When all the stars blaze out on a clear, moonless night, it seams as if it would be impossible to count them; and yet it is seldom that more than 2000 are visible together to the unaided eye. The number, however, depends very much upon climate and sharpness of sight. Argelander enumerated at Bonn, where rather more than eight-tenths of the sphere come successively into view, 3256 stars.^[1] But of these no more than 2000 could be, at any one time, above the horizon, and so many would not be perceptibly above it, owing to the quenching power of the air in its neighbourhood. Heis, with exceptionally keen sight, distinguished at Münster 1445 more stars than Argelander had seen at Bonn;^[2] Houzeau recorded 5719 at Jamaica;^[3] Gould 7756 within 100° of the south pole at Cordoba in South America.^[4] The discrepancies of these figures are due to the multitude of small stars always, it might be said, hovering on the verge of visibility. If, indeed, the atmosphere could be wholly withdrawn, fully 25,000 stars would, according to a trustworthy estimate, become apparent to moderately good eyes.^[5]

Our system of designating the stars has come down to us from a hoar antiquity. It is a very embarrassing one. "The constellations," Sir John Herschel remarks,^[6] "seem to have been almost purposely named and delineated to cause as much confusion and inconvenience as possible. Innumerable snakes twine through long and contorted areas of the heavens where no memory can follow them; bears, lions, and fishes, large and small, northern and southern, confuse all nomenclature." And yet we could ill afford to dispense with the picturesque associations of a menagerie, largely stocked from the banks of the Euphrates. The signs of the Zodiac, which are undoubtedly of Chaldean origin, embody legendary cycles of thought already, some four thousand years ago, the worse for the wear and dilapidated by time. Homer and Hesiod were familiar with the Bear, Arcturus, and the Dog-star, with "the Hyades, and the Pleiades, and the strength of Orion." The Little Bear was introduced from Phoenicia, when the Pole-star became the mariner's "cynosure." Finally, a number of individual stars have Arabic appellations, dating from the epoch of Saracen supremacy over science. Thus "Vega," the current name of the brightest star in the Greek constellation of the Lyre, is the remnant of an Arabic phrase signifying the "Falling Eagle," while "Altair" stands for the "Flying Eagle;" "Deneb" means the Tail of the Swan; "Fomalhaut," the "Mouth of the Fish;" "Rigel" in Orion is the Leg, "Betelgeux," the "Shoulder of the Giant," and so on.

The constellations^[7] now generally recognised are eighty-six in number, of which forty-eight are found in Ptolemy's "Almagest." From Ptolemy, too, is derived the method of classifying the stars by "magnitudes." This is a most inappropriate term, since none of the stars have any perceptible dimensions, They are literally what Shelley calls them, "atoms of intensest light"—globes shrunken by distance to the semblance of mere shining needle-points. Our own sun, removed to the place of the nearest fixed star, would be in the same condition; contracted to 1/143″, its diameter would be utterly inappreciable with the largest telescope. It is true that the telescopic images of the stars appear to be of measurable size; but this is a purely optical effect, and the "spurious discs" shown by them actually grow smaller instead of larger as the power of the instrument is increased.

Thus 'magnitude' has nothing to do with apparent size, but refers entirely to apparent lustre, which depends upon distance and intensity of shining, as well as upon actual dimensions. The faintest stars have the highest numerical magnitudes; and it has been found that the gap between each successive order, as represented by the stars traditionally belonging to it, corresponds to a falling-off of light in the proportion of about 21/2 to 1. The arrangement by magnitudes is, of course, entirely arbitrary; natural gradations are not by a flight of steps, but along an inclined plane. Stars classed as of the first magnitude (of which there are ten in each hemisphere)^[8] differ accordingly very much among themselves. Sirius exceeds Fomalhaut no less than twelve times; Vega is more than twice as brilliant as α Crucis. Arcturus outshines every other northern star, but three southern luminaries—Sirius, Canopus, and α Centauri—are superior to it. Of second magnitude are the seven stars grouped to form 'Charles's Wain,' the Pole-star, and some of the most vivid gems in Perseus, Cassiopeia, and the Swan. Stars of the sixth magnitude are the faintest ordinarily visible to the naked eye; but those of the seventh can be seen under advantageous circumstances. The plan, introduced by Bayer in 1603, of naming the stars of the several constellations roughly in order of brightness by the letters of the Greek alphabet, established for each a kind of light-sequence, useful, though far from exact. The smaller stars are usually distinguished by the numbers attached to them in various catalogues.

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One of the most notorious circumstances about the stars is their 'twinkling.' They undergo, especially when near the horizon, extremely rapid changes of lustre, attended sometimes by the glinting of prismatic colours. Nor do all stand, in this respect, on the same level. White stars twinkle more than red ones. Even early and untutored observers noticed how

 The fiery Sirius alters hue,
And bickers into red and emerald.

whence it was called by Aratus ποικίλος, the "many-coloured"; and chromatic unsteadiness was a marked peculiarity of the "new stars" of 1572 and 1604.

It is easy to see that this effect is in some way due to the atmosphere. Like refraction, it vanishes at the zenith; it varies in intensity with weather and climate. The first rational conjecture as to its cause was made in 1667 by Robert Hooke, who attributed it to irregular refraction in the various air-strata. More exact inquiries on the subject have, in recent times, led to some curious results.

The impressions of light on the retina last, according to Plateau's careful determination, 0.34—say one-third—of a second. This is the limit of their individual perceptibility. With more frequent recurrence, they become merged indistinguishably together. But the changes producing scintillation succeed each other much more rapidly than three times in a second. Hence the need of some means of separating and analysing them.

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These were provided by M. Montigny's 'scintillometer,'^[9] in which the sensibility of different parts of the retina was skilfully turned to account. By the rotation of a glass-plate obliquely inserted in front of the eye-piece of a refracting telescope, the image of a star viewed with it is made to describe an exact circle in the field. The line of light traced out is, in perfectly steady air, continuous and of a uniform hue, but breaks up, under the influence of scintillation, into vividly tinted arcs, at times into prismatic " pearls." The addition of a pair of crossed wires facilitates the reckoning of the colour- fluctuations thus rendered separately visible; and they are found to occur, on an average, in white stars standing thirty degrees above the horizon, seventy-eight times in a second, in yellow and red stars similarly placed, sixty-eight and fifty-six times respectively.^[10]

The explanation of these appearances is evidently to be sought in the refractive power, combined with the disturbed condition or our atmosphere. ...

THE TASK OF SIDEREAL ASTRONOMY

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of our aedftl envelope. For a different path through its strata ia neoeasarily pursued by each of the differently refrangible beams united to form the itnage of a atar. The violet ent«ra them higher up, eiuce it is more bent in transit tlian the red ; and &o proportionately of the rest. Each then ia liable to ^^mcouBter different vicissitudes on the way, betrayed to our ^Bght by rapid flashes of colour. Each is affected by innumer^ ^Blle small deviations and momentary capncea of refraction ; PHb that the bundle of rays picturing a star at a given instant jjt, as it were, a fortuitous and eminently unstable combination. It id dissolved, and a new one constituted, sixty or seventy titD€s in a second ; and the elements temporarily missing determine the reeultiug tint. The fundamental fact of the matter, in short, i8 that the light of every star near the horizon is drawn out into a tiny spectrum by the chromatic dispersion of the atmosphere ; and Respighi's study of the ^actuations in these prismatic images ^ provided, accordingly, ^^le ifirsct secure basis for a scientific theory of scintillation. ^B That white twinkle more than red stats becomes intelligible ^^■ben we consider that the sheaf of their beams being fuller, ^^Kteroeptiona of them are more frequent But planets which are radiating discs, and not merely points, rarely show the effect, because the absence of rays from one part is compeasated by the arrival of rays from other parts of their surfaces. Similarly, the steady radiance of stars in large telescopes ia k^ie to the neutralisation of each casual stoppage by the P^^eat number of the beams collected together. Instead of a twinkling image, however, a blurred and distended one is formed under perturbed conditions, and observation gains nothing by the exchange. And since the degree to which this phenomenon is present varies very much with locality, regard should be had to its prevalence in choosing ^ sites for power- jil instruments. It diminishes, on the whole, with altitude. it the configuration of adjacent mountain-ranges is strongly uential, and Dr. Pemter found Sirius actually to scintillate

^ Le* Monda, t lix. p. 6S8 {isa&) \ Lard Rayleigh, Phil M'ag. toL xxxvj. > 129, 1»03. ' ExDCTf Attr, Xtteh, Tjlo. 37$! ; A. E. DouglaflB, Fopttlnr Aatronontt/, Jwae ' ; I«oweU, Monthly Koti«s, Tol< Ixiii. p> 4D ; Eincr abd Villiger, Mtroph. Jowm. vqL xxL p. 36S.

THE SYSTEM OF THE STABS

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more at the summit than at the foot of the Sounblick (10,C feet high).^

Scintiltatioti, like astronoooical refmction, augments as the thermometer falls and as the barometer rises. Thia is inevit- able, since the first requisite for its occurrence is differential refractiTe action on the various Ught-rays,* But it haa other leas obviously accountable meteorological relations. M. Montigny ascertained, by the experience of nearly forty years, that with the quantity of moisture in the air the twinkling of the stars increases so markedly as to serve for a useful pTOgnostic of rain. Cyclonic conditions promote it;^ and it is extremely sensitive to magnetic disturbances.* Useher was struck, in the eighteenth century, with the surprising vividness of scintillation during aurorse ; Montigny extended the coincidence to magnetic commotions perceptible only instrumentally. Moreover, Weber remarked at Peckeloh in 1880, that stars aitoated near the magnetic meridian twinkled more than elsewhere in the sky;^ and although little attention has of late been paid to the possible dependence of the effect upon the points of the compass, yet the theoretical interest of scintillation would be much enhanced should it turn out to be one of the many terrestrial phenomena associated with vicisaitudes in the physical condition of the sun.

The world of stars thrown open by the telescope may fairly be called boundless. Using a glass only two and a half inches across. Argelander registered 324,189 down to 9i magnitude, aU in the northern hemisphere, with the addition of a southern zone one degree wide. The work was extended to the southern tropic by Schonfeld, and completed to the southern pole under Sir David Gill's direction. At the Cape, the photographic method was employed, and the resulting enrolment, published 1896-1900, comprises 454,875 stars nearly to the tenth magnitude; while the Cordoba visual Dnrchmuaterung, executed by Thome and Tucker, 1835-1904, is a still more comprehensive register of southern objects. Yet

• Obttrmtory, rol, lii, p. 1&4.

• Montigny, Bull, de I'Acad, BrunBUeft, t. xlri. p. 813, 2t]d senea.

' Roaentlial. Meicor. 2iitschr\/t, Bd. xi. p. 145.

• CQmpltf R*nd%tMt t. icvi* p. 673-

' lyoehtntchr^t/tlr Astrtmmut, 1360, p. 2&4.

THE TASK OF SIDEREAL ASTKONOMY

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greftt works are merely preliminary to the International 'chartJBg opCTationa in progress. Their accomplishment will yi«ld a " Catalogue of precision " including at least three million stars; aad somewhere about thirty millioas, taken from the submerged population of space^ will, by their record on the chart-plates, be admitted to the citizenship of astronomy. They can never thenceforward be excluded from the scope of research* Their light - changes, their movementB, their distribution, will present an inexhaustible, and probably a moot fruitM field of inquiry.

Mr. Plommer showed in 1877^ that the lucid atare (those visible to the naked eye) in the Bonn Durchmusterung give as mach light as 7349, the telescopic stars as 23,337 sixth magnitude atara Those singly imperceptible thus really illuminate the sky just three lirues more than those in- dividually seen. Stamaing up, with the aid of the beet photometric data, the entire light of Argelander's 324,000 atara, we get for its equivalent j^ full moonlight ; and we y roughly estimate the total light of all those similarly inumerated in both hemispheres, to the number of about 900,000^ at -pj^ the lunar brightness. The amount of scattered effulgence dispensed by still fainter stars is exceed- ingly difficult to evaluate. Sir William Abney, using a photographic method, rated in 1896 the sum of starlight in both hemispheres at ^iv ^^ moonlight. Professor Newcomb, in 1901,^ from visual observations of diffused sky-radiance, condaded the light-power of all the stars to be just 723 timea that of CapeUa;* and 728 stars like Capella give ^ the light of the full moon.* But it is far from certain that the I T anlt of heaven would seem absolutely black if the stars were ^^lotted out.^ Our tipper air is the seat of processes by which ^^■iniinosity is at times strongly developed ; and we cannot be ^^pore that they are ever entii'ely suspended. Hence, Frofe&aor [ Kewcomb's experiments ajford no assurance regarding the

^^H 1 Monthly J^fotias, vol, xxxvii. p, 43(3.

^V ^ Astrophytioil Jtmrnal, vol, xiv. p. 310; see iho OaTla J. Buras, ibid^ ToL iri. p. 169.

' Newomab'B 600 stars a{ O'O fflftgnttude are «qatl to 723 of th« pbQtoniBtriQ bright&6» (0^1] 4f CApeUft.

• VmtT, flictomarit dtr Qt^TM, p. 340.

■ Cf. Bttrn», Jov.rn. BrU. Aatr, Au. toI. iv. p. 91.

8

THE 8TSTEM OF THE STABS

iiudividi^B

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eqoiTaleat, to mf *****^f d Urn mitml BiuiLct of itaa oomuig dinctfy, or iadtnetly wHfata o«r fcea. Thaae last caa- QOt well &U ifaat of ntj ™ittwT«_ aad tbej m«« profaablj mun 1^ to OBie hazidred aillioBo; bat the aet* of iDqaiij can at prueeDi scmroefy be djswn ckser Yet the foot io Doto- worth^ that each clan of stsn sendi itt ^ppncnl^ more light tban the dam next sbofv iL The I^it-Aggx«g«ta of BBOond magmtode ttaxs exeeeds that 'of ink, of third that of aeooud, and so oil The ftinter the aUzs, in sbortj the gieatcr is their total luminooe power/ >iw»iiiift their uumbers more than «yMin t .f*' t;Miti^"fg tb^r diniiiiiabed i lufiti^ But this pfogreanao, it is eridetit^ caanoi go on iodefiuitelj, EJoce otbenrise an indtifinitelj inteoae radiaaca would fill the sky. Daricnen wonld be abolished through the fihiaing of invisible stara. It £ollo«a other Uiat the obeerred order of the etellar vorid baa aaagnabk limits — that the alar- depths, however proloand, are not abaolotelj tm&tbomable ; or that space, for whaterer reasoo, is not abeolately tiaaspareat

The Laak of exploration, at anj rate, does not aeem to be altogether hopeless. It can neTer indeed be exhausted; bat it can fairly be grappled with by finite minds. It does aofc evade their efibrte with the paasve ecom of material infini- tude. Geauine. if partial, saooeases have otiwned them in the pastf and will, it may be hoped, continne to ciowu ihem in the future.

We must not. however, in seeking enconittgement from the thought that it does not utterly defy oor powera^ under- rate the difficulty of the enterprise we have taken in hand. The nature of our own sun oflers a vast and intricate problem, still very far from being solved ; but stellar epoce contains many millions of suns, variously couEtituted, varioualy circum- atanced, frequently snrpaesing our magmficent orb in size and splendour. Now each of these millions of suns challenges the closest pergonal attention ; no single one of them is exactly like any other, and their diffcrenoee and resemblances open endleaa vistas of instruction and interest. Their incon- ceivable remoteDese in no way derogates from their real dignity. An all but evaaescent speck of light in the field of the great Lick refractor may bo the life-giving centre of a

• L*AttTonoini*t t. v, p. 400*

THE TASK OF SIDEREAL ASTROXOMY

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of worldB, each aboanding as manrelloudj vitb proofs of creaiiwe wisdom and goodness aa the little planet in which our tempoml deetuues &re imprisoned. Such ligbtr-specks are Aea equally deeerring of stody with the moet effulgent orb«  in the ekj, although it may never be practicable to bestow it apoD them. We can indeed hardly imagine the amount of telescopic improvement which would be needed in order to hnng them within the range of critical examination. For the preeent, accordingly, physical research must be confined to eome thonstuids of the brighter etare which may serve as Bpecuneiu of the rest. Nor need we lament the restriction. GenerAtioiiS of workers migbt expend their energies in ^tbering facta from the field actually open to them, and yet leave a fiill harvest for their successors. In all ei- perimentAl inquiiies, it may with truth be said that the reaper, aa be gamers one crop of knowledge, sows another : Bo endless are the secrets of nature, ao untiring the inqniai- ivenese of man.

The stars in their combinations demand inquiry no less ■n the stare in themselves. Stellar systems are to be met with in indescribable profusion and variety^ from mutually drcUng pairs, through groupa including thousands of phyei- caUy related objects, to the stupendous integrated collection which we call the Milky Way. But as yet investigation has barely aldrted the edge of this well-nigh infinite region. Before it can be penetrated by so much as a plausible conjecturej BMtati»tics are wanted of the distances and movements of r thousands, nay millions of stars.

I Nor is the amassing of them any longer the Sisyphean

I labour it seemed a short time ainca By the unhoped-for development of novel methods, the pace of inquiry has been quickened all along the line. Particulars are accumulated faster than they can be assorted and arranged. Time has virtually expanded, as if for the purpose of gratifying curiosity ^^phich becomes keener a3 its sublime objects loom more ^Histtnctly above the horizon of thought Ten years now connt for a century of the old plodding advance. Express trains carry passengers on erraada of research, aa well as of btmnesa or pleasure. Problems ripen as if in a forcing- bousej and 80 numerously as almost to bewilder the attention.

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THE SYSTEM OF THE STAPS

The whole sabjecc af ddereal natural historx ia and intricat« beyond wbAt il is eAsy to coarej to the approaching it for the fitBt time. TheT« ia ac»rcely a topic iti physical aHtronomy with which it is unconcected. The pro- gress of discovery has giadtially drawn ctoaer the geuenc relationships of the hearenly bodies The mm has oome to be recognised ns the grand exemplar of the Atars ; meteoritea show themseWes to be intimately associated with comets ; comets are perhaps the shreds of uaed-np nebnlfe ', while the stellar and nebular realms blend one with the other aa indift' tinguishably as the animal and vegetable kingdomB of organic nature.

The strange cloud-like objects called "^ nebulie " may be considered as wholly of telescopic revelation. Only one of them — the famous object in the girdle of Andromeda — can be at all easily seen with the naked eye ; and even that escaped the notice of aU the Greek, and most of the mediseval astronomers. The " nebuloase " of the ancients were many of them small groups of stars accidentally set close together ; but among the seven enumerated by Ttoiemj were two real clusters like the Pleiades, only (presumably) much farther away, one in Perseus, and the other in Cancer. Indeed, to extremely ehort-aighted persons, the Pleiades them- selves put on a nebulous appearance, the individual ertars nmning together into one wide blot of light

Halley was the first to form anything like an adequate conception of the importance of nebular observations. He was acquainted in 1716 with six -'luminous spots or patches, which discover themselves only by the telescope, and appear to the naked eye like small fixed stars ; but in reality are nothing else but the light coming from an extraordinary great space in the ether, through which a lucid medium is diffused, that shines with its own proper lustre." ^ Only two of Halley's half-dozen objects, however — those in Orion and Andromeda — were genuine nebnke ; the rest when viewed with better instru- ments than hia " six-foot tube," proved to be magnificent star clusters.

This small beginning of knowledge was followed up by Lacaille in the southern, by Messier in the northern hemi- ^ Phil. Trafit, vol, izLi. p. 3»0.

THE TASK OF SIDEBEAL ASTRONOMY

11

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ilibere. Then Hencbers great tdescopes opened the modern epoch in the sciesce of nebole. A» the result of his labours thef came to be reckoned by the tbooeaud instead of by the Kore. Portioofi of the Eky were foand to be crowded with them. Yet the vaat tnajoritj must always, owing to their ^ctzeme faintness, remain imperceptible without powerful opCiical Aid, only sixty-four coming into view with the same tfllaoeope which showed Ar^lander 324,000 atara. Thua Aeoees to the nebular heavens can be gained only by making the yerj most of the little light they send iia> I^u-ge teleeoopes and prolonged photographic exposures are indeed pie-eminently uBeful in this department of celestial phyBic8» which, mainly through the application of the camera, has of late incalculably widened A pioneering survey with the Crosaley reflector led Profeseor Keeler to estimate at 120,000 the number of nebulae which might be chemically recorded by it8 meftns ; and the specimen sheaves garnered by Dr. Max Wolf at Eomg;3tuhl in 1901 are no lees promising for a rich future harvest. But discoveries are of email account if Bucceeded by neglect. Assiduous and prolonged observation is indispensable for the detection of the cyclical or progressive changes doubtleas proceeding in these inchoate systems.

This then is the task of sidereal astronomy — to investigate the nature^ origin, and relationships of 30,000,000 stars and of 120,000 nebalse — to inquire into their movements among themselves, and that of our sun among them — to assign to each ita place and rank in the universal order, and, gathering hinU) of what has been and what will be from what is, distinguish hierarchies of cele.^^tial systems, aud thus at last rise to the higher eynth^is embracing the grand mechaniBm of the entire — the sublime idea of Omnipoteuce, to which the stars conform their courses, while " they shine forth with joy Him that made them."