolf and Charlois have detected a large number of asteroids by photography. If an ecliptic star is accurately followed the asteroids in the region will come out as lines on account of their motion during the exposure.

The various results of focal length, aperture, and exposure may be shown from the following table, condensed from one given in the Harvard College Observatory Annals, vol. xviii. No. 7.

Aperture. Focal length. Exposure. Limiting

Inches. Inches. Minutes. Magnitude.

2'I — ... 15 ... II'I

4 — — 25 ... 115

8(Bache) ... 44 ... 25 ... 135

8 (Bache) ... 44 ... 61 ... 147

13 (Henry) ... 131 ... 180 ... 147

8 (Bache) ... 44 ... 82 ... 15-1

13 (Henry) ... 131 ... 240 ... 15-5

20 (Robert's reflector) 144 ... 120 ... 15-5

This table shows the great advantage of large aperture and short focal length.

22 THE INSTRUMENT AND THE OBSERVER.

II.

NOTE ON THE SPECTROSCOPE APPLIED TO THE TJKLESCOPE.

TJte Sun. To see the prominences the slit should be nearly closed and placed tangentially to the edge of the sun's image. The instru- ment should then be focussed on one of the hydrogen lines. Now open the slit very gradually and the prominence will be seen on the line. Clapham has seen the prominences with a 5-prism direct vision on 3|-inch refractor; J. Evershed, jun., has done excellent work with 2j-inch achromatic and six small prisms, the observations being made on the C line, which Mr. Evershed finds preferable to F. Sidgreaves sees the prominences easily with a single dense glass prism of 60° on 5^-inch refi actor.

The Moon and Planets, as might be expected, give a solar spectrum with some slight differences. In Mars there are evidences of aqueous vapour ; as also in the case of the brighter asteroids. The spectrum of Uranus is crossed by absorption bands of great intensity, which may be easily observed without any slit, and Huggins, by photo- graphy, finds solar lines, showing reflected sunlight.

Tlie Stars. Here we dispense with a slit, and substitute a cylindrical lens. An elaborate instrument, however, is unnecessary. Huggins long since showed that the stellar spectra may be well seen by holding a small direct vision prism between the eye-piece and the eye. A better plan is to mount the prisms, not less than five — those of the rainband spectroscope answer admirably — in a sliding adapter before the eye-piece. Some star with banded spectrum, such as a Oriouis and a Herculis, should be focussed to a line, and then the spectroscope and eye-piece moved slowly outwards till the bauds are distinctly seen. By separating the distance between the prisms and the eye-piece increased dispersion will be obtained. The observer should practise on stars with banded spectra, like a Oriouis and a Herculis, till he has thoroughly mastered the instrument. Stellar spectra were divided by Secchi into four types, and the later researches of Pickering have shown that this is the best division. Vogel, however, adopts only three. Classification may be made as follows : —

Type I. — (a) The hydrogen lines and some other lines are dark, as in

Sirius and Vega.

(1) Lines are wanting, and the spectrum is perfectly con- tinuous, (c) The hydrogen lines are bright, as in 7 Cassiopeiao.

THE MODE OF OBSERVATION. 23

Type II. — The stars show a strong resemblance to the solar type ; as in Aldebaran, Arcturus, etc.

Type III. — The spectrum is columnar, the bands being sharply defined on the more refrangible side and fading away on the less. In some cases, as, for instance, in a Orionis, the bands are resolved into innumerable fine lines. Most of the long-period variable stars belong to this type, but have in addition bright lines of hydrogen, etc., like Mira.

Type IV. — The stars have large absorption bands, due to carbon; the bands are sharply defined on the less refrangible side. 19 Piscium is the brightest of this class. The greater part are irregularly variable to the extent of one magnitude, and some are long-period variable stars, as U Cygni ; V Cygni ; T Cancri.

Type V. — These stars consist of bright lines and bands and resemble the nebular spectra.

The spectrum of Novae is a complex one, having bright lines flanked with dark ones, and in some respects seems to resemble the spectra of third-type variable stars. Motion in the line of sight displaces the lines of the spectrum. If the body is moving towards the observer the wave length is shortened, and the line moved towards the violet, if it is moving away the wave length is increased and the line moved towards the red. This is analogous in sound to a train passing rapidly when the locomotive is whistling. It will be noticed that the note is sharpened as it approaches and flattened as it recedes. Whatever be the explanation of the duplicity of the lines in a nova, the displacement clearly shows motion in the line of sight. Much valuable information has been obtained as to motion of the stars in the line of sight by this method. The doubling of the lines shows that the star consists of two bodies in rapid revolution. The first star of this class discovered was £ Ursae, the observers at Harvard finding that the K line in the stellar photographs was doubled every fifty-two days. £ Aurigae in the same way shows a period of revolu- tion of four days. Professor Vogel has detected orbital motion in Spica. Thus a new field is opened out, and when the telescope will no longer separate stars on account of their proximity, the spectro- scope comes in and shows us Binaries where the period is no longer of years but of days, we may perhaps go further, and say of hours, since photographs show that the displacement of the lines in Algol agree wilh the period of variation, and some of the Algol stars have periods of less than a day. For photographing the spectra of stars a prism

24 THE INSTRUMENT AND THE OBSERVER.

before the O.G. is often used. This turns the star's image into a line, and by slightly altering the rate of the driving-clock, the line is expanded on the photographic plate into a band.

Nebulas. The spectra of nebulse usually consists of three pro- minent lines grouped in the bluish green. One is the F line, the line that is the brightest lias been referred to magnesium by Lockyer, but this is denied by Huggins and Keeler. The third line Lockyer finds due to iron. Other lines have been seen from time to time.

Comets. When the comet is a great distance from the sun, the spectrum shows three bands due to hydro-carbon. At Perihelion many other lines appear, notably those of sodium and iron.

PART II.

THE SOLAR SYSTEM.

O domus luminopa et speciosa, dilexi decorem tuum, et locum habitations glorise Domiui inci, fabricatoris et possessoris tui ! — ST. AUGUSTINE.

THE SUN.1

THE solar phenomena are specially wonderful. The un- rivalled pre-eminence of that glorious sphere, the dependence of our whole system upon the mysterious processes de- veloped at its surface, the rapid and extensive disturbances of which it is the scene, as well as (in fine weather) the daily visibility of the object, all combine to invite research. But the student had better not begin here : more than one astro- nomer has suffered from that piercing blaze : Galileo pro- bably thus blinded himself wholly, and Herschell I. in part. With due precaution, there is no danger; but the eye and hand had better first acquire experience elsewhere. Much depends on the dark glass of the solar cap which is to be screwed on the eye-piece ; red is often used, but is not always dark enough, and transmits too much heat ; green is cooler, but seldom sufficiently thick. The Germans have employed deep yellow. Herschel I. adopted, with great success, a trough containing a filtered mixture of ink and

1 The new matter is from manuscript notes furnished me by Miss Brown. — [EDITOR.]

26 THE SOLAR SYSTEM.

water. Cooper, at Markree Castle, Ireland, used a ' drum ' of alum water and dark spectacles, and could thus endure the whole aperture, 13^ inches, of his great 25-foot achro- matic.1 With large apertures, a plane surface of unsilvered glass placed diagonally, as originally suggested by Herschel II., so as to reflect only a small fraction of the light and heat, is found of eminent service. Merz, of Munich, has so reduced the light by polarisation at four such surfaces, that a dark glass, the tint of which is of course better dis- pensed with, becomes unnecessary; and the same end is attained in a very ingenious double-prism eye-piece devised by Prof. Pickering : neither apparatus, however, is free from accidental colour. An eye-piece constructed by Andrews with two lenses of complementary tints, burnished in loosely to avoid fracture from expansion, has succeeded with a small aperture. In screen-glasses combinations of colour are good. Red succeeds perfectly with green, or with green and blue. Herschel II. used green and cobalt blue.2 A Barlow lens, carefully silvered, is said to act admirably, though a light screen might still be necessary. If there is to be only one solar .cap, deep bluish grey, or neutral tint, will be quite satisfactory ; if several, it would be worth while to have different colours, Secchi's observations at Rome seeming to show that the visibility of very delicate details may depend on the tint. In the absence of a proper screen, smoked glass may be used : it is said to intercept heat very perfectly, by Prince, who places it within the eye-

1 Not, however, an example to be imitated. Dawes thought that with a focus of 30 inches, 2 inches of aperture were enough for perfect security. A 4f-inch silvered mirror is not safe for screen-glasses.

2 The value of complementary, or at any rate dissimilar, tints in protecting the eye was known before the telescope. Fabricius ob- served a solar eclipse in 1590 * per duplex diversi coloris vitrum; ' and Apian speaks of them 50 years earlier.

THE SUN. 27

piece, close to the * stop/ or circular opening, which bounds the field ; but thus it can have only one degree of depth, and must be taken out to view other objects. A strip of glass may be smoked to different densities in different parts, and held between the eye and eye-piece ; but it should be protected from rubbing by a similar strip of glass placed over it, and kept from touching by bits of card at the corners, the edges of the two strips being bound round with gummed slips of paper, or tape.1 Where expense is not regarded, an optician will provide a delightful graduated screen with two wedges of glass, plain and coloured. A more complete command may be obtained by two such wedges sliding behind one another in a brass cap. In any case wo should not begin with too faint a shade, but try the deepest first, and change it if necessary. The thickness of any external screen will contract the field much, unless the eye is brought as close up as possible. A pleasanter view is obtained by placing the screen within the adapter, but from its larger surface it will require to be worked very true.

To bring the Sun into the field, do not attempt to look for it with the finder unless it has a solar cap : point the telescope till the finder shows it centrally on the hand, or on a paper held behind it; or bring it to shine through the eye-piece before the dark cap is screwed on.

With these precautions, there need be no fear for an ordinary sight, though long and uninterrupted observation is not desirable, from the heat of the screen so near the eye ; on which account, when not in actual use, the telescope should be pointed a little on one side. Should the light or heat be still unpleasant, the aperture may be contracted

1 Gum-water or mucilage should always be made with cold water. It is far btrouger, and keeps for a loug time without growing mouldy.

28 THE SOLAR SYSTEM.

as recommended for defective glasses,1 or, for a very sensi- tive eye, or a whole company at once, the image of the Sun may be received direct from the eye-piece, without any screen, on card. Choose a field large enough to take in the whole disc, and alter the focus till the image on the card is quite sharp, and at a convenient distance as to size ; any spots then visible will be easily and, with due precau- tion, very fairly seen. And so will specks of dirt in the eye-piece ; but these may be detected by moving the tube, is the true spots alone will keep their places in the image. If the eye-piece includes only part of the Sun, do not mis- take the edge of the field as shown on the card for the Sun's limb ; both are circular, but the latter only will move so long as the telescope is fixed. If a circle of suitable size is drawn on the card, and crossed by lines forming small squares, the image may be adjusted to coincide with it, and the progress of the spots may be marked and recorded day after day.2 Noble has found that plaster of Paris, smoothed

1 Schwabo used a contracted aperture and light screen ; Her- echel I. and Dawes preferred full apertures and deep screens, for sharper definition. A small aperture ha?, however, one material advantage in preserving the screens from cracking. I have known a double screen demolished in a few seconds; partial fusion also, or blistering, may be produced by a large aperture.

2 A simple method of determining the heliographic latitude and longitude of spots has been invented by Professor Thompson. It consists of a set of caidboard discs, 8 inches in diameter with lines ruled on them to correspond with the varying positions of the sun's axis at different times of the year. They require to be used with an equatorially mounted telescope, and should be attached by a light frame to the eye end of the instrument in order that the sun's image may be projected on the disc, and made to coincide with the 8-inch circle. The necessary data for reducing the Observations and finding the position angle of the sun's axis, and its inclination to the ecliptic are given in the " Companion to the Observatory" for every fifth day,

THE SUN. 29

while wet on plate-glass, gives a most beautiful picture ; ho fixes a disc of it inside the base of a pasteboard cone, black- ened within, i foot long, and 6 inches across the large end : the small end being opened so as to fit close on the eye- piece, with a hole in the side of the cone to look at the image. Hewlett prefers the projected picture to a direct view. It has at any rate the advantage of avoiding the tinge given by a screen-glass. In either mode of observa- tion, if an achromatic is employed, it is an excellent plan to shade the face in the one case, or the screen in the other, by a large piece of pasteboard with a hole in it, through which the tube passes.

All being arranged, we shall find four points especially worthy of attention : i, the dark spots ; 2, the faculae ; 3, the mottled appearance ; 4, the transparent atmosphere.

i. The Dark Spots. These are not always visible; the disc is occasionally entirely free from them, but more fre- quently one or more will be in sight. Unless very small, they generally consist of two perfectly distinct parts — a dark ' umbra,' usually termed the ' nucleus ' by the older observers, often very irregular in its outline,1 which resembles, as Secchi remarks, the creeping of a very dense luminous material over an extremely rough surface ; and a surrounding * penumbra ' (the * umbra ' of former days), a fainter shade with an equally definite, but in general less angular boundary, usually, according to Schwa be, in proportion to the umbra as 2 to i, or as 7 to 3. The umbra appears black from contrast, but is

the intervening days being easily calculated. These discs may be obtained from Miss E. Brown, of Cirencester, 28. 6d. the set.

1 A double flexure, like that of the letter S, is not unfrequent. It was noticed by Messier in the great spot of 1759 (the colour of which, he says was *brun fonce'') ; and in modern times by Chacor- DMC and others. Howlott remarks this curve in the grouping of famuli spots.

30 THE SOLAR SYSTEM.

not quite so, as is evident when Mercury in transit, or the limb of the Moon in an eclipse, passes near it. In such a juxtaposition, during the eclipse of July 1860, Dembowski found the lunar disc perfectly black, the umbra * brun fonce.' Frequently the umbra is slightly and unequally illuminated, as if partially overspread by a thin haze, which Secchi com- pares to cirri, or mare's-tail clouds, and finds to be the harbinger of its decrease and extinction; sometimes it is intersected by narrow white veins, or bridges ; these Huggins has occasionally found of especial brilliancy, and in one of them Secchi detected by the spectroscope a different consti- tution from that of the photosphere. In Dawes's very ingenious solar eye-piece,1 a sliding plate, or wheel, of metal contains a series of holes gradually decreasing in size, each of which may limit the field in turn, while the whole is insulated by ivory, so as to prevent the eye-piece from getting heated ; thus most of the luminous part may be shut off, and the spot alone viewed with a very light screen-glass. In this way, he detected in all large 2 and many small urnbne, a per- fectly black spot, or opening, of much smaller size, which he termed the * nucleus,' and the presence of which, he believed, might point out an important difference in the origin of the spots. Secchi, who concurs in this discovery, finds that holes in a glazed visiting-card (which may be burnt with a red-hot needle) answer well : the card, however, should be placed a little way from the focus, or the edges of the hole may bo

» A similar idea had been entertained by Professor Wilson, of Glasgow, in the last century, who thought a screen would not be necessary. But Langley says that even the darkest part would bo insupportable to the naked eye.

2 He mentions, however, one large and unusually changeable spot, in which he could detect no nucleus (1859). Brayley observes that they had been already figured by Herschel II. at the Cape.

THE SUN. 31

charred. A Barlow or even common concave lens, inter- posed before the rays reach the focus, will divert a great portion of the heat, and an excellent amateur arrangement may be made by a combination of concave lens, card diaphragm, and screen -glass. Howlett and others have occasionally perceived the nuclei without any such assistance, and Buffham has seen them frequently with a 2y^-in. object- glass; often multiple ; once (1870, June 20) 5 in a single umbra : Huggins finds frequently 3. The feeble illumination of the umbra Dawes ascribed to the presence of a « cloudy stratum ' beneath the photosphere : this Secchi, with Merz's polarising apparatus, finds tinged with a rosy hue. The most diligent of solar observers, Schwabe of Dessau, has seen an occasional reddish-brown colour in spots, whose immediate contiguity to others of the ordinary greyish-black precluded deception; in one instance, three telescopes, and several by-standers, agreed as to this fact. Capocci, in 1826, perceived a violet haze issuing from each side of the bright central streak of a great double umbra : Secchi, during the eclipse, i858,March 15, remarked a rose-coloured promontory in a spot visible to the naked eye. Schmidt records many tints, chiefly violet umbra) and yellowish penumbras, especially as cast on paper ; Howlett and others have noticed brown, and Lockyer copper-coloured and violet, tints in umbraB.1 Birmingham has also seen a red cloud suspended, apparently, across an umbra and nucleus. The penumbra — which in most cases encompasses considerable umbraB, occasionally comprises a group of them, and frequently z outlasts them —

1 Miss Brown sees red tints occnsionally, but never copper-colour or violet. An * over-corrected ' object-glass might possibly cause a violet tinge ; and the strong influence of contrast must, at least in some cases, be taken into account.

8 Miss Brown's long series of careful observations do not confirm this.

32 THE SOLAR SYSTEM.

is made up, according to Schwabe, of a multitude of black dots usually radiating in straight lines from the umbra. Sccchi, with greater optical power, finds these radiations to be alternate streaks of the bright light of the photosphere and dark veins converging to the umbra. The penumbra, Herschel II. observes, occasionally shows * definite spaces of a second depth of shade;' it is generally darkest at the outside ; l an appearance which is proved by photography to be no illusion from contrast ; sometimes it includes brilliant specks, or streaks, even close to the umbra. Schmidt describes one of these insulated specks as the brightest portion at that time visible. They have been frequently seen to disappear in floating over the umbra. Sun-spots are of all shapes and sizes, up to enormous dimensions, the umbra frequently surpassing the earth greatly in magnitude. The penumbra, especially of a group, is often much larger. Herschel II., at the Cape of Good Hope, estimated the area of one to be 3,780,000,000 square miles. Schwabe and Schmidt speak of groups which have extended across more than a quarter of the disc. The length of one, observed by Hevel in 1643, is said to have occupied one-third of it. Spots exceeding 50", Schwabe finds visible to the naked eye through a fog, or dark glass: he has often recorded such instances, sometimes repeatedly in 12 months; and Dawes states that a year seldom, if ever, passes without them. When thus perceptible they surpass the earth at least 3 times — if conspicuous, much more. A gregarious tendency ia obvious, and the groups are apt, especially in certain seasons, to be nearly parallel with the solar equator.

' This was figured by La Hire as far back as 1700. Dawes found it to be the case with his cloudy stratum also, but only in opening spots. Buff ham and Andrews have seen a nucleus encompassed by a narrow grey ring.

THE SUN. 33

Herschel II. says that, if they converge, it will be towards the preceding side of the disc. They are absent from the poles, and infrequent and of short duration for about 10° on each side of the equator : beyond this region are two fertile zones, reaching as far as 30° or 35° each way; sometimes they exceed these bounds. Peters (U.S.) saw one (June, 1846) in 50° 55', La Hire in 70°, of solar latitude (if cor- rectly reduced, which Carrington thinks questionable). The observations of Peters and Carrington tend to unsettle these limits, which may be subject to change: the latter astronomer has ascertained that, previous to the minimum epoch, the spots break out nearer to the equator, and the reverse afterwards.1 The numbers are said to be greater in the N. than S. hemispheres. Schwabe finds that the W. members of a group disappear first, and new ones are apt to form on the other side, on which are the greatest number of minute companions, and on which the spots themselves generally increase, decreasing the opposite way ; also that the small points are usually arranged in pairs; and that, near the edge of the Sun, the penumbraB are much brighter on the side next the limb. Herschel II. saw the penumbras often best defined on the preceding side ; and Capocci found that the principal spot of a group leads the way, and that the umbrae are better defined in their increase than diminution. Peters and Carrington observe a remarkable tendency to divergence in adjacent umbras.2 Groups are frequently

1 This is now an established fact, the spots not only breaking out, but gradually approaching the equator as the minimum period draws near. When it has passed they suddenly appear in high latitudes, showing that a new cycle has commenced. So that after the minimum there are for a time, as it were, two spot zones, one in the high latitudes and one nearer the equator.

2 Miss Brown divides spots into eleven classes : (i) Normal ; (2) VOL. I. D

34 THE SOLAR SYSTEM.

elliptical, curvilinear, or bifurcated. The extraordinary mutability of the spots will be obvious ; frequently they are in continual change, varying from hour to hour, and even more rapidly. Herschel I. lost a group while merely turning away his eye for a moment : Biela has found spots disappear while he looked at them : Krone has observed them to form within a single minute: Schwabe saw a penumbra increase from i' 3" to 5' 2'' in 24*. Lubbock has seen spots with the naked eye, of which a telescope would show no trace next day. Capocci noticed the temporary reduction of an umbra, four times as large as the earth, to the dimensions of Europe, ' under his eyes : ' an unfortunately vague expression, as the Academic des Sciences has remarked, but characteristic of that surprising fluctuation which must strike every observer. Dawes has alluded to the probability that the state of our own atmosphere may be concerned in many of these apparent variations.

The inquiry into their nature is very perplexing, from the absence of terrestrial analogies, the Sun evidently belonging to a wholly different and entirely unknown class of bodies. The theory of Professor Wilson of Glasgow, modified by Herschel I., has been very generally adopted, that the spots are openings l in a blazing envelope or * photosphere,' 2 through which we see, in the penumbra, a deeper and less brilliant region ; at a still greater depth, in the interior of the umbra, a feeble luminosity, marking the ' cloudy stratum ' of Dawes (or ' cirri ' of Secchi) ; and below both these, in

Compound; (3) Pairs; (4) Clusters; (5) Trains; (6) Streams; (7) Zigzags; (8) Elliptical; (9) Vertical; (10) Nebulous; (n) Dots.

1 According to Klein, this was first suggested by Schulen in 1770. Lynn has, however, shown that Klein was mistaken.

2 Schroter used this very appropriate and now universally-admitted term as far back as 1792.

THE SUN. 35

the nucleus, the non-luminous body of the Sun. This view rests on the perspective appearance of the penumbra, when near the limb, which usually 1 is more contracted on the side next the Sun's centre ; and the depression has been supposed to be corroborated by several observations of actual notches in the limb.2 Herschel I. thought these openings might be caused by invisible elastic vapour, rising from the dark body of the Sun, and expanding in its ascent ; such is also the view of Secchi, Chacornac, and Dawes, who refer the brighter edges of the openings to their being * folded back,' as it were, by the rush from beneath. On two occasions, in 1 86 1, Schwabe found the limb faint and indistinct beyond spots recently entered. Herschel II. inclined to the idea that a transparent atmosphere above the luminous stratum may be subject near its equatorial regions, like that of the earth, to hurricanes, forcing their way downwards to the surface. Circular movements are occasionally traceable: they were noticed by Silberschlag a century ago; Elvins and Knobel observed them in 1867 ; and Dawes detected them in two nuclei, one rotating through 100° in 6d, the

1 De la Ruo, Stewart, and Loewy have found, in longitude, 75 cases having the penumbra equal on both sides, 456 giving tne per- spective of depression, 74 the reverse; in latitude, 72 cases for, 17 against, depression, which they consider sufficiently established. It has been said, however (Klein, Anleitung, 60), that larger instruments and higher powers do not confirm the idea.

8 An indentation on a globe will disappear in profile, unless its breadth and depth are considerable : hence such observations would be rare; they have been recorded by La Hire, 1703; Cassini, 1719; Herschel I., 1800; Dollond and others, 1846; Lowe, 1849; Newall, 1850, 1859; observers at Kew and Dessau, 1868; but some of these may have been due to inferiority in optical means. If the spots were masses suspended above the photosphere, as Kirchhoff and a few others still maintain, they would, as Howlett well observes, be often seen as notches in the limb.

3<5 THE SOLAR SYSTEM.

other through 70° in 24**. Secchi has also perceived, besides several cases of rotation, a spiral structure in the penumbne and nuclei of certain spots. Something of the kind has likewise been t \vic3 delineated by Birt, and noticed by several other observers, especially Lohse in 1872; and, though Sporer thinks it deceptive, it deserves careful atten- tion. The excentricifcy or lateral deficiency occasionally noticed in the penumbra seems to indicate an oblique direction of disturbance. Secchi has revived Wilson's idea,, that the penumbra may slope inwards : he calculates that their depth is about ^ the semi-diameter of the earth, or upwards of 1300 miles— a depression which the subsequent computations of Faye have extended through a space of from 2000 to 3800 miles. From the nature of the photo- sphere, we might conjecture that of the spots, were it not equally unknown. Dawes, Huggins, and Schwabe, like Herschel I., infer an irregular distribution of luminous clonds: Arago's polariscope experiments were thought to have shown that the light is that of flame, not of white-hot solid or fluid matter ; but the result is questionable. Secchi and Henry have shown that the spots are relatively cool. Herschel II. deduced the partial removal of definite films, floating on a dark or transparent ocean, rather than the melting of mist or mutual dilution of gaseous media, and the analogy of the Aurora Borealis has also been alluded to by him and his father. At present, however, the idea seems to be obtaining currency that the darkness results from the absorption of the light of the photosphere in traversing vapour below a certain degree of temperature, and that the spots are produced by descending currents of gaseous material which have become cooled from their propulsion into a higher region. It is known at least that an intimate connection exists between the number of spots and the

THE SUN, 37

number and magnitude of the upward currents which cause the prominences surrounding the solar limb. It does not fall within our province to treat of these marvellous appen- dages, at once beautiful in tint, fantastic in form, and astonishing in mutability. The spectroscope, through which alone they can be studied (excepting during a total solar eclipse), is very seldom attached to ' common telescopes ; ' and those inclined to pursue this most interesting branch of research would find far better guidance than could be given in these pages in Schellen's ' Spectrum Analysis ' translated by Miss Lassell, Roscoe's ' Spectrum Analysis,' Secchi's ' Le Soleil,' or Schellen's enlarged German edition of it ' Die Soune,' or Proctor's ' Spectroscope.'

In confirmation of the theory last referred to, it is observed that the prominences on the Sun's limb in the vicinity of spots appear to curl over towards them; but there is still much unexplained ; the prominences occur in all latitudes, even in the neighbourhood of the poles ; but the spots are chiefly confined, as we have seen, to much narrower limits: and on the whole we are far from any satisfactory knowledge of the processes by which they are formed and maintained; our interest, however, in the phaenomena is increasing, since there is now more than a suspicion that they influence the whole dependent system. The extraordinary perseverance of Schwabe l has shown that*

1 The late lamented President of the Astronomical Society, Mr. Johnson, thus refers to the presentation of their Gold Medal to this observer: — 'It was not ... for any special difficulty attending the research, that your Council has thought fit to confer on M. Schwabe this highest tribute of the Society's applause. What they wish most emphatically to exprc-s^ is their admiration of the indomitable zeal and untiling energy which he has displayed in bringing that research to a successful issue. Twelve years, as I have said, he spent to satisfy himself — six more years to satisfy, and still thirteen more to

38 THE SOLAR SYSTEM.

the spots have regular maxima and minima, with a period averaging about 10, or, according to Schmidt and Wolf (from a much more extended comparison),1 ii'ii years; which corresponds so exactly with the period of all magnetic variations, that both, as well as auroras and electrical earth- currents, are now ascribed to the same unknown power, and the spots are no longer objects of mere curiosity, but indica- tions of a mighty force, one of the prime laws of the universe. The revelations of the spectroscope, which, according to Balfour Stewart, have indicated the existence of 23 terres- trial elements in the Sun, are among the most surprising of modern astronomical discoveries, and stand on evidence which seems incontrovertible ; 2 but they still leave much to

convince, mankind. For thirty years never has the Sun exhibited liis disc above the horizon of Dessau without being confronted by Schwabe's imperturbable telescope, and that appears to have happened on an average about 300 days a year. So, supposing that he observed but once a day, he has made 9000 observations, in the course of which he discovered about 4700 groups. This is, I believe, an instance of devoted persistence (if the word were not equivocal, I should say, pertinacity) unsurpassed in the annals of astronomy. The energy of one man has revealed a phsenomenon that had eluded even the suspicion of astronomers for 200 years 1 '

1 An abstract of Dr. Wolfs results may be found in Memoirs of the Eoyal Astronomical Society, vol. xliii., showing that though the average length between successive maxima is irn years, a period

| of 1 6' i years is found between those of 1788 and 1804, while only 7 '3 years elapsed between those of 1829 and 1837. The intervals of remarkable auroral displays correspond fairly with those of sun-spot maxima, and Wolf finds evidence for a longer period, — 56 years, in the latter ; and that their increase is more rapid than their decrease, with a second subsidiary maximum a year or two after their greatest development.

2 The probability resulting from one or two coincidences in the position of lines would of course be but slight, but it rises rapidly with the multiplication of comparisons. Kirchhoff says, that about 60 lines are common to the \apour of iron and the light of the photo-

THE SUN. 39

be investigated; much that we can never reasonably hope to explain.

Concurrent authorities have justified our assuming that the spots are at least depressions, if not openings ; yet observations exist, looking another way ; and it may be well to insert them from their curiosity, as well as their being seldom referred to. Dr. Long, who published a Treatise on Astronomy in 1764, states that he, 'many years since, while he was viewing the image of the Sun, cast through a telescope upon white paper, saw one roundish spot, by estimation not much less in diameter than our earth, break into two, which immediately receded from one another with a prodigious velocity.' Dr. Wollaston says: 'Once I saw, with a 12 -inch reflector, a spot burst to pieces while I was looking at it. I could not expect such an event, and therefore cannot be certain of the exact particulars ; but the appearance, as it struck me at the time, was like that of a piece of ice when dashed on a frozen pond, which breaks to pieces and slides on the surface in various directions. I was then a very young astronomer, but I think I may be sure of the fact.' It is also stated that Bayley, who sailed twice with Captain Cook, saw a spot split in two.1 From such appearances, an observer, unacquainted with the ordinary theory, might easily have inferred the solidity, from the disruption, of the dark object. On chemical grounds, connected (but not as a necessary consequence) with spectrum analysis, Kirchhoff and Bunsen have deduced a constitution analogous to floating

sphere, and the consequent chance in favour of its presence is more tliuu I,OCK),OOO,OOO,OOO,OOO,OOO to one. Yet this is but one set out of a combination of corresponding lines.

1 Some strange separations and oscillations of umbrae, recorded at Lawson's observatory in 1849, were probably due to the unsteadiness of our atmosphere ; but a better observer, Robinson of Armagh, saw a luminous bridge shot across some thousands of miles of umbra in a lew minutes.

40 THE SOLAE SYSTEM.

clouds ; an opinion adopted by Donati and Sporer. Zollner ascribes to them a scoriaccous character. Kaschig (1816), Weiss (1864), Hallaschka, and Haag (1869), observed what they considered a passage of one spot over another.1

Notwithstanding their changeable nature, the larger spots are possessed of some permanency.2 After describing straight lines about June n and Dec. i2,8 but elliptical paths at other times, in consequence of the position of the Sun's equator towards our eye,4 they go out of sight at the W. limb, and, if not dissipated, return at the E. edge after about i3§d to run the same course. Some have thus persisted through many revolutions.5 In 1779^ large spot continued visible for 6 months, and in 1840 and 1841 Schwabe ob- served 1 8 returns (though not consecutive) of the same group : the most permanent, he says, arc usually round, of moderate size, and not sharply denned. Carrington thinks there can be no question of their occasional reappearance in the same places ; as indeed had been supposed by La Hire as far back as 1700: and this reduces within certain limits their proper motion, which was perceived at an early period by Schroter and others, and micrometrically established in a lateral direction by Challis in 1857. Carrington has subsequently made known his very interesting discovery that the period of rotation varies with tho latitude ; the backward drift of the photosphere in the higher latitudes causing a difference of more than 2 days between the respective periods derived

1 Schellen, SpecJrum Analyst*, 273.

2 Hewlett has also found some of the smaller and more isolated ones very persistent.

* So Herschel II., Outlines of Astronomy, § 390. But in the next paragraph they stand as July 12 and Dec. n.

4 Inclination 70° 15', Carrington ; 6° 57', Spore?.

5 Miss Brown, however, finds that it is the exception rather tliau the rule for even large spots to return more than two or three times.

THE SUN. 41

from spots in latitudes ± 50°, and those situated near the equator; so that the period of rotation of a spot may be determined by its latitude, if unchanged : in this respect, however, it may exhibit slight deviation.1 With these shifting landmarks, it is not surprising that the Sun's period of rotation should have been variously estimated. Laugier's value, 25* 8h iom, was formerly adopted; Sporer has given 24d i4h 59m, Carrington 24d 23** i8m 23% for spots in lati- tude 15°. Possibly, as the latter suggests, the anterior mass may revolve with greater speed. Relative displacement in groups would be an interesting study, requiring neither micrometer nor clock, only careful drawing. The applica- tion of photography to solar delineation, as at Kew and Ely, is not likely to be within the reach of our readers : it is, how- ever, too important to be passed over in silence. Hewlett and several others have found that spots near the limb require a different focus from those in the centre ; arising, no doubt, as Dawes says, from the effect on the retina of very different degrees of brightness.

2. The Faculse, or bright streaks. Less obvious than the dark spots, and requiring more power, these are not difficult objects, and were detected by Galileo : they are to be looked for in the spot-bearing regions, but only near the limbs.2

1 Peters (1846) ascribed to all the spots a set towards the equator; Laugier, from it. Carrington's valuable results were the fruit of un- wearied perseverance through n years in the use of a very simple apparatus, consisting only of cross wires in an equatorially mounted telescope, without driving-clock or micrometer. It is to be regretted that his work has not been continued by some possessor of ' a common telescope,' which, with the requisite command of time, would be quite sufficient. A full account of his method is given in Carrington's Observations of Solar Spots (1863)— often to be met with second-hand.

2 They were observed by Carrington and Noble nearer the centre than usual in 1870, a maximum spot-period.

42 THE SOLAR SYSTEM.

They are irregular, curved, and branching, considerably more luminous than their vicinity, but not, according to Secchi, than the centre of the Sun. He observed one in 1858, at least 60° in length. They are proved to be what they appear, ridges in the photosphere, by an observation of Dawes, who once saw a facula projecting above the limb as it turned across it into the other hemisphere, and at another time the luminous border of a large spot which had just entered, forming an irregular low ridge upon the limb : Secchi also has seen a similar appearance. We seldom, however, find them visible close to the limb,1 or far from it, as they are changed in the centre into bright tufts and specks ; an effect, as Secchi has pointed out, of perspective, if their height much exceeds their breadth ; or of the elevation of their crests above an absorbent atmosphere. Hewlett sees them beautifully projected on a screen, and thus detects them in the centre of the disc. They are nearly as variable as the spots, and are probably connected with them, surrounding them usually near the limb, and sometimes as they cross the centre, attending their development,2 and succeeding their dissolution; as though they were temporary accumulations of the displaced matter of the photosphere. Secchi com- pares them to immense waves raised by the outburst of the spots, of which they are commonly the harbingers. Schwabe thinks them rather more obvious when spots are few,3 and suspects that one at least, consisting of 5 to 7 connected ovals, and enclosing occasionally a great group of spots, may

1 Miss Brown sees them often very neaj the lirnb, but finds that there is sometimes an intervening space without any.

2 The greatest authorities are now inclined to think that faculse do not precede spots.

3 He has since adopted an opposite opinion. — Monthly Notices, xxvii. 286.

THE SUN. 43

be a permanent or recurring feature. The authors of ' Solar Physics' consider them (and probably the whole photo- sphere) as consisting of solid or liquid bodies suspended or slowly sinking in a gaseous medium. They find that on an average they follow the accompanying spots, and infer that they have probably been uplifted out of them, and have fallen behind from being thrown up into a more swiftly rotating region.

3. The Mottled Surface. An object-glass of only 2 inches will exhibit a curdled or marbled appearance over the whole disc, caused by the intermixture of spaces of different bright- ness. The earliest mention I have noticed of this mottling is during the solar eclipse, 1748, July 14 (O.S.), when it was clearly described by Mr. J. Short (the eminent optician ?), to whom it was quite new: since that time it has been a familiar object ; and it must have been this coarser mottling which Schwabe described in 1831, as showing itself strongest in the spotted zones, like two freckled girdles round the Sun. Should this marbled appearance not be at once detected, a slight shaking of the image by tapping the telescope may render it perceptible. Increase of magnifying power, how- ever, brings out a far more delicate mottling overspreading the coarser irregularity, and occasioned by the juxtaposition of minute patches of greater brightness on a greyer ground. This was first observed by Herschel I., and described in two very valuable but not perhaps sufficiently known papers on the Sun ('Phil. Trans.' 1795, 1801), in the former of which the whole solar surface is said to have l the appearance of a mixture of small points of an unequal light/ and in the latter is described as f studded with nodules,' which are also referred to a stratum of self-luminous clouds of unequal thickness, of which the higher and lower regions tend respectively to form faculae and penumbrse. It has more

44 THE SOLAR SYSTEM.

recently been interpreted by Nasiuyth (1861) as the inter- lacing of a multitude of lenticular masses resembling willow leaves in form, the length of each being ten times its breadth. Considerable discussion has arisen as to the point ; but the supporters of this idea are few, and the prevalent opinion is that of Daw83, who had been familiar with the phaenomenon since 1830 ; and found, from very careful examination of the photosphere, that it was composed of minute ( granules,' or luminous clouds, irregular in form and size, and separated by less brilliant interstices ; thus agreeing precisely with the view given in 1792 by Herschel I. The less luminous interstices are frequently stippled with dusky or nearly black points, seldom circular and often arranged in rows. In the faculoe and round the penumbrfe, where the photo- sphere appears to be heaped up or rolled together, these interstices usually disappear, and the compressed and elongated granules, like very slender faculae, extend over the penumbra, and often project, like irregular thatch, on to the outer border of the umbra ; or, many being joined together, cross the umbra and nucleus as luminous bridges. The grey interstices were thought by Herschel I. to be portions of a lower stratum ; but, as he remarked, the de- pression must be very slight, since they are visible in extreme foreshortening towards the limb. This view is adopted by many of our best observers, including Fletcher, Lockyer, Huggins, and Knott ; and is supported by 0. Struve, Secchi, and Le Verrier. Huggins adds his impression, that the general surface on which the granules are strewed is itself ' corrugated into irregular ridges and vales,' not unlike a stormy sea ; and coincides with the idea of Herschel IT. as to ' a luminous medium intermixed but not confounded with a transparent and non-luminous atmosphere.' In the use of a i3-iuch achromatic of very fine definition, Langley hus

THE SUN. 45

found that each of these granules is composed of a varying number, 3 to 10, of brilliant points (his granules), which on the whole occupy less than -^ of the Sun, and are the chief source of its light. He thinks that they are the upper ends of elongated filaments, the length of which is seen in an inclined position in the penumbraB ; and he finds evidence in the spots not only of cyclonic action, frequently right and left-handed whirls in juxtaposition, but also of horizontal currents. The same structure is also shown in some fine photographs by Janssen.

4. The Transparent Atmosphere.1 That this exists, and is of far greater proportionate extent than planetary atmo- spheres, is demonstrated in total eclipses by the red pro- minences which then surprise the spectators, and which may, as Janssen and Lockyer have discovered, be detected at any time by the spectroscope : but the presence, and the comparative shallowness, of an absorbent envelope forming its interior stratum is at once evident from the faintness of the limb compared with the centre of the disc. This was early remarked at Rome by Luca Valerio, called by Galileo the Archimedes of his time : it was denied by the inventor of the telescope, but may be easily perceived when the image is cast on paper; and its effect is evident in the

1 It may be well to note that astronomers divide this atmosphere into the Corona, and Chromosphere. Professor Young thus describes them. 'It (the atmosphere) is divided into two portions, separated by a boundary as definite, though not so regular, as that which parts them both from the photosphere. The outer, and far more extensive portion, which, in texture and rarity, seems to resemble the tails of <omets * * * * is known as the "coronal atmosphere" since to it is chiefly due the corona or glory which surrounds the darkened sun during an eclipse * * *. At its base and in contact with the photo- sphere is what resembles a sheet of scarlet fire * * * *, this is the chromosphere.'

46 THE SOLAR SYSTEM.

Kew, and especially the Rutherfurd photographs.1 By means of a doubly refracting prism, Secchi has found the limb not more luminous than the penumbras of spots in the middle of the Sun — that is, deprived of about half the light;2 and his previous discovery of a similar decrease of heat confirms the existence of such an absorbent stratum. Hence also may result the inferior distinctness of the solar limb as compared with that of the Moon in eclipses : a fact remarked by Airy, and exhibited by photography ; and Secchi has observed that the occasional bad definition of the spots may be due to the solar atmosphere instead of our own. Some measures of his, and observations by Sporer, seem to indicate a refractive power in this atmosphere, producing an apparent displacement of the spots towards the limb.

Something must be said of the Sun as a background for the exhibition of intervening bodies. Partial solar eclipses seldom gain much interest in the use of the telescope, excepting from the occasional projection of mountains on the Moon's limb. The total eclipse is so wonderful and so fugitive as to require special preparation, and is too rare in England to require notice here. The transits of Mercury will be found under the head of that planet : those of Yenus are too uncommon and distant to be admitted in our limited space ; the next visible in England not occurring till 2004.

1 Arago's polariscope foiled to exhibit this difference; it has been shown by the later observations of Schwabe, that it does not invari- ably exist, and that when it is not perceptible, there is also an absence of faculse, « scars,' and pores. Sporer, however, is disposed to refer such observations to dew or hoar-frost on the object-glass.

* He has subsequently reduced this to £ or J, and finds the limb of a very decided smoky red tint. Elger has seen a portion of the disc near the limb intensely brown, where large spots have dis- appeared.

MERCURY. 47

Comets, from the endless diversity of their paths, must occasionally traverse the solar disc ; and the opportunity of such a background would be most valuable for acquiring more information as to their nature. If anything unusual should ever be noticed on the disc, it should be carefully watched; and should its rate of progress show that it is not an ordinary spot, its appearance ought to be most critically examined with various powers and screen-glasses, and information telegraphed instantly, where practicable, to some of our principal observatories.

It may never be granted to our readers to witness such a wonderful outburst of light in front of the Sun as was observed by Carrington and Hodgson, 1859, Sept. i ; and repeated, according to Brodie, on a minor scale, 1864, Oct. i.1 But should any of them be so fortunate, the propriety need scarcely be mentioned of the greatest care in the record, and readiness in the publication, of such an event.

MERCURY*

THIS planet, though at times readily visible3 to the naked eye, is but seldom seen from its nearness to the Sun, and often lies too near the horizon for the telescope. A well- adjusted equatorial stand will find it by day, but its small diameter of about 3060 miles subtends at a mean only 7-5" ; and it was sometimes missed, as well as once noticed extremely faint, even by the experienced Schroter.

1 And to Hale and others, July 15, 1892.

9 The new matter is by Mr. A. Stanley Williams.

8 Often seen by Sehroter and Heis with the naked eye " luoenta sole." Brightest from 10 to 14 days hefore greatest eastern (evening) elongations, much fainter at actual elongations.

48 THE SOLAR SYSTEM.

Ordinary observers will not see much, where professed astronomers have usually found little ; but, as these pages may possibly fall into the hands of some whose advantages or enterprise may lead them to attack a neglected object, the following points may be specified : —

1. The Phases. These will be easily seen, and are only remarkable because the breadth of the enlightened part has been sometimes found loss than it should have been from calculation. Schroter noticed this: Beer and Madler con- firm it; but their explanation of a dense atmosphere en- feebling the terminator, or boundary of light and darkness, is inadequate, as their observation was before sunrise, when even the dullest part of the disc would be very luminous. See this again in Venus.1

2. The Mountains. At the close of the last and begin- ing of this century, Schroter, of Lilienthal in Hanover, a most diligent observer, and his assistant Harding, obtained what they deemed sufficient evidence of a mountainous surface in the alternate blunting of the horns,2 some minute projections from the limb, and an irregular curve of the terminator; they gave the inferred elevations a height of nearly 1 1 miles.

3. The Atmosphere. The decrease of light towards the terminator, and the occasional presence of dark streaks and spots, indicated to the same astronomers a vaporous envelope,

1 According to Trouvelot the terminator becomes straight at eastern elongations before, and at western elongations after the calculated times, as is found in the case of Venus.

2 Noble also has suspected this (1864) as to S. horn, and Burton ; and both saw an ill-defined terminator. Franks has seen (1877) the S. cusp blunted. Trouvelot confirms (1876-81) the blunting of tlie S. horn, and the irregular curve of the terminator. Guiot at Soissons has observed the S. horn rounded with a 3f-in. refractor.

MKHCURY. 49

where they even imagined traces of the action of winds. From a combination of their observations Bessel deduced a rotation in 24** om 53" on an axis inclined about 70° to the ecliptic of Mercury. A bright spot with faint lines diverg- ing from it, NE. and S., was seen by Prince a little S. of the centre, in unusually clear air, 1867, June n ; Birmingham was pretty certain that there was a large white spot near E. limb, 1870, March 13; and Vogel at Bothkamp observed spots, 1871, April 14 and 22. In De La Hue's Newtonian, ro-ft. focus, I3~in. aperture, constructed by himself, and now at Oxford, the planet has a rosy tinge, noticed also by Burton, and once by Harding; Franks, however, sees it very white. Secchi has remarked that the disc is always very ill-terminated, and very faint at the edges, but Burton's admirable silvered specula give a sharply defined limb. Venus in the same field appeared to Nasmyth fully twice as reflective as Mercury.1

1 Recent observations have added much to our knowledge of this planet. In 1888 Denning published an important series of observa- tions made with a loj-in. With-Browning reflector. In the autumn of 1882 he obtained some good views of Mercury, and saw tha mark- ings on the disc so pronounced as to suggest an analogy to those of TMars. His observations also led this observer to conclude that the rotation period of the planet must certainly be longer than that usually accepted from Schroter's observations. Schiaparelli at Milan has obtained even more decisive results. His observations were made usually in the daytime, and chiefly with an 8^-m- refractor between the years 1881-89. In the latter year he made the rather startling announcement that the planet performs a rotation in the same time that it completes a revolution round the sun (88 days). So that as the axis of Mercury is apparently nearly perpendicular to his orbit, the planet must always turn the same face to the sun. But owing to a considerable motion of * libration,' resembling that which occurs in the case of the Moon, as much as f of the planet's surface is at times exposed to the sun's rays, the remaining | remaining for ever in darkness. Schiaparelli also iound the markings on the disc so

VOL. I. E

50 THE SOLAR SYSTEM.

Transits of Mercury are comparatively frequent; one will be visible in Europe, 1894, Nov. 10. The planet breaks in upon the Sun as a dark notch, sometimes preceded, it is said, by a penumbral shade ; but the earliest impression will be missed, unless the exact point of ingress is known, and kept central in the field. As it advances, the part of Mercury not yet entered may become visible by being pro- jected upon the f corona/ which is so conspicuous in total solar eclipses, and has been known to relieve dark bodies in front of it, such as an inferior planet, or even a portion of the Moon. (Pratt, 1873.) On finally entering the Sun, or beginning to leave it, the planet has been seen lengthened towards the limb from irradiation : fully on the disc, where Mercury appears intensely black, some astronomers have given it a slight dusky border, others a narrow luminous ring: both, probably, deceptions from the violent contrast and the fatigue of the eye,1 especially as others have re- marked nothing of the kind. A similar explanation may be applied to a whitish or grey spot on the dark planet, seen by Wurzelbauer, 1697 ; Schroter, Harding, and Kohler, 1799; Fritsch and others, 1802; Moll and his assistants, 1832 (when Harding clearly distinguished two spots, and Gruithuisen suspected one); and recognised in England and America, 1848, and by Huggins and Elger, 1868, when Browning perceived two spots again. In 1878 also a bright

definite and of such a degree of permanency, that he was able to construct a map of them.

According to Zollner's photometric researches the 'albedo,' or reflecting power of Mercury is very low, only 13 per cent, of the incident light being reflected. In 1890 the disc of Venus appeared to Denning like newly-polished silver, whilst that of Mercury was of a dull leaden hue. The planets were only 2° apart at the time.

1 Pratt, however, in 1868, saw the 'halo' finely by projection in a darkened room.

VENUS. 5 I

speck was noticed by several English and Belgian observers (in one case, two specks), though others entirely failed to detect it ; and every attempt to recognise it in America was unsuccessful. No terrestrial analogy will explain a lumin- osity thus visible close to the splendour of the Sun ; and it seems natural to refer it to the exhausted state of tho retina: an artificial disc, however, subsequently tried by Huggins, showed nothing of the kind. And everything that is seen, however improbable, should always be recorded : an opposite procedure would have effectually precluded many an important discovery. We may remark that Schroter and Harding ascribed to these spots a motion corresponding with the rotation which they subsequently inferred from other indications.1

VENUS*

Fairest of stars, last in the train of night,

If better thou belong not to the dawn,

Sure pledge of day, that erown'st the smiling morn

With thy bright circlet, praise HIM in thy sphere.

MILTON.

THE most beautiful of all the heavenly bodies to the unaided eye is often a source of disappointment in the telescope. For the most part it resists all questioning beyond that of Galileo, to whom its phrases revealed the confirmation of the Copernican theory ; an important discovery, which he

1 A similar phenomenon was observed on Venus in transit in 1761 (Append, ad. Ephem. Astron. 1766,62); and by some observers in 1874.

8 The new matter is by Mr. A. Stanley Williams.

52 THE SOLAK SYSTEM.

involved for a season in the following ingenious Latin transposition —

" Haec immatura & me jam frustra leguntur, o. y." the letters in their original order forming the words ( " Cynthia) figuras semulatur mater amorum."

Observers in general will subscribe to the experience of Herschel II., who says it is the most difficult of all the planets to define with telescopes. ' The intense lustre of its illuminated part dazzles the sight, and exaggerates every imperfection of the telescope : yet we see clearly that its surface is not mottled over with permanent spots like the Moon ; we notice in it neither mountains nor shadows, but a uniform brightness, in which sometimes we may indeed fancy, or perhaps more than fancy, brighter or obscurer portions, but can seldom or never rest fully satisfied of the fact ; ' and he infers, like his father, and Huygens long before, that * we do not see, as in the Moon, the real surface of these planets' (Venus and Mercury) 'but only their atmospheres, much loaded with clouds, and which may serve to mitigate the otherwise intense glare of their sunshine.1 Notwithstanding, however, the authority of this opinion, perseverance has of late years rendered it doubtful : mode- rate-sized instruments have proved of service : and the silvered reflector, from its colourless image, is here greatly

1 Chacornac was of the same opinion, from the absence of polarisa- tion in the light of Venus. His apparatus showed her brightness to be 10 times greater than that of the most luminous parts of the Moon. The reduction of her light to 5^3 perceptibly diminished the image from removal of irradiation. Jupiter the same. Zollner makes the 4 albedo ' of Venus 0*50, so that the planet reflects about half the light that falls upon it. This high reflective power favours tks view of a 'l«T) atraosphere.

VENUS. 53

superior to the achromatic. The following are the chief points to be noticed —

i. The Phases. With a mean diameter of i8"'8, these are of course easily seen, and very beautiful, excepting the dull gibbous form in the superior or farther part of the orbit, where the disc is also small ; near the greatest elongation from the Sun towards the E. when she is an evening, or W. when a morning star, Venus puts on a beautiful shape — that of the moon in quadrature ; between these points in the inferior or nearer part of the orbit, she is a most elegant crescent, larger, sharper, and thinner in proportion as she is nearer really to the Earth and apparently to the Sun. This crescent has been seen even with the naked eye in the sky of Chile,1 and with a dark glass in Persia, and a very small telescope will show it. When quite slender it should not be looked for after sunset or before sunrise, as it lies too low in the vapour ; but an equatorial stand will find it in the middle of the day — an exquisite object ; and thus, or in a transit instrument, it has been many times traced as a mere curved thread extremely near the Sun.2 Ordinary observers may succeed without an equatorial mounting in seeing it a very delicate crescent soon after it has passed its inferior conjunction, by watching for its earliest appearance

" Under the opening eyelids of the morn,"

setting the finder, or the telescope with the lowest power, upon it, and following it at intervals sufficient to keep it in the field, till it has cleared the vapours of the horizon ; in this way it may be readily viewed for hours. In fact, the

1 Theodore Parker saw it in America when 12 years old, and ignorant of its existence, and when no one else could perceive it.

2 Dawes considers that, with due precaution, Venus might be seen in her upper conjunction within i' of the Sun's limb.

54 THE SOLAK SYSTEM.

planet is best seen for many purposes in the day-time ; its light, unpleasantly dazzling 1 in a dark sky, so as to bear a screen-glass, is subdued by day to a beautiful pearly lustre. Nor is it very difficult to find. For some time about its greatest brightness, at 40° from the Sun in the inferior part of the orbit, it not merely casts a shade by night, but is visible to the naked eye at noon-day : if its position is pretty well known, a little careful steady gazing will bring it out as an intense white point, when in good air it will be a charming telescopic object. At other seasons, a little hand- telescope with a large field will show it much sooner in the evening or later in the morning than might have been ex- pected.2 Like that of Mercury, the phasis often disagrees with calculation ; at its greatest elongations it ought to be exactly ' dichotomised ' as a half-moon ; but in August, 1793, Schrotcr found the terminator slightly concave in that posi- tion, and not straight till 8 days afterwards ; and Beer and Miidlcr iully confirmed this by many observations in 1836, proving that the apparent * half-moon ' takes place 6 days earlier or later than the computed, according to the direction of the planet's motion. They found also that there is a similar defect in the breadth of the crescent-phasis. Neither their explanation of this, through the shadows of mountains, nor Schroter's, through the fading of light towards the termi- nator, is satisfactory as far as night observations are con- cerned. In 1839 De-Vico and his assistants at Rome found

1 Olbers at brightest 19 to 23 times above Aldebaran ; Bond 5 times brighter than Jupiter at mean opposition ; Plummer 9 times brighter than Sirius.

2 Trouvelot concludes that in the pure atmosphere of Cambridge, U.S.A., Venus is visible to the naked eye in full sunshine in all parts of its orbit, provided its angular distance from the sun is not lessi than 10° at inferior, or 5° at superior conjunction.

VENUS. 55

a similar discrepancy of about 3 days. This curious phae- nomenon is so easily seen, that I perceived it with an inferior fluid achromatic, on Barlow's plan, 1833, March 6, before I knew that it had been noticed by others.1

2. The Mountainous Surface. La Hire, in 1700, with an old i6-ft. refractor, power 90, professed to have seen on several occasions in the crescent, when very narrow, 'des iuegalitez beaucoup plus considerables quo celles de la Lune ; ' and Briga not only described an indented termina- tor, but unjustifiably altered Cassini's figures accordingly. Fontana is also stated to have perceived it in earlier days. But Schroter's observations at the end of the last century are far more trustworthy. With several fine reflectors and a very good achromatic, he and others found the boundary sometimes slightly jagged, sometimes irregular in curvature, so as to vary the relative thickness of the horns ; and the.se would occasionally pass through such changes as to show a rotation in 23*" 2im 8&. At the quadrature, the N. cusp would frequently project, while the S. was blunted, with sometimes a minute point cut off from it by a narrow black line, the shadow apparently of a lofty ridge ; some of these mountains Schroter supposed to be 27 or 28 miles high, but of course with great uncertainty. Herschel L attacked these dis- coveries in the 'Philosophical Transactions' for 1793, in what Arago justly terms ' une critique fort vive, et, en apparence du moins, quelque peu passionnee.' Schroter calmly and satisfactorily vindicated himself through the same medium in 1795 ; an<^ Beer and Miidler, in 1833 and 1836, have fully established his accuracy as to an irregularly curved terminator, causing great and rapid changes in the

1 From observations of 9 elongations, Trouvelot found that ' dichotomy ' occurred at east elongation 4 to 8 days before, and at west elongation 6 to 1 1 days after the time of greatest elongation.

56 THE SOLAR SYSTEM.

shape of the cusps. They also saw an occasional bending off oi the S. horn, corresponding with Schroter's flattening of the limb in that direction,1 as Langdon has done at the other cusp. Gruithuisen in 1847 saw the S. horn blunted. Fritsch, in 1799 an^ 1801, witnessed several of these ap- pearances with small telescopes. Gruithuisen and Pastorff have drawn an indented terminator ; Flaugergues and Yalz noticed its irregularity, but deny its changes. Breen, with the great Northumberland telescope at Cambridge, of i9J-ft. focus, and n^-in. aperture, has often seen the unequal curve of the terminator, and the blunted S. horn, and so have several recent observers : either cusp being at times blunted, or nearly or quite cut off. Pratt especially with a fine 8|-in. ' With * mirror has fully confirmed Schroter's pro- jections and indentations on the terminator. Safarik has also seen them, 1868 ; and in 1876 Baron van Ertborn repeatedly observed a bright point detached from the S. horn.2 But the most curious observations are those made by De-Vico and his assistants at Rome, in April and May, 1841. An achromatic by Cauchoix, 6^-in. aperture, with powers sometimes up to 1128, enabled them to trace the approach towards the terminator, night after nighf, of a valley surrounded by mountains like a lunar crater, 4^5 in diameter. The crescent was narrow, and near the N. horn

1 It is a pity that the figures in their * Beitrage ' are BO bad. Schroter's, though not good, are more intelligible. The Roman observers also have noticed this strange temporary flattening of the circular limb near the S. horn, and consider it a profile view of one of the large grey spots, which, if so, must be deeply depressed.

2 In 1886 a bright narrow line of light, about fa of the diameter of Venus in length, was observed by I. G. Lohse in prolongation of the lower horn, but perfectly separated from it. Trouvelot, who between the years 1876 and 1891 made 744 observations of the planet and 295 drawings, fully confirms the irregular curve of the terminator.

VENUS. 57

they first saw an oblong black spot, which afterwards was bordered with stronger light, in the sequel encroached with half its ring upon the dark side, and ultimately formed a black notch between two bright projections, giving the appearance of a triply-pointed horn ; a longer black streak was seen near the other cusp at the same time. Gruithuisen and Pastorff seetn to have noticed in part the same irregu- larity. Secchi, in 1857, with the 9T6¥-in. Merz achr., when the crescent was only o"'4 broad, observed it to be still further contracted in one part. Safarik and Lyman have seen two contractions ; Buckingham has found it when very thin not merely irregular, but interrupted at 3 points ; and Arcimis at Cadiz saw an indentation at the S. horn in 1876. 3. The Spots. These have occasioned much controversy, from their indistinctness, which is such that the Virgiliau 'aut videt, aut vidisse putat* is often the observer's con- clusion. There have been, however, many exceptions. In 1645 Foutaua detected a dark spot;1 in 1666 and 1667 Domenico Cassini, at Bologna, repeatedly saw one bright and several dusky spots, the former giving a rotation in 23h 2im ; 2 subsequently, in the air of France, he never could recover them, nor were J. J. Cassini or Maraldi at Paris more successful afterwards. In 1726 and 1727 Bianchini, at Home, observed spots many times with a 66-ft. refractor, aperture a little more than 21-in., power 112, like the " seas " in the Moon to the naked eye, though less distinct ; and not till after sunset, from want of light in his glass.

1 The spot observed by Fontana was however probably due to the optical imperfections of the telescope used by him, as Flammariou has pointed out.

2 So almost invariably stated, but Schiaparelli has pointed out that the period of rotation actually found by Cassini himself from his observations is 23 days.

THE SOLAK SYSTEM.

His figures show them surrounding the equator of Venus, forming, as it were, three oceans — one tolerably circular, the others much lengthened, and each of the latter subdivided into three portions connected by narrower openings ; besides two spots, one occupying the S. polar region, the other forming a horse-shoe round the N. pole. He did his work well, but, for want of sky-room, did not perceive the rapid motion of the spots, and gave a wrong rotation of 24* 8h. A copy of this diagram is here given which the student may easily transfer to a small globe. Schroter and Herschel I., half a century later, with much finer instruments, but in less pellucid skies, could only make out through many years a few faint markings, with suspicions of motion. Bode saw some dark spots in 1788 ; at last, 1801, Aug. 29, Schroter

360 330 300 270 240 210 l8o 150 120

360 33O 3OO 27O 240 2IO l8o 150 120

detected a dim oblique dusky streak, like one on Mercury, giving a rotation in about 24**. Several other astronomers had occasional glimpses of darker shado wings ; Gruithuisen perceived repeatedly long vertical shales, and minute bril- liant round specks, and Schumacher a dusky spot in the twilight, which in half an hour was lust in increasing glare : others, visible with a small telescope, he found effaced in a

VENUS. 59

larger one — a caution for future observers. Lamont failed entirely with an n-in. achr. at Munich in 1836: but more effective results were obtained at Eome, 1839-1841, by De-Vico, who had been instigated by Schumacher to verify Bianchini's assertions in the same atmosphere. He used the Cauchoix achromatic chiefly by day, since in a dark sky the glare overpowered the spots so much as to render the micrometer useless, and account for Bianchini's erroneous rotation : the drawings of the latter were, however, found, save in the omission of one small spot, remarkably exact. Of 6 observers, the most successful in seeing these faint clouds were those who had most difficulty in catching very minute companions of large stars ; l but all agreed in the figures, and witnessed a progression, giving a rotation in 23h 2im 22s on an axis greatly inclined, though less so than B-ianchini had supposed.* They have given no less than 145

1 De-Vico assigns no reason, but it seems obvious enough, and worthy of notice. A very sensitive eye, which would detect the spots more readily, would be more easily overpowered by tho light of a brilliant star, so as to miss a very minute one in its neighbour- hood. There is abundant industry in these Roman observations : Palomba, the assistant, made 11,800 measures in 1839, of which 10,000 were employed in determining tho rotation. But De- Vice's style wants explicitness, and there are strange traces of inexperience or inattention in the Jesuit College, rendering the memoirs of that date far less satisfactory than those of the succeeding and now to be lamented Director, Secchi.

2 The question of the duration of the rotation of Venus is still a disputed one, and several very elaborate though contradictory investi- gations upon the subject have been published within the last few years. In 1890 Schiaparelli came to the conclusion after a very ex- haustive discussion of all the existing material, including some observations of his own, that the rotation of Venus is very slow; and that whilst being very probably equal to the time taken by the planet in making one revolution round the Sun (225 days), it is certainly not less than 6 months or greater than 9 months. Perrotin confirms this

60 THE SOLAR SYSTEM.

delineations ; many of these are very coarsely executed, but, as they are little known in England, copies of a few of the more characteristic are given here. Of late these spots, invisible at Greenwich and to several of our first observers, have been recognised by others — De la Rue, Huggins, Worthington with a I3~in. mirror, Seabroke with the 8^-in.

achr. belonging to Dawes, Terby, Denning, Safarik, Baron van Ertborn, and others, and described as bright patches and specks, and cloudy and crater-like markings of different forms and sizes ; according to Ormesher, much like the spots

slow rotation from observations of the markings on the disc made by him at Nice in 1890, and fixes it at from 195 to 225 days. In the same year Terby published a number of observations made by him in 1887- 89, which appeared to him to still further confirm the slow duration of the rotation. But in 1891 appeared an important memoir by Niesten, giving the results of many observations made by himself and Stuyvaert at Brussels between 1881 and 1890. These observers, instead of supporting the slow rotation, strongly confirm the short rotation period of De-Vico. They also found the markings so evident and apparently permanent, that they were able to construct a map of them. This map, however, does not bear the slightest resemblance to that of Bianchini on p. 58. More recently still Trouvelot has dis- cussed the subject, and decides that the rotation is performed in about 24 hours. His observations of a large dark spot are referred to on p. 61. With results so contradictory, and obtained too by some of our very best observers, it is difficult to come to a satisfactory conclusion upon the subject. The balance of the evidence appears at present, however, to be in favour of a rotation in about 24 hours.

VENUS. 6 1

of Mars.1 With describes the disc (April 6, 1868) as marbled or veined all over, and on the same occasion per- ceived with a I2|-in. unsilvered mirror, a small bright projection on the circular limb,2 abont 40° from S. cusp : this was confirmed by Key, at about 36°, April 12 and subsequent days, with granulations towards a terminator more deeply serrated than that of the Moon: March 15, Browning had seen a bright patch of some extent 80° from that cusp, so luminous as to show projection like the snow on Mars. Could we but see these features more readily, what an interesting object would this lovely planet become, especially as in point of size it is the only companion to the Earth in the whole system ! 3 And the possessors of even common telescopes need not despair, though their chances may not be great : at Rome the spots have been seen even with a little telescope of 2-in. aperture, and the following recital shows that the chief difficulty lies in our own atmo- sphere ; it is so curious that it must be given entire from the 'Philosophical Transactions': — 'January 23, 1749-50, there was a splendid Aurora Borealis. About 6h P.M., the Rev. Dr. Miles, at Tooting, had been viewing Jupiter and Venus, and showing them to some friends, with one of Short's reflectors, greatest power 200, when a small red

1 Trouvclot recorded a number of greyish or whitish spots between the years 1876 and 1891. They were usually of small size and little permanence, and generally only visible near the terminator. But in 1876, and again in 1891, he observed a large dark marking of com- paratively well-defined nature and characteristic form, which remained visible for a number of days. In 1891 the spot \\as observed to arrive at the same position on the disc at a later and later hour on each succeeding day. This circumstance indicates a rotation period slightly longer than 24 hours.

2 Compare Schroter and Harding on Mercury, p. 48.

3 Diameter, 7850 miles.

62 THE SOLAE SYSTEM.

cloud of the Aurora appeared, rising up from the SW. (as one of a deeper red had done before), which proceeded in a line with the planets, and soon surrounded both. Venus appearing still in full lustre, he viewed her again with the telescope, without altering the focus, and saw her much more distinctly than ever he had done upon any occasion. All his friends were of the same opinion as to the sight they had of her on that occasion. They all saw her spots plain, resembling those in the Moon, which he had never seen before, and this while the cloud seemed to surround it as much as ever; but whether the vapour might be rarer nearer the planet, no judgment could be made, because of her too powerful light.' In curious accordance with this, Langdon saw the irregularity of the terminator beautifully through an Aurora in 1873.

4. The Atmosphere. The bright border noticed by some observers as attending the circular limb may be a deception ; and so probably were the polar snows which Gruithuisen imagined that he frequently saw ; l but there is very sufficient proof of the existence of a vaporous envelope. Schroter's dusky belt, already mentioned, indicates it.2 He has ascribed

1 But Trouvelot, from a great many observations, confirms these white polar spots, and has also pointed out that they have been seen in some form or other by a great many observers. According to Trouvelot they consist of brilliant white spots nearly always visible close to the two cusps, and never departing much from these positions. From their nearly invariable position he concludes that they mark the poles of rotation, and that the equator is only inclined 10° or 12° to the plane of the planet's orbit. He considers them to be elevated mountain masses, ice or snow covered ; and that to their presence are due the deformations frequently observed in the horns. Under favourable conditions the margins of these polar spots appear formed of minute brilliant points, or detached mountain peaks.

2 Similar ones have been detected by Buffham, and a long, narrow

VENUS. 63

to it the great decrease of light towards the terminator and cusps ; and he and Herschel I. agreed as to the extension of the horns beyond a semicircle, which may be due in part to the penumbra, or additional daylight caused by the Sun's not being a point but a great disc, but more to refraction through an atmosphere. Schroter also perceived in 1790 a faint gleam along the limb beyond the horns, a true twilight produced by an atmosphere which must be denser than our own. In May, 1849, Madler at Dorpat found the horns projecting to 200° and even to 240°, showing a refraction about ^ stronger than ours; and Lyman with a 9~in. achromatic in America obtained a similar result ; but from the calculation of Neison, who detected an error in the formula they employed, the true density comes out 1*892, or nearly twice as great as that of our atmosphere. From this cause Cassini in 1692, and Drew in 1854, found the crescent too broad near the conjunction. Secchi, in 1857, saw in that position the cusps much prolonged, and the twilight extending 19^°, even through our strongly illumi- nated atmosphere. Noble has seen the whole disc so bordered in almost every inferior conjunction ; it had been recorded in the transit of 1769 ; and in 1864 and 1866, and at the recent transit, in common with several other observers, Lyman found the delicate ring complete, and considers that it will always be so within i° 50' from the Sun's centre ; a segment only, of an extent proportioned to the distance, being visible beyond that limit. When entering the Sun in 1875, this light was especially conspicuous in one spot.1

line, possibly atmospherical, by Do La Rue and Lord Rosse in the great Melbourne Cassegrain reflector.

1 At the transit of 1882 the atmosphere very beautifully revealed itself, when the planet was about half on the sun, in the form of a brilliant half ring of light, marking the outline of that portion of

64 THE SOLAR SYSTEM.

Tacchini considers that this atmosphere contains traces of aqueous vapour.

5. The Phosphorescence of the Dark Side. This truly unaccountable appearance l is remarkably well attested ; though it is but fair to state that it has been questioned by several good observers, even Dawes himself. It was noticed as far back as 1715, in the 'Astro-Theology' of Derham, who says that ' this sphaericity, or rotundity, is manifest in our Moon, yea and in Venus too, in whose greatest Falcations the dark part of their Globes may be perceived, exhibiting themselves under the appearance of a dull, and rusty colour.' 1721, June 7, Kirch, junior, believed that he saw it, the crescent being then extremely narrow ; and again, with two others, 1726, March 8. Herschel I. perceived traces of it. In 1806 it displayed itself beautifully to Harding three times (once with a companion), and to Schroter once, within five weeks. Pastorff also witnessed it twice. Guthrie and others noticed it in 1842, with small reflectors, in Scotland; Purchas, at Ross, in England ; De-Vico and Palomba, many times, in Italy. Berry at Liverpool saw it in 1862 with no previous recollection of its visibility; it was remarked in that year, and especially in 1863, by many observers in England, and by one in 1865, as well as by four at Leipzig with the 8~iri. achromatic. To these we may add Petty, 1868; Winnecke, 1871, Nov. 6; Elger, Erck, 1873; Banks,

Venus not yet entered on the sun. This semi-ring gleamed with a beautiful silvery or pearly light, and was of uneven brightness and breadth, probably owing to the planet's atmosphere being of varying degrees of opacity at different parts of the limb.

1 Arago's 'negative visibility' is but a perplexing attempt at solution. The faint illumination which renders some of our terrestrial nights lighter than others, remarked by Schroter, Arago, and, I think, by myself, scarcely affords an adequate comparison. Humboldt gives a striking instance, Cosmos, I., 131 (Bohn).

VENUS. 65

Grover, Arcimis, 1876; Mills, 1878. On Jan. 31 of that year, after having many times looked for it in vain in former years, I saw it, without specially thinking about it, with powers of about 90 and 212 on my 9'38-in. mirror, coming out at intervals rather paler and browner than the twilight sky, and equally visible when the bright crescent was hidden by a field-bar. Strange to say, it has been seen even in the daytime, by Andreas Mayer, 1759, Oct. 20, through merely a transit instrument, 44™ after noon: 'etsi pars lucida Veneris tennis admodum erat, nihilominns integer discus apparuit, instar lunae crescentis, quse acceptum a terra lumen reflectit:' and Winnecke records a similar observation, though very faint, 1871, Sept. 25, a little before noon.1 Von Hahn also says he made it out repeatedly, by day as well as by night, and with several instruments; he was, however, an inferior observer. Gruithuisen saw it once at sunrise; Mad. Safarik in a broad crescent, 1871, Aug. 9, nh A.M. ; Browning many times, 1870, Feb. and Mar. with io^-in. silv. mirror, very distinct, brighter than the bright

1 LangJoii and several others perceived it by day a little before inferior conjunction, 1870, Feb. 5; but this, as well as some other instances, may have been merely the atmospheric ring of Lymaii and Noble ; and such appears to have been the case in some day-observa- tions of Baron van Ertborn in 1876. A curious observation of my wife's may be cited here, made June 30, 1880, 7h 30™ A.M., between the Lago Maggiore and Domo D'Ossola. The waning Moon, 2i£h past Last Quarter, was still at some height in W., but pale in the strong sunshine, when she noticed that the circle appeared complete, the dark side being smaller than the bright, of a more lilac tint than the deep blue Italian sky, and irregularly shaded, being brighter towards the SW. limb, as it would be, though she was not in the least aware of it. The phenomenon was confirmed to her by a good binocular glass; but neither I nor her servant could perceive it. She was not thinking at the time of the lumiere cendrfe, and still less of the dark side of Venus.

VOL. I. F

66 THE SOLAR SYSTEM.

twilight of 5b P.M., and even when gibbons Baron van Ertborn saw it in 1876. The dark side is often too small in propor- tion (as I saw it), like that of the crescent Moon to the naked eye; and from the same cause, — the irradiation of the luminous part ; it is sometimes described as grey, some- times reddish. It would be well worth looking for when the crescent is narrow, but Venus should have high N. latitude to clear the vaponrs of the horizon: the bright part should be put behind a bar in the field ; and it should be noted whether the dark side is lighter or darker than the background. Noble has repeatedly seen the latter, projected, perhaps, on the solar corona.

THE MOON.1

(Abbreviations: — Schr., Schroter. — G., Gruithuisen. — L., Lofirmann. — B. & M., Beer 2 £ Mddler.— ;Ne., Neison.— Schm., Schmidt.— B.A. A., Jlritish Astronomical Association. — L.A.S., Liverpool Astronomical Society. — Sel. Journal., Selenographical Journal.)

THE comparatively small distance of our satellite, 240,000 miles,3 renders it the easiest of telescopic objects. Its shadowed and irregular surface, visible to the naked eye,4 is well brought out even with a low magnifier ; hence Galileo

1 The new matter is by Mr. Elger, who wishes it to be understood that to bring this part up to date would have necessitated many alterations and insertions in the text.

2 This name is retained, as originally associated with the work, which, however, is known to have been chiefly executed by his colleague.

3 More accurately 238,840, varying, from its elliptical orbit, between 252,972 and 221,614 miles.

4 Klein observes that an immense amount of detail may be made out in the Full Moon with the naked eye, especially if the light of a flame enters it at the same time.

THE MOON. 67

readily comprehended the nature of what his new and imperfect invention disclosed to him, and the smallest instrument will show that freckled aspect, arising from numberless craters, which he compared to the eyes in a peacock's tail. Many a pleasant hour awaits the student in these wonderful regions ; only let him not expect that what he sees so plainly will be equally intelligible, excepting in its unquestionable relief from the effect of light and shade. Greatly overstrained ideas, as to the possibility of making out the minute details of the surface, have been entertained, not much more reasonable than those of the islanders of Teneriffe, whose simplicity led them to imagine that the telescope of Piazzi Smyth would show their favourite goats in our satellite; a very little consideration, however, will detect the absurdity of such anticipations. The first * Moon Committee ' of the British Association recommended a power of 1000; few indeed are the instruments or the nights that will bear it ; but when employed, what will be the result ? Since increase of magnifying is equivalent to decrease of distance, we shall see the Moon as large (though not as distinct) as if it were 240 miles off, and any one can judge what could be made of the grandest building on earth , at that distance : very small objects, it is true, are discoverable there with the finest instruments, possibly 150 feet broad, or from their shadows one-third as much in height; but their nature remains unknown. Much difficulty, too, arises from the want of terrestrial analogies. It may be reasonably supposed that Venus or Mars, at the like distance, might be far more intelligible. We should certainly not find them mere transcripts of our own planet, for the conditions of existence as to temperature and air, if not in other respects, are very different, and, as Schr. often remarks, variety of detail in unity of design is characteristic of creation ; bntH

UNIVERSITY )

68 THE SOLAR SYSTEM.

we might have a fair chance of understanding something of what we saw. It is quite otherwise with the Moon. It is, in B. and M.'s words, no copy of the Earth ; the absence of seas, rivers, atmosphere, vapours, and seasons l bespeaks the absence of * the busy haunts of men ; ' indeed of all terrestrial vitality, unless it be that of an insect or reptile. Whatever may be the features of the averted hemisphere, on which, as G. and Hansen have suggested, other relations may exist, we perceive on this side a mere alternation of level deserts and craggy wildernesses. The hope which cheered on G. and others, of discovering the footsteps of human intelligence, must be abandoned.2 If it should be thought probable, as it very reasonably may, that the lunar surface is habitable in some way of its own, we have reason to suppose that, where the conditions of life are so extremely dissimilar, its traces would be as undecipherable by our experience as a brief inscription in a character utterly unknown. We ought not, in fact, to bo surprised at such a difference between bodies belonging to distinct classes: it would have been unreasonable to have looked for a duplicate of a primary planet in its attendant. Waiving, then, any disappointment from this cause, we shall find the Moon a wonderful object of study. It presents to us a surface

1 This assertion must be limited to the subject in hand. It is not intended to deny the possibility of some kind of atmosphere, or fluid; and a trifling change of seasons would result from the slight inclina- tion of the lunar axis (i° 32' 9").

2 The existence of many natural wonders on our own globe — for example, the canons, or river-gorges, of NW. America, one of which has a length of 550 miles, an extreme depth of 7000 ft, and a closest contraction of 100 ft. ; or the obelisk of limestone, near Lanslebourg, 360 ft. high with a base of 40 ft. (Weld., Auvergne, etc., 305), shows how cautiously inferences should be drawn as to the artificial origin of extraordinary appearances.

THE MOON 69

convulsed, upturned, and desolated by forces of the highest activity, the results of whose earliest outbreaks remain, not like those of the Earth, levelled by the fury of tempests, and smoothed by the flow of waters, but comparatively undegraded from their primitive sharpness even to the present hour. The ruggedness of the details, as old Hovel anticipated, becomes more evident with each increase of optical power, and we cannot doubt that we look upon the unchanged results of those gigantic operations which have stereotyped their record on nearly every region of the lunar globe.1

A brief general description of the phenomena of the Moon will prepare us for an examination of its topography. We have then —

i. The Grey Plains, or Seas as they were formerly be- lieved to be, and are still termed for convenience.2 These are evidently dry flats — if the term 'flat' can be applied to surfaces showing visibly the convexity of the globe — analogous to the deserts and prairies and pampas of the Earth. B. and M. find that they do not form portions of the same sphere, some lying deeper than others: they are usually of a darker hue than the elevated regions which bound them, but, with a strong general resemblance, each has frequently some peculiar characteristic of its own.

1 This is sufficiently correct as a popular view of the subject ; but ( unre careful examination may require it to be somewhat modified. Tue varieties of colour, if not arising from vegetation, may indicate an amount of 'weathering' which must bo great from the distance at which it is perceptible: nor can we confidently affirm that with powerful telescopes no traces can be detected of the hand of time.

2 Kiccioli, when he recast the lunar nomenclature, and substi- tuted the names of philosophers for the feeble geographical analogies of Hevel, retained the generic titles of ' seas,' though he altered their designations. The reform attempted by G., who would have had them called ; surfaces,' has never Ukeu effect.

70 THE SOLAR SYSTEM.

2. The Mountain Chains, Hills, and Ridges. These are of very various kiiids : some are of vast continuous height and extent, some flattened into plateaux intersected by ravines, some rough with clouds of hillocks, some sharpened into detached and precipitous peaks. The common feature of the mountain chains on the Earth — a greater steepness along one side — is very perceptible here, as though the strata had been tilted in a similar manner. Detached masses and solitary pyramids are scattered here and there upon the plains, frequently of a height and abruptness paralleled only in the most craggy regions of the Earth.1 Every gradation of cliff and ridge and hillock succeeds : among them a large number of narrow banks of comparatively slight elevation but surprising length,2 extending for vast distances through level surfaces ; these so frequently form lines of communica- tion between more important objects, — uniting distant craters or mountains, and crowned at intervals by insulated hills, — that Schr. formerly, and B. and M. in modern times, have ascribed them to the horizontal working of an elastic force, which, when it reached a weaker portion of the • surface, issued forth in a vertical upheaval or explosion. The fact of the communication is more obvious than the probability of the explanation.

1 Sehm. ascribes less rapidity to the gradients than is here supposed ; very seldom exceeding 60°, never attaining in any extent 90°. Phillips was of an opposite opinion; but Ne. concurs with Schin.

2 Schr. gives a length of 630 or 640 miles to a ridge connecting the spots Copernicus and Kirch. The great serpentine ridge, traversing the M. Serenitatis on the western side, is one of the most prominent of these features, and is fully 300 miles in length. That they usually appear to take the direction of the meridian is simply due to the E. or W. projection of the shadows which make them prominently visible.

THE MOON. 71

3. The Crater-Mountains, comprising both the mound or wall, and the included cavity. These are the grand peculiarities of the Moon: commonly, and probably with correctness, ascribed to volcanic agency : yet differing in several respects from the foci of eruption on our own globe : on the Earth, they are usually openings on the summits or sides of mountains — on the Moon, depressions below the adjacent surface, even when it is a plain or valley ; on the Earth, the mass of the cone usually far exceeds the capacity of the crater — on the Moon, they are much nearer equality ; on the Earth, they are commonly the sources of long lava- streams — on the Moon, traces of such outpourings are rare ; on the Earth, their number as well as dimensions are com- paratively inconsiderable — on the Moon, M. considers that there are upwards of 1000 visible with a breadth of 9 miles, and Schm. has drawn nearly 33,000 in all, so that they are the greatest characteristic of its surface, and, the grey plains excepted, among the largest of its features. When, however, allowance has been made for the inferior power of gravity on the- Moon, through which a six-fold displacement in height or distance would be caused by the same amount of force,1 — for the possible difference of materials, — and for the more rapid cooling produced by radiation in the absence of an atmosphere, it is quite conceivable that volcanic force, similar to that on the Earth, may have been the real agent, though in a gently modified form. Any one may see, with the ingenious Hooke, a strong resemblance to the rings left by gaseous bubbles ; 2 but to this impression mechanical

1 Mount Tongariro in New Zealand was, in 1875, throwing stones 8 miles from the crater.

2 Scrope lias suggested a somewhat similar hypothesis. He thinks that the lunar craters have derived their peculiar features * from the explosions of vapour that produced them, breaking through a surface

72 THE SOLAR SYSTEM.

difficulties arising from the cohesion of materials have been opposed, and a more consistent explanation sought in the idea that the larger craters may be the remains of molten lakes; in these, left for a while unfrozen in the general cooling and crusting over of the once-fiery globe, an alter- nate shrinking and overflowing of lava, from a fluctuating pressure from beneath, might gradually produce the existing forms. This theory has been recently extended and advo- cated with much ingenuity in the * Selenographical Journal ' by Green, who would substitute erosive for eruptive agency, even in the smaller craters; the undermining and re-dis- solving power of fluid lava pressed upwards through the surface, and subsequently in many cases receding, being substituted for actual explosion. On this hypothesis the brighter and more elevated parts of the globe would, as a rule, date from a more ancient epoch than the darker and once fluid plains.1

We have nothing on the Earth to be compared with the gigantic scale of the greater circles, though an extinct crater-plain, 20 miles in diameter, with peaks on its 'ring, is said to exist on the island of Mauritius: and the c aters of the Sandwich Islands, Kirauea and Haleakala— the one a fused, the other a frozen, lake of lava, with the small 'blowing-cones' which eject only cinders and ashes— afford

of soft and semi-liquid matter in successive bubbles, whose bursting would throw off all round a concentric ridge formed of repeated layers of this substance.' — Volcanoes, 2nd Ed., 1872, p. 233.

1 Mr. S. E. Peal of Assam advocates, very ingeniously, 'A Theory of Lunar Surfacing by Glaciation,' in which he contends that the walled- plain s, ring-plains, etc., 'have all been lakes or lagoons left for a time in a slowly glaciating crust, kept liquid by quasi- volcanic orifices;' and that the aqueous vapour rising from these areas, forming a local and dome-shaped atmosphere, ultimately condensed round the margin as snow and formed the ring.

THE MOON.

73

an analogy,1 the striking nature of which will be apparent from the following representation of the latter, taken from a view in Elwes's ' Sketcher's Tour.'2 Difficulties no doubt remain ; but we can hardly wonder at them, while geologists are still so little agreed about 'elevation-craters' and sub- marine volcanoes on our own globe. The circular cavities

Extinct Crater of Haleakala.

of the Moon are arranged in three classes, — Walled (or Bulwark) Plains, Ring-Mountains, and Craters: a fourth includes little pits without, or with scarcely a visible ring. The second and third differ chiefly in size ; but the first have a character of their own ; — the perfect resemblance of

1 The craters of Java are also said to bear out this comparison.

2 Haleakala, ' the house of the Sun,' in E. Maui, is of an oblong form, "with two great openings in the wall, more than 30 miles in circumference, 10,000 feet above the sea, and about 2000 feet deep. On the floor are 12 or 13 small red or yellow cones. The highest summits in the view are the two snowy volcanoes of Hawaii, seen over the clouds in a very faint distance: our sketch gives only the general effect.

74 THE SOLAR SYSTEM.

their interiors to the grey plains, as though they had been originally deeper, but filled in subsequently with the same material ; many of them, in fact, bearing evident marks of having been broken down and overflowed from the outside. Their colour is often suggestive of some kind of vegetation, though it is difficult to reconcile this with the apparent deficiency of air and water. It has been ingeniously sug- gested that a low stratum of carbonic acid gas — the frequent product of volcanoes, and long surviving their activity — may in such situations support the life of some kind of plants : and the idea deserves to be borne in mind in studying the changes of relative brightness in some of these spots.1 The deeper are usually the more concave craters ; but the bottom is often flat, sometimes convex ; and frequently shows sub- sequent disturbance, in ridges, hillocks, minute craters, or more generally, as the last effort of the eruption, central hills of various heights, but seldom attaining that of the wall, or even, according to Schni., the external level. The ring is usually steepest within (as in terrestrial craters),2 and many times built up in vast terraces, or in parallel walls, frequently lying, Schm. says, in pairs divided by narrow ravines. Nasmyth refers these terraces — not very probably — to successively decreasing explosions ; in other cases he more reasonably ascribes them to the slipping down of materials piled up too steeply to stand, and undermined

1 The supposition, however, is said to be incompatible with the law of the diffusion of gases.

G-. thought he perceived, in the grey tints of depressed surfaces, gome of which vary with the amount of solar light, traces of several kinds of vegetation, comprised between 65° N. and 55° S. latitude, and preserving the correspondence observed on the Earth between increasing latitude and elevation.

2 Schm. gives 4° to 15°, and 20° to 50°, as the respective average inclinations without and within.

THE MOON. 75

by lava at their base, leaving visible breaches in the wall above : they would bo well explained on the supposition just mentioned of fluctuating levels in a molten surface. Small transverse ridges occasionally descend from the ring — chiefly on the outside : great peaks often spring up like towers upon the wall ; gateways at times break through the rampart, and in some cases are multiplied till the remaining piers of wall resemble the stones of a huge megalithic circle.1 A succession of eruptions may be constantly traced, in the repeated encroachment of rings on each other, where, as Schm. says, the ejected materials seem to have been disturbed before they had time to harden: the largest are thus pointed out as the oldest craters, and the gradual decay of the explosive force, like that of many terrestrial volcanoes, becomes unquestionable. The peculiar whiteness of the smaller craters may indicate something analogous to the difference between the earlier and later lavas of the Earth, or to the decomposition caused, as at Teneriffe, by acid vapours : in the grey levels we thus perhaps obtain an indication of the superficial character of their colouring.2

4. Valleys, of ordinary character, are not infrequent ; some of grand dimensions ; others mere gorges. The most contracted of these latter are entitled to be classed apart as

5. Clefts (or Hills8). These were discovered by Schr. : G. and L. added to their number, which B. and M. raised to 90 in the text of their work ; but the latter astronomer, on

1 Or Druidical Avenue, as, for example, between Guerike and Parry.

* In addition to these more prominent objects, traces of obscure und ancient rings are found in almost every part of the lunar surface, but perhaps more commonly on the Maria.

3 An incorrect term adopted from the German, in which it does not imply, aa in English, the presence of \\ater.

76 THE SOLAR SYSTEM.

taking charge of the noble Dorpat achromatic, Q^-in. aper- ture, perceived more than 150, and thought it might be pos- sible to descry 1000. Schm. published in 1866 a catalogue of 425, the greater part discovered by himself; and has since brought the number up to nearly icoo. These most singular furrows pass chiefly through levels, intersect craters (proving a more recent date), reappear occasionally beyond obstructing mountains, as though carried through by a tunnel, and commence and terminate with little reference to any conspicuous feature of the neighbourhood. Ne. observes that they are frequently connected with valley-systems open- ing out of high lands, and often run down the centre of valleys. Klein says they are not common in the great plains. The idea of artificial formation is negatived by their magni- tude ; l they have been more probably referred to cracks in a shrinking surface ; a process which M. and others think may even now be in operation. The observations of Ku- nowsky, confirmed by M. at Dorpat and Schm., seem in some instances to point to a less intelligible origin in rows of minute contiguous craters ; but a more rigorous scrutiny with the highest optical aid is yet required.2 To these wo may add

6. Faults, or closed, not open, cracks, sometimes of con-

1 Schm. gives them 1 8 to 92 miles long, £ mile to 2T45 miles broad, and 100 to 430 yards deep. O., following out his strange theory that the Moon was once surrounded by an immense ocean which has wholly disappeared, considered the larger of thtse furrows as the beds of dried-up rivers ; the smaller he referred to artificial clearings in the forests, answering the purpose of roads. It is to be regretted that the extravagance of his fancy should in many instances have brought discredit upon the unquestionable precision of his sight.

2 Klein, with a very sharp image and power upwards of 400, finds their breadth much more irregular than would be suspected with inferior means.

THE MOON. ' 77

siderable length, where the surface on one side is more ele- vated than on the other. These familiar geological features have been recognised in abundance by Birt and With, and are more readily traced on the Moon than on the Earth, from the absence of superincumbent alluvial deposits and denudation.

Wonders are here in abundance for the student : but he will find it impossible to pursue them far from the termi- nator,—they must be viewed under the oblique rays of a rising or setting Sun. As the angle of illumination in- creases, a fresh aspect of things creeps in, and extends itself successively over the whole disc, and in its progress the inexperienced observer will find himself astonished at the change, and frequently bewildered in the attempt to trace out the landmarks of the surface. Objects recently well recog- nised under the relief of light and shade will become con- fused by a novel effect of local illumination, and the eye will wander over a wilderness of streaks and specks of light, and spots and clouds of darkness, where it may sometimes catch the whole, sometimes a portion, sometimes nothing, of many a familiar feature ; while unknown configurations will stand boldly out defying all scrutiny, and keeping their post immovably till the decreasing angle of illumination warns them to withdraw. Nothing can be more perplexing than this optical metamorphosis, so complete in parts as utterly to efface well-defined objects ; so capricious as in some instances to obliterate one, and leave unaffected the other, of two similar and adjacent forms. G., carrying out an idea of Schr.'s, referred some of these changes to the progress of vegetation which, if existing, will naturally, in default of a change of seasons, run its whole course in a single lunation : even the cautious B. and M. have admitted that some variations of colour may possibly point in this

78 THE SOLAE SYSTEM.

direction; and photographic results seem to indicate the presence of green light not cognisable by the eye : but the general change demands a more universal solution ; and probably a wide range of colours in the soil may be con- cerned in the effect. The subject calls for careful study, but would involve much laborious application.1

The most obvious features of the Moon under a high illumination are the Systems of Bright Sir calf s which issue, though in widely differing proportions, chiefly from seven different centres: all craters, few inconsiderable, but none of the very largest class. In some cases the streaks proceed from a circular grey border surrounding the crater; in others they cross irregularly at its centre. They pass alike over mountain and valley, and even through the rings and cavi- ties of craters, occasionally reach the terminator,2 and seem to defy all scrutiny. Nichol asserts that in some contiguous systems, the order of formation may be detected from the mode oiNtheir intersection ; 3 a statement well deserving the notice of those whose telescopes will carry them through the inquiry. But one thing is certain, that they are very seldom accompanied by any visible deviation in the super- ficial level ; a fact irreconcilable with Nasmyth's conjecture, that they are cracks diverging from a central explosion, filled up with molten matter from beneath ; trap-dykes on the Earth are, indeed, apt to assume the form of the surface, but the chances against so general and exact a restoration of

1 The polarisation of light by the Moon was found by Secchi to be entirely different in the lowlands and in the mountainous parts. Birt sees a greenish tint in the plains under very oblique lights.

2 Ne. remarks this, in opposition to B. and M., and says that some of the Tycho streaks shift a little with varying libration.

3 The following, according to him, is the chronological order of three great systems : Copernicus, Aristarchus, Kepler.

THE MOON. 79

level, all along such multiplied and most irregular lines of exposure, would be incalculable ; many of the rays are also far too long and broad for this supposition, or for that of B and M., that they may be stains arising from highly heated subterraneous vapour on its way to the point of escape.1

1 Schwabe suggests that they may be due to contrast: a shade being formed in their intervals as the day increases by a multitude of thin converging or parallel grey lines, which may be seen by a good instrument, and which may possibly result from some kind of vegetation.

In a paper recently published in the Attronomische Nachrichten, Professor Pickering gives the results of his observations of the bright streaks with a 13-inch telescope at Arequipa. He finds: (i) That the streaks round Tycho do not radiate from the apparent centre of this formation, but point towards a multitude of minute craterlets on its SE. or N. rim. Similar craterlets occur on the rims of other great craters, forming streak-centres. (2) Generally speaking, a very minute and brilliant crater is located at the end of the streak nearest the radiant point, the streak spreading out and becoming fainter towards the other end. The great majority of lunar streaks appear to issue from one or more of these minute craters, which rarely exceed a mile in diameter. (3) The streaks which do not issue from minute craters usually lie upon or across ridges, or in other similarly exposed situations, sometimes apparently coming through notches in the mountain walls. (4) Many of the Copernicus streaks start from craterlets within the rim, flow up the inside and down the outside of the walls. (5) Though there are similar craters within Tycho, the streaks from them do not extend far beyond the walls of this forma- tion. All the conspicuous streaks about Tycho originate outside its walls. (6) The streak-systems of Copernicus, Kepler, and Aristarchus are greyish in colour and much less white than that associated with Tycho. Some white lines extending SE. from Aristarchus do not apparently belong to the streak-system. In the case of craterlets lying between Aristarchus and Copernicus, the streaks point away from the latter. (7) There are no very long streaks. They vary from 10 to 50 miles in length, and are rarely more than a quarter of a mile in width at the crater. From this point they gradually widen out and become fainter. Their width, however, at the end farthest troni the crater is seldom more than 5 miles.

80 THE SOLAR SYSTEM.

The extraordinary brilliancy of some portions of the Full Moon is less difficult of explanation, when we bear in mind the effect of chalky strata, or that peculiar kind of granite which on the loftiest peaks of the Himalayas may readily be confounded with the adjacent snow. In many cases the form of the surface gives us no key to the distribution of this vivid reflection; mothers it occupies a marked position, as on the summit of rings and central mountains : the idea of a mirror-like glaze reflecting an image of the Sun, though entertained by such authorities as B. and M., seems difficult to be reconciled with the ever-varying angle of illumination.

A scale of reflective power has been adopted from Schr. by later selenographers, in which (blackness being o) the darkest grey shades rank as i°, the brightest portions 10°. This is of some use, though merely arbitrary. A more reliable plan, especially in combination with such a scale, would be the mode of comparative values, in which the light of any given spot is referred to others above and below it in brightness. Changes are more than suspected in the reflec- tive power of portions of the lunar surface, and it would be interesting to examine it in that aspect; but we have no good map of the Full Moon, and if we bad, it never could adequately express the various gradations of brightness, which must be the object of topographical study.

A few peculiarities of arrangement deserve to be men- tioned here. The remarkable tendency to circular forms,1 even where explosive action seems not to have been con- cerned, as in the bays of the so-called seas, is very obvious ;

1 It has recently been pointed out by Elger that the borders of a large number of formations are polygonal in shape, and that many exhibit a tendency to a hexagonal figure, e.g. Copernicus, Langrenus, Vieta, Ptolemaus, Wargentin, Hevel, Hausteen, etc. (see Journal B.A.A., vol. iii. p. 314).

THE MOON. 8 1

and so are the horizontal lines of communication already mentioned. The gigantic craters or walled-plains often affect a meridional arrangement: three huge rows of this kind are very conspicuous, near the centre, and the E. and W. limb. A tendency to parallel direction has often a curious influence on the position of smaller objects ; in many regions these chiefly point to the same quarter, usually N. and S. or NE. and SW. ; thus in one vicinity (between G, L, and M on the accompanying map) B. and M. speak of 30 objects following a parallel arrangement, for one turned any other way; even small craters entangled in such general pressures (as round L) have been squeezed into an oval form ; and the effect is like that of an oblique strain upon the pattern of a loosely woven fabric : an instance (near 27, 28) of double parallelism, like that of a net, is mentioned, with crossing lines from SSW. and SE. Local repetitions frequently occur; one region (between 290 and 292) is characterised by exaggerated central hills of craters; another (A) is without them; in another (185), the walls themselves fail. Incomplete rings are much more common towards the N. than the S. pole ; the defect is usually in the N., seldom in the W., part of the circle ; sometimes a cluster of craters are all breached on the same side (near 23, 32).1 Two similar craters often lie N. and S. of each other, and near them is frequently a corresponding duplicate. Two large craters occasionally lie N. and S., of greatly resembling character, S. usually £ of size of N. from 18 to 36 miles apart, and connected by ridges pointing in a SW. direction (20, 19: 78, 77: 83, 84: 102, 103: 208, 207, 204: 239, 242: 261, 260: 262, 263: 340, 345). Several of these

1 Compare Elwes's account of the small cones on the floor of the ureat extinct crater Haleakala, some of them "broken down at the side, nearly always on the NE." — Sketcher's Tour, 214.

VOL. I. O

82 THE SOLAR SYSTEM.

arrangements are the more remarkable, as we know of nothing similar on the Earth.

The question as to the continuance of eruptive action on the Moon is one of great interest. It is now well known that the volcanos, which Herschel I. and others thought they saw in activity on the dark side, were only the brighter spots reflecting back to us the earth-shine of the lunar night with the same proportional vivacity as the sunshine of the day. No valid reason, indeed, has been assigned for the fact, witnessed by many observers, especially Sm., that one at least of these spots — Aristarchus — varies remarkably in nightly luminosity at different periods,1 nor for the specks of light which more than once Schr, caught sight of on the dark side for a short time : 2 but in these cases there has been no subsequent perceptible alteration of surface ; and they furnish no reply to the inquiry respecting present changes. Such were abundantly recorded by Schr. in his day, chiefly variations in the visibility or form of minute objects ; but a great majority of them he, and subsequently G., who witnessed many such appearances, referred to the lunar atmosphere: and B. and M. are disposed to discard them all as the result of inaccuracy or varying illumination.

1 Elger saw a spot on the dark side, apparently Aristarchus, 1867, April 12, 7h 30™ to 8h 30™, nearly as bright as a 7 m? star, with a 4-in. aperture, and too conspicuous to be overlooked by the most careless observer. It was much fainter during the last 15™, and scarcely perceptible at 9h. The Moon was id 5h after I. Qu. He had seen something similar on former occasions. Schr.'s conjecture that the variations, which he observed in a minor degree in several parts ef the disc, may be due to atmospheric condensation during the lunar night, is more elegant than probable : it may, however, deserve consideration.

8 Such a phenomenon, more extended, but very faint, was seen, or fancied, by G. ; and witnessed in our own day, with great distinct- ness, by Grover and by Williams.

THE MOON. 83

The extraordinary influence of the latter upon the aspect of distant and unknown objects may be estimated by any one who will sketch the changed effects of light and shade on any familiar terrestrial object at different times of day; still there seems to be a residuum of minute variations not thus disposed of, and in some cases possibly indicating actual local change. Terrestrial analogy is in favour of the idea that disturbing agency may have greatly diminished without having become extinct : but observation, not assertion, must decide the point, There are no traces of any grand convul- sion since the date of the first lunar map ; and we are only now becoming possessed of the means of detecting smaller changes. Schr.'s drawings are very rough ; the much more careful ones of L., and B. and M., would be too recent to warrant great expectations, even were they more reliable — especially the latter — as to minute details ; and they are sub- ject to the same material drawback as the recently published great map of Schm. — admirable as it is in its own way — • that of representing objects in only one aspect, and that a conventional one. This defect is of course inherent in every general map, and can only be supplied by topographical sketches, after the manner of Schr., giving repeated views of the same object under differing angles of illumination and reflection. As yet, this has been only partially attempted. We have no monograph representing any one spot for each successive night of its visibility, and in varied conditions of libration: but such a series would be very instructive, as well as valuable for the future. But we must not omit to do justice to the indefatigable as well as accurate labours of Birt, continued through many years, and adding very materially to our knowledge of lunar detail ; and much is to be hoped from the researches of the Selenographical Society, established in London in 1878, which, comprising many

84 TFIE SOLAR SYSTEM.

eminent English and foreign astronomers, has for its object the collection and preservation of observations, and the indication of the most promising lines of research.1

Little of a satisfactory nature can be said as to a Lunar Atmosphere. That its density cannot amount to y^ of that of our own is demonstrable from theory, and proved by observation, which shows us a sharp outline, and detects no change in the aspect of stars in contact with the limb : 2 and hence its entire absence has been maintained by great astronomers : Schr. asserted its existence from many changes of aspect in minute objects, and from a very dim twilight which he traced through 9 years beyond the points of the horns; his inferences are supported and in part exceeded by G., who frequently saw — or imagined — fogs and clouds

rooting on tlic surface ; nor is Klein disposed to deny the probability of such obscurations, especially in deep places. The defined limb and the absence of refraction are accounted

1 This Society was dissolved at the close of the year, 1882, but its work was subsequently resumed by the Liverpool Astronomical Society, and is now being actively carried on by the Lunar Section of the British Astronomical Association.

2 It has been ascertained of late years that the Moon's observed diameter exceeds by 4" that computed from occultation of stara This appears too much to be explained by irradiation.

THE MOON. 85

for by Schr. by limiting the sensible atmosphere to the inferior regions. B. and M., while explaining away — not very satisfactorily — Schr.'s twilight, which they could never distinctly find, do not deny the possibility of a very rarefied gaseous envelope.1 Those traces of twilight, which have been confirmed by G. and recently by MM. Henry with a large telescope at Paris, and which I imagined I saw, 1855, June 20, but only doubtfully from want of better optical means, might well engage the student's attention : in order to assist him two figures are here given from among Schr.'s numerous delineations. They represent the cusps of the Moon as seen by him, 1792, Feb. 24, with a 7 -ft. reflector by Herschel L, powers 74, 161.

Occultations of planets or stars, which are important to the professed astronomer, are interesting to the amateur as bringing out distances and motions of which the naked eye takes little cognisance : the comparative nearness of our satellite becomes self-evident, and its orbital movement is made apparent from minute to minute. A grand effect is produced by the visible sailing of this ponderous globe through immeasurable space ; and it may well convey a

1 Some very curious photographic experiments by De la Rue have been thought to tend the same way. It seems hardly probable that oxygen, which forms £ of the weight of terrestrial earths, should be entirely absent from the Moon; and the whiteness of the small craters in the grey plains is a suggestive fact. Schm. has in a few instances (to which he might have added one from Schr.) seen a grey border to the black shadow in craters, but this he ascribes to its being thrown from a very ragged edge. Schr. and other observers have noted occasional want of definition in certain spots as compared with others. In a few cases M. and Schm. have perceived a blue tint round bright points on the terminator, probably, as Schm. says, due to the * secondary spectrum' of all achromatics. De la Rue's remark is important, that it is difficult to conceive any chemical formation of matter without an atmosphere.

86 THE SOLAR SYSTEM.

deep impression of the omnipotent Power and consummate Wisdom which orders its undeviating course. The instan- taneous extinction, too, and sudden flashing out of a large star in these circumstances are very striking, though less instructive than might have been expected as to the question of an atmosphere : the immersion and emersion are usually as sudden as if none existed ; but this may be accounted for by the rapid motion of the Moon, or, on Schr.'s theory, by the elevation of that portion of the limb : the gradual extinction sometimes recorded may be only due to irregu- larities on the edge : the distortions or flattenings occasion- ally noticed in the shape of planets at occultation are too extensive for any such cause, and must be referred to optical illusion.1 All occultations of importance are predicted in the Nautical Almanac ; the most beautiful are immersions behind the darJc limb when the latter receives a strong earth-light, as between New Moon and I. Quarter.2

The projection of a star upon the Moon, just before immersion, as if it advanced in front of it, is very difficult of explanation. South, after a careful examination of the instances on record, was unable to come to any satisfactory conclusion : it is an occurrence of the most capricious kind, which can never be predicted for any star, eye, or telescope ; and is evidently an optical, not astronomical, phenomenon.

Libration must be well understood, before proceeding to topography. This is an apparent displacement of the spots with respect to the limb (or centre), arising in one direction (that of lunar longitude) from the equable rotation of the

1 See Proctor's very ingenious illustration of such deceptions under the head of Saturn.

2 Kunovvsky has seen the dark liinb sharply defined 3d 4" after I. Quarter. G. professed that he was able to trace the larger suas on the dark side with the naked eye.

THE MOON. 87

Moon on its axis combined with its unequable velocity in an elliptical orbit, in another direction (that of latitude) from the inclination of its orbit to the ecliptic in conjunction with a slight inclination of the axis to the plane of the orbit : it completes its changes in about four weeks, though an exact restoration of the position called ' mean (or medium) libration' does not take place till the end of three years. Hence, the usual statement that we see always the same hemisphere is only approximately true. The spots are constantly swinging a little backwards and forwards, and those near the E. and W. limbs going alternately out of sight ; while, as our eye rises successively above each pole, a little more or less of those regions is seen, — the whole area thus concealed and exposed by turns amounting to T2T of the surface of the globe.1 No map of the Moon, there- fore, can correctly represent its whole aspect on two follow- ing nights, and they are of course constructed to correspond with a state of mean libration. The displacement of tha spots, which amounts at a maximum to 6° 47' N. and S., 7° 55' E. and W., and 10° 24' from a concurrence of both, in either direction from a normal position, produces little effect upon the aspect of the central parts, which are merely shifted with regard to the Moon's equator or Ist meridian ; but in the foreshortened neighbourhood of the limb its results are very obvious. Fresh regions constantly take the outline of the disc, and the mountainous projections of to-night may be all out of sight to-morrow. One effect of libration is, that the spots have no fixed position with respect to the Moon's age, being sometimes earlier, sometimes later, on the terminator, so that no precise instructions can be given when to look for them ; it may be useful, however, to know that, as Birt has pointed out, the terminator passes 1 Proctor, The Moon, 197.

88 THE SOLAR SYSTEM.

through very nearly the same part of the surface at recurring intervals of 59d ih 28™. A few approximations alone will be found in these pages, especially as a little practice with the map will render identification easy : the following con- siderations, however, may be useful to the beginner: —

The relief of the surface will be stronger as it is nearer to the terminator, and all delicate and difficult objects are best seen near the sunrise or sunset of the Moon, which, like the corresponding times of the Earth, abound with grand and beautiful effects of light and shade. Every little irregu- larity then assumes temporary importance ; inconsiderable hillocks, minute craters, low banks, narrow canals, become visible in the horizontal ray ; rising grounds or soft valleys seem to start into existence, the outer slopes of craters advance into the surrounding levels, larger masses appear in exaggerated prominence,

Majoresque cadunt de montibus umbrea.

These broad and deep shadows often taper off to such slender points, that a caution may be requisite, not to infer anything extravagant respecting the sharpness of the form which casts them. It may be questioned whether anything in the Moon exceeds the acuteness of the Swiss Finsteraarhorn, or the abruptness of the Pic du Midi d'Ossau in the Pyrenees. An oblique light cast upon a rough surface, modelled in clay or dough, will show how disproportioned oftentimes is the shadow to the reality ; and the same experiment will prove to us that in many cases the true relief will be unknown till the shadow on either side of the object has been examined. The intense blackness of the lunar shadows gives an effect which must be strangely contrasted with a distant prospect of our Earth. Here, a highly reflective and vapour-charged atmosphere surrounds all objects with a constant illumina-

THE MOON. 89

tion, beginning before sunrise and outlasting sunset ; there, the Sun goes up and down in noonday strength, and the shadows are unrelieved by any reflection from a sky which must be almost black all day: every mountain produces utter darkness where it intercepts the Sun ; and every crater, while its ring is glittering like snow in the rising or setting beam, is filled with midnight shade. Dawes, alone, by em- ploying the contracted field of his solar eye-piece, has traced a very faint glimmering in these depths, produced by reflection from the opposite sunlit cliffs.1 The termi- nator is, indeed, marked through the grey plains by a narrow shadowy border, and the tops of the mountains just appear- ing or vanishing in the night-side are somewhat deficient in brightness ; but this is the ' penumbra,' or partial illumina- tion, due to a portion of the Sun's disc, while the rest is beneath the horizon. In other respects the glittering sharp- ness of the Moon's sunrise, or sunset, is widely contrasted with the softness of our own skies. But though it is then that we must watch for minute details, for large objects we shall find the relief too broad and partial ; unbroken night conceals the greater cavities, and the shade of loftier summits renders lower ranges invisible. Not till day descends among the terraces of the one, or creeps down over the shoulders and along the slopes of the other, does their true structure come out; and their distinctness increases while their minuter neighbours decline into insignificance.

L.,2 and B. and M., have done admirably well in their

1 Schm. twice saw the innermost cavity of the peculiarly shaped crater 399 feebly illuminated at sunrise, as though filled with mist. He prefers an explanation identical with that of Dawes; but qu. as to its adequacy under the circumstances? Another large crater near S. pole also twice showed a similar internal twilight. — Compare appearance of 132, Eng. Mech. 1871, Dec. 25 : 1872, Mar. 15.

3 His general map is valuable, and not expensive; but the scale.

90 THE SOLAR SYSTEM.

delineations. Yet a little experience will show that they have not represented all that may sometimes be seen with a good common telescope. My own opportunities, even when limited to a 3r7¥-in. aperture, satisfied me, not only how much remains to be done, but how much a little willing perseverance might do, provided there were some knowledge of the laws of perspective and shadow, and a due attention to the direction of the incident and reflected light: some facility in design would also be very desirable, and, if pro- portion is tolerably preserved, a number of rough sketches under varied lights taken at the eye-piece, and carefully compared with the original, would be more serviceable than one or two finished drawings. Detail being the great object, a small portion only should be attempted at once ; this will not merely be easier and more pleasant, but will avoid the change in the shadows, which is considerable near the terminator during a long-continued delineation. The record, to be of value, must possess four data: i. Hour of Observa- tion;— 2. Moon's Aye, reckoned from or to the nearest change ; — 3. Position of Terminator referred to well-marked spots, some nearly in tlie latitude of the observed region ; others a good way N. and S. of it, so as to show the direction of illumination, which is seldom exactly at right angles to the lunar meridian ; — 4. Libration, indicated (i.) in longitude, by time reckoned to or from nearest perige or apoge; or both may be specified ; (ii.) in latitude, by Moon's latitude at the time ; these particulars being taken from ' Nautical Almanac,' and the last reduced to the hour of observation.

15^ inches, is rather small, and the deviations from circularity in the craters exaggerated. The discontinuance of his * Sections,' on a 3-ft. (French) scale, from failing sight, was long a serious hindrance to selenography ; but the missing portions have been recovered, and recently published under the careful editorship of Schm.

THE MOON. . QI

More than this cannot bo ascertained without a micrometer or calculation; but this is enough for the comparison of delineations. If the last two data are nearly coincident in any observations, the angles under which the landscape is illuminated and viewed differ so little that any variation in its details must be referred to either (i) inadvertence or mistake in the observer; (2) actual change in the Moon's surface ; or (3) obscuration or deception in her atmosphere. Schr. relates so many instances of the latter kind that, even if we reject the confirmation of the keen-eyed but fanciful G., it seems difficult to follow B. and M. in disposing of them all as errors of observation ; nor does Klein, a very accurate living observer, think the inquiry by any means unworthy of special notice. One important caution seems not to have occurred to Schr. : the change of lunar seasons, slight as it is, somewhat affects the direction of the shadows ; and the variations of foreshortening from libration may make them more or less visible; and thus some apparent discrepancies might be reconciled. With large apertures, a lightly tinted screen-glass works well ; possibly the use of different colours might bring out seme curious result.1

A good popular Map of the Moon has been hitherto a desideratum in England. B. and M.'s smaller map, though very inexpensive, is scarcely known here ; and is rather crowded. Russell's and Blunt's, striking as to general effect, break down in details. Those in ordinary books of astronomy are especially useless. The one here given will, it is hoped, be found less defective : it makes no claim to pictorial resemblance, and professes to be merely a guide to such of the more interesting features as common telescopes will reach. It is carefully reduced from the ' Mappa Sele-

1 See De la Kue, on increased distinctness during advance of lunar eclipse, Monthly Notices, xxv 276.

92 • THE SOLAR SYSTEM.

nographica ' of B. and M., published in four sheets in 1834, and subsequently re-edited, on a scale of 3 ft. if in. (3 French ft.), omitting an immense mass of detail accumu- lated by their diligent perseverance,1 which would only serve to perplex the beginner. Selection was difficult in such a crowd ; on the whole it seemed best to include every object distinguished by an independent name, including the recent additions ; many of little interest thus creep in, and many sufficiently remarkable ones drop out ; but tho line must have been drawn somewhere, and perhaps would have been nowhere better chosen for the student. Other spots, however, have been admitted from their conspicuousness, to which B. and M. have given only a subordinate name ; minuter details come in, in places, for ready identification ; elsewhere, larger objects are passed by, as less useful for the purpose of the map. The nomenclature is almost identically that established by B. and M., as rectified and extended by Neison. Hevel (or Hevelius), in the earliest attempt, designated different regions of the Moon from supposed geographical analogies ; but this system has long been abandoned, except in the case of some mountain ranges. Riccioli, a far inferior observer, adopted a more available method, originally adopted by Langrenus, of affixing to the larger spots the names of distinguished philosophers ; his list was increased by Schr., who made each name include the adjacent objects, by adding the letters of the alphabet ; and this system, improved and generalised, has been applied by B. and M. to the whole disc ; the name used alone dis- tinguishes the principal object ; Greek or Roman letters

1 Some idea may be formed of this, from the 919 nricrometrical measures of the position of spots, and 1095 of heights and depths, contained in their work Der Mond. Schrn., however, has left them far behind.

THE MOON. 93

added to it signify respectively the elevations or hollows in the vicinity. To avoid crowding our map, letters and numbers are substituted for names; every object in the descriptive notices will be thus referred to, but, as this selec- tion is limited, a complete list of the names givon by Ne. is subjoined, which will also secure, in all cases, the important object of identification as far as it goes. The alphabetical arrangement which follows it may be found convenient.

The remark is scarcely needed that the student who aims at more than a superficial knowledge of this very interesting subject will require furthor aid than he can derive from these pages, or from the accompanying map. The larger of the maps of B. and M. will help him greatly, especially if, as a German student, he can avail himself of their corresponding work, ' Der Mond ' ; l or, if economy is an object, their own abridgment of it. In point of compre- hensiveness, nothing can equal the magnificent map of Schm., 6 French ft. in diameter, in 25 sections, published, to their great honour, by the Prussian Government. It is the fruit of 34 years' labour, and the surprising number of 32,856 craters which it is said to contain sufficiently bespeaks its minuteness. It has, indeed, one very serious fault — namely, the exaggeration of the importance of low banks and ridges by a disproportioned depth of shade ; and in this respect it is inferior to the map of B. and M. But its mass of detail, unique as it is, cannot supersede the necessity of verbal description ; and here the ' Moon ' of Neison will render such invaluable service that it can hardly be dispensed with. It is illustrated by a map in sections, less minute than that of Schm., but quite sufficient for the

1 A little work by Schm. under the same title could not be sub- stituted for it as a text-book, but is very pleasing and interesting, ami well worthy of translation.

94 THE SOLAE SYSTEM.

majority of students ; and our choice, if limited to a single manual, would fall without hesitation on this eminently careful and faithful work.1

The points of the Lunar Compass must be mastered before we can use the map. Astronomers have fixed these from their relative, not intrinsic, position; that is, the several portions of the disc are named from the adjacent quarter of the sky when the moon is on the meridian. Hence N. and S. occupy the top and bottom, but E. and W. are reversed, as compared with terrestrial maps : the former being to the left, the latter to the right. But maps of tho Moon usually represent its telescopic — that is, inverted — appearance, so that we shall find S. at top, N. at bottom, E. to right, W. to left. The meridians and parallels of latitude have been omitted, except the Ist Meridian and Equator, which divide it, like the original, into Four Quadrants: these are called by B. and M., the i8t or NW., 2nd or NE., 3rd or SE., 4th or SW. Quadrant. A selection of the most interesting objects in each follows, from materials furnished by the ' Mond ' of these astronomers and retaining their arrangement : additions are made from other sources ; but all statements not otherwise authenticated depend upon their authority. In the more remarkable instances, their measures of height and depth are given ; these were ascer- tained by the lengths of the shadows — a method previously employed with less accuracy by Schr. ; capable of much precision on favourable ground, but elsewhere uncertain. For our present purpose round numbers will be a fully sufficient approximation.2

1 A good map has recently been prepared by Gaudiberfc ; and, for beginners, Mellor's " Handy Map," mounted on card, will be found useful.

8 Minuteness of numerical detail in astronomical results should not

THE MOOX (FIRST QUADRANT). 95

FIKST, OR NORTH-WEST, QUADRANT.

MARE CRISIUM (A on the map). We begin with a con- spicuous dark plain, the most completely bounded on the Moon, and visible to the naked eye : apparently elliptical from foreshortening, but really oval the other way, being about 28 im. from N. to S., 355 from E. to W., and contain- ing about 66,000 square m., or -g^th part of the visible hemisphere — more than half as much again as the area of England and Wales. Its grey hue has a trace of green in the Full; this has also been represented by the late talented Astronomer Royal for Scotland, C. Piazzi Smyth, in two beautiful figures taken during the increase and wane. On rare occasions it has been seen by Schr., and in part by B. and M., speckled with mimite dots and streaks of light : something of this kind I saw with a fluid achromatic, 1832, July 4, near I. Quarter. A similar appearance was noticed by Slack, and Ingall, 1865. It would be difficult to say why, if these are permanent, they are so seldom visible — a suggestive, though at present unintelligible, phenomenon. The surface is deeply depressed, lower than Mare Foecundi- tatis and M. Tranquillitatis. Many low ridges cross it in a N. and S. direction ; but in this, as in many other cases, it must be remembered that those lying E. and W. would be imperceptible for want of shadow. The boundary moun- tains are in part very steep and lofty. The PROMONTORIUM AGARUM (1) rises about 1 1,000 ft. ; a mountain SE. of Picard, 15,600 ft., rivalling our Mont Blanc. On W. edge, Schr. delineated a crater called by him ALHAZEN, which he con- stantly employed to measure the existing libration : he saw

be misunderstood. It represents no natural fact, but only the care bestowed upon measurement.

96 THE SOLAR SYSTEM.

in it after a time unaccountable changes, and since, it has been said, it cannot be made out. B. and M. think he confounded it with a crater (2) lying farther S. ; G., how- ever, drew it in 1824; the question, which was debated among Kunowsky, Kohler, and others, has been cleared up by Birt, who has recovered Schr.'s crater, between two lofty mountains ; the cause, however, of its greater obviousness in Schr.'s earlier days still offers some difficulty. The plain contains several moderate-sized craters — the largest, PICARD (4), S. of which G. saw some curious regular white- ridges like ramparts : while on the W., Birt and Ingall have pointed out a remarkable bright patch, which subsequently faded to a great extent, and which, according to Espin, marks a depression. The next crater N. (PEIRCE) has a very minute interior craterlet, discovered by Schm. — a test- object. The next crater N. of this (PEIRCE A) Birt has sometimes been unable to find. There are also several very minute craters in the level. Near the E. edge where there is a pass in the great surrounding ridge between 450 and 451, lie several small, but in part lofty mountains,1 — islands, as it were ; — among these Schr. describes singular changes, which he refers to an atmosphere; B. and M. consider them merely varied illumination, or pass them by as unworthy of attention. Succeeding observers may revise, even if they ultimately acquiesce in, this summary decision. Schr. was a bad draughtsman, used an inferior measuring apparatus, and now and then made considerable mistakes ; but I have never closed the simple and candid record of his most zealous labours with any feeling approaching to contempt ; and though there may be truth in the assertion of B. and M., that he was biassed by the desire of discover-

1 These are ill-figured in the great map, and B. and M. have given a separate representation of them in their Mond.

THE MOON (FIRST QUADRANT). 97

ing changes, they, possibly, were not themselves free from an opposite prepossession.

A long winding cleft is suspected to cross the centre of A.

Central hills seldom occur in craters here.

A may be well seen about 5a after New, or 3d after Full ; in the latter case it is a magnificent spectacle when crossed by the terminator and partially covered in by the vast shadows of the mountains, from which Schr. considered that those on NE. side must be at least 16,000 or 17,000 ft. high.

This astronomer has inserted in his work a marvellous observation by Eysenhard, a pupil of Lambert, 1774, July 25. The night being perfectly clear, he saw with a common 4-ft. refractor four bright spots in A, then intersected by the terminator, two of which only — those on the day-side — can be identified ; after noticing them at times for 2h, he found all at once that the part of the terminator in A had a slow reciprocating motion, completed in 5 or 6m, between these pairs of spots, each pair being touched by it in turn. Two other refractors of 7 and 12 ft. showed this appearance with equal distinctness, and it was observed for 2h, the terminator in X remaining perfectly still. A very strange story, yet Lambert seems to have believed it ; and perhaps we cannot pronounce it wholly incredible in the face of an equally wonderful and perfectly well-attested retrogression in a satellite of Jupiter, to be described hereafter.

Between FIRMICUS (7) and the limb, halfway in mean libration, are some curved dark streaks (Paludes Amarse, Hevel), in which B. and M. found singular variations, re- sulting, as they admit, possibly (not probably) from periodical vegetation.

CLEOMEDES (12), a walled plain 78 m. in diameter, in- cludes a small crater (Cleomedes A), brilliant, but not always VOL. i. H

98 THE SOLAR SYSTEM.

alike defined, in Full ; Schr. had found it not always equally visible. He speaks of many variations in the interior level, which he represents rather differently from B. and M. — G. found its W. part marked out into many rhomboids — squares in perspective, since confirmed in part by Ne.

BURCKHARDT (19), 35 m. in diameter, lies 12,700 ft. below its E. wall.

GEMINUS (20), 54 m. broad, has a ring 12,300* ft. high on E., 16,700 ft. on W. side.2

BERNOUILLI (21), equally deep, is very precipitous.

GAUSS (22) is a walled plain, no m. long. B. and M. describe the fine effect of sunset upon its ring. It has a grand central mountain, which must at times command a glorious view, across a plain of 50 m. covered with night, to illuminated peaks all round the horizon, above which the Sun on one side, and the Earth on the other, are slowly coming into sight.3

STRUVE (25), a slight depression, is remarkably dark in Full.

ENDYMION (27), a walled plain 78 m. in diameter, is, in some states of libration, very dark in Full ; the irregular wall rises W. to more than 15,000 ft., overtopping all but the very highest peaks of our Alps. I have seen it in grand

1 By a singular coincidence, in reducing as usual French io English measure, the resulting number was i, 2, 3, 4, 5 with 6 in the first place of decimals. The chances against such a sequence must have been extremely great; but it exemplifies a principle, not always kept in mind, that so long as a thing is possible, it must sometimes occur.

2 And two distinct clefts traversing the inner slope of the SW. and NE. walls respectively, and extending for some distance on the floor.

8 There is a long range of hills running in a meridional direction on the floor, and two large rings on the S. side of it.

THE MOON (FIRST QUADRANT). 99

relief 3d 7 h after New, 2d Qh after Full. Between 27, 28, and 29 is a curious double parallelism, objects all ranging SSW. or SE.

ATLAS (28), a superb amphitheatre 55 m. broad, 460 square m. in area ; its ring rich in terraces and towers, rising 1 1,000 ft. on N. ; a very dark speck in interior, where I have also seen some clefts with Bird's i2-in. silvered reflector.1