In the arduous matter of determining star distances progress has been steady, and bids fair to become rapidly accelerated. Together, yet independently, Gill and Elkin carried out, at the Cape Observatory in 1882-83, an investigation of remarkable accuracy into the parallaxes of nine southern stars. One of these was the famous Alpha Centauri, the distance of which from the earth was ascertained to be just one-third greater than Henderson had made it. The parallax of Sirius, on the other hand, was doubled, or its distance halved; while Canopus proved to be quite immeasurably remote—a circumstance which, considering that, among all the stellar multitude, it is outshone only by the radiant Dog-star, gives a stupendous idea of its real splendour and dimensions.

Inquiries of this kind were, for some years, successfully pursued at the observatory of Dunsink, near Dublin. Annual perspective displacements were by Dr. Bruennow detected in several stars, and in others remeasured with a care which inspired just confidence. His parallax for Alpha Lyrae (0.13") was authentic, though slightly too large (Elkin's final results gave Pi = 0.082"); and the received value for the parallax of the swiftly travelling star "Groombridge 1,830" scarcely differs from that arrived at by him in 1871 (Pi = 0.09"). His successor as Astronomer-Royal for Ireland, Sir Robert Stawell Ball (now Lowndean Professor of Astronomy in the University of Cambridge), has done good service in the same department. For besides verifying approximately Struve's parallax of half a second of arc for 61 Cygni, he refuted, in 1811, by a sweeping search for (so-called) "large" parallaxes, certain baseless conjectures of comparative nearness to the earth, in the case of red and temporary stars.[1580] Of 450 objects thus cursorily examined, only one star of the seventh magnitude, numbered 1,618 in Groombridge's Circumpolar Catalogue, gave signs of measurable vicinity. Similarly, a reconnaissance among rapidly moving stars lately made by Dr. Chase with the Yale heliometer[1581] yielded no really large, and only eight appreciable parallaxes among the 92 subjects of his experiments.

A second campaign in stellar parallax was undertaken by Gill and Elkin in 1887. But this time the two observers were in opposite hemispheres. Both used heliometers. Dr. Elkin had charge of the fine instrument then recently erected in Yale College Observatory; Sir David Gill employed one of seven inches, just constructed under his directions, in first-rate style, by the Repsolds of Hamburg. Dr. Elkin completed in 1888 his share of the more immediate joint programme, which consisted in the determination, by direct measurement, of the average parallax of stars of the first magnitude. It came out, for the ten northern luminaries, after several revisions, 0.098", equivalent to a light-journey of thirty-three years. The deviations from this average were, indeed, exceedingly wide. Two of the stars, Betelgeux and Alpha Cygni, gave no certain sign of any perspective shifting; of the rest, Procyon, with a parallax of 0.334", proved the nearest to our system. At the mean distance concluded for these ten brilliant stars, the sun would show as of only fifth magnitude; hence it claims a very subordinate rank among the suns of space. Sir David Gill's definitive results were published in 1900.[1582] As the average parallax of the eleven brightest stars in the southern hemisphere, they gave 0.13", a value enhanced by the exceptional proximity of Alpha Centauri. Yet four of these conspicuous objects—Canopus, Rigel, Spica, and Beta Crucis—gave no sign of perspective response to the annual change in our point of view. The list included eleven fainter stars with notable proper motions, and most of these proved to have fairly large parallaxes. Among other valuable contributions to this difficult branch may be instanced Bruno Peter's measurements of eleven stars with the Leipzig heliometer, 1887-92;[1583] Kapteyn's application of the method by differences in right ascension to fifteen stars observed on the meridian 1885-89;[1584] and Flint's more recent similar determinations at Madison, Wisconsin.[1585]

The great merit of having rendered photography available for the sounding of the celestial depths belongs to Professor Pritchard. The subject of his initial experiment was 61 Cygni. From measurements of 200 negatives taken in 1886, he derived for that classic star a parallax of 0.438", in satisfactory agreement with Ball's of 0.468". A detailed examination convinced the Astronomer-Royal of its superior accuracy to Bessel's result with the heliometer. The Savilian Professor carried out his project of determining all second magnitude stars to the number of about thirty,[1586] conveniently observable at Oxford, obtaining as the general outcome of the research an average parallax of 0.056", for objects of that rank. But this value, though in itself probable, cannot be accepted as authoritative, in view of certain inaccuracies in the work adverted to by Jacoby,[1587] Hermann Davis, and Gill. The method has, nevertheless, very large capabilities. Professor Kapteyn showed, in 1889,[1588] the practicability of deriving parallaxes wholesale from plates exposed at due intervals, and applied his system, in 1900, with encouraging success, to a group of 248 stars.[1589] The apparent absence of spurious shiftings justified the proposal to follow up the completion of the Astrographic Chart with the initiation of a photographic "Parallax Durchmusterung."

Observers of double stars are among the most meritorious, and need to be among the most patient and painstaking workers in sidereal astronomy. They are scarcely as numerous as could be wished. Dr. Doberck, distinguished as a computer of stellar orbits, complained in 1882[1590] that data sufficient for the purpose had not been collected for above 30 or 40 binaries out of between five and six hundred certainly or probably within reach. The progress since made is illustrated by Mr. Gore's useful Catalogue of Computed Binaries, including fifty-nine entries, presented to the Royal Irish Academy, June 9, 1890.[1591] Few have done more towards supplying the deficiency of materials than the late Baron Ercole Dembowski of Milan. He devoted the last thirty years of his life, which came to an end January 19, 1881, to the revision of the Dorpat Catalogue, and left behind him a store of micrometrical measures as numerous as they are precise.

Of living observers in this branch, Mr. S. W. Burnham is beyond question the foremost. While pursuing legal avocations at Chicago, he diverted his scanty leisure by exploring the skies with a 6-inch telescope mounted in his back-yard; and had discovered, in May, 1882, one thousand close and mostly very difficult double stars.[1592] Summoned as chief assistant to the new Lick Observatory in 1888, he resumed the work of his predilection with the 36-inch and 12-inch refractors of that establishment. But although devoting most of his attention to much-needed remeasurements of known pairs, he incidentally divided no less than 274 stars, the majority of which lay beyond the resolving power of less keen and effectually aided eyesight. One of his many interesting discoveries was that of a minute companion to Alpha Ursae Majoris (the first Pointer), which already gives unmistakable signs of orbital movement round the shining orb it is attached to. Another pair, Kappa Pegasi, detected in 1880, was found in 1892 to have more than completed a circuit in the interim.[1593] Its period of a little over eleven years is the shortest attributable to a visible binary system, except that of Delta Equulei, provisionally determined by Professor Hussey in 1900 at 5.7 years,[1594] and indicated by spectroscopic evidence to be of uncommon brevity.[1595] Burnham's Catalogue of 1,290 Double Stars, discovered by him from 1871 to 1899,[1596] is a record of unprecedented interest. Nearly all the 690 pairs included in it, 2" or less than 2" apart, must be physically connected; and they offer a practically unlimited field for investigation; while the notes, diagrams, and orbits appended profusely to the various entries, are eminently helpful to students and computers. The author is continuing his researches at the Yerkes Observatory, having quitted the Lick establishment in 1892. The first complete enrolment of southern double stars was made by Mr. R. T. A. Innes in 1899.[1597] The couples enumerated, twenty-one per cent. of which are separated by less than one second of arc, are 2,140 in number. They include 305 discovered by himself. Dr. See gathered a rich harvest of nearly 500 new southern pairs with the Lowell 24-inch refractor in 1897.[1598] Professor Hough's discoveries in more northerly zones amount to 623;[1599] Hussey's at Lick to 350; and Aitken's already to over 300.

There is as yet no certainty that the stars of 61 Cygni form a true binary combination. Mr. Burnham, indeed, holds them to be in course of definitive separation; and Professor Hall's observations at Washington, 1879 to 1891, although favouring their physical connection, are far from decisive on the point.[1600] Dr. Wilsing, from certain anomalous displacements of their photographed images, concluded in 1893[1601] the presence of an invisible third member of the system, revolving in a period of twenty-two months; but the effects noticed by him were probably illusory.

Important series of double-star observations were made by Perrotin at Nice in 1883-4;[1602] by Hall, with the 26-inch Washington equatoreal, 1874 to 1891;[1603] by Schiaparelli from 1875 onward; by Glasenapp, O. Stone, Leavenworth, Seabroke, and many besides. Finally, Professor Hussey's revision of the Pulkowa Catalogue[1604] is a work of the teres atque rotundus kind, which leaves little or nothing to be desired. The methods employed in double-star determinations remain, at the beginning of the twentieth century, essentially unchanged. The camera has scarcely encroached upon this part of the micrometer's domain.[1605]

A research of striking merit into the origin of binary stars was published in 1892 by Dr. T. J. J. See, in the form of an Inaugural Dissertation for his doctor's degree in the University of Berlin. The main result was to show the powerful effects of tidal friction in prescribing the course of their development from double nebulae, revolving almost in contact, to double suns, far apart, yet inseparable. The high eccentricities of their eventual orbits were shown to result necessarily from this mode of action, which must operate with enormous strength on closely conjoined, nearly equal masses, such as the rapidly revolving pairs disclosed by the spectroscope. That these are still in an early stage of their life-history is probable in itself, and is re-affirmed by the exceedingly small density indicated for eclipsing stars by the ratio of phase-duration to period.

Stellar photometry, initiated by the elder Herschel, and provided with exact methods by his son at the Cape, by Steinheil and Seidel at Munich, has of late years assumed the importance of a separate department of astronomical research. Two monumental works on the subject, compiled on opposite sides of the Atlantic, were thus appropriately coupled in the bestowal of the Royal Astronomical Society's Gold Medal in 1886. Harvard College Observatory led the way under the able direction of Professor E. C. Pickering. His photometric catalogue of 4,260 stars,[1606] constructed from nearly 95,000 observations of light-intensity during the years 1879-82, constitutes a record of incalculable value for the detection and estimation of stellar variability. It was succeeded in 1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of 2,784. The instrument employed was the "wedge photometer," which measures brightness by resistance to extinction. A wedge of neutral-tint glass, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. Professor Pickering's "meridian photometer," on the other hand, is based upon Zoellner's principle of equalization effected by a polarising apparatus. After all, however, as Professor Pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range.[1607] Absolute uniformity, then, in estimates made by various means, under varying conditions, and by different observers, is not to be looked for; and it is satisfactory to find substantial agreement attainable and attained. Only in an insignificant fraction of the stars common to the Harvard and Oxford catalogues discordances are found exceeding one-third of a magnitude; a large proportion (71 per cent.) agree within one-fourth, a considerable minority (31 per cent.) within one-tenth of a magnitude.[1608] The Harvard photometry was extended, on the same scale, to the opposite pole in a catalogue of the magnitudes of 7,922 southern stars,[1609] founded on Professor Bailey's observations in Peru, 1889-91. Measurements still more comprehensive were subsequently executed at the primary establishment. With a meridian photometer of augmented power, the surprising number of 473,216 settings were made during the years 1891-98, nearly all by the indefatigable director himself, and they afforded materials for a "Photometric Durchmusterung," published in 1901, including all stars to 7.5 magnitude north of declination -40 deg.[1610] A photometric zone, 20 deg. wide, has for some time been in course of observation at Potsdam by MM. Mueller and Kempf. The instrument employed by them is constructed on the polarising principle as adapted by Zoellner.

Photographic photometry has meanwhile risen to an importance if anything exceeding that of visual photometry. For the usefulness of the great international star-chart now being prepared would be gravely compromised by systematic mistakes regarding the magnitudes of the stars registered upon it. No entirely trustworthy means of determining them have, however, yet been found. There is no certainty as to the relative times of exposure needed to get images of stars representative of successive photometric ranks. All that can be done is to measure the proportionate diameters of such images, and to infer, by the application of a law learned from experience, the varied intensities of light to which they correspond. The law is, indeed, neither simple nor constant. Different investigators have arrived at different formulae, which, being purely empirical, vary their nature with the conditions of experiment. Probably the best expedient for overcoming the difficulty is that devised by Pickering, of simultaneously photographing a star and its secondary image, reduced in brightness by a known amount.[1611] The results of its use will be exhibited in a catalogue of 40,000 stars to the tenth magnitude, one for each square degree of the heavens. A photographic photometry of all the lucid stars, modelled on the visual photometry of 1884, is promised from the same copious source of novelties. The magnitudes of the stars in the Draper Catalogue were determined, so to speak, spectrographically. The quantity measured in all cases was the intensity of the hydrogen line near G. By the employment of this definite and uniform test, results were obtained, of special value indeed, but in strong disaccord with those given by less exclusive determinations.

Thought, meantime, cannot be held aloof from the great subject upon the future illustration of which so much patient industry is being expended. Nor are partial glimpses denied to us of relations fully discoverable, perhaps, only through centuries of toil. Some important points in cosmical economy have, indeed, become quite clear within the last fifty years, and scarcely any longer admit of a difference of opinion. One of these is that of the true status of nebulae.

This was virtually settled by Sir J. Herschel's description in 1847 of the structure of the Magellanic clouds; but it was not until Whewell, in 1853, and Herbert Spencer, in 1858,[1612] enforced the conclusions necessarily to be derived therefrom that the conception of the nebulae as remote galaxies, which Lord Rosse's resolution of many into stellar points had appeared to support, began to withdraw into the region of discarded and half-forgotten speculations. In the Nubeculae, as Whewell insisted,[1613] "there coexist, in a limited compass and in indiscriminate position, stars, clusters of stars, nebulae, regular and irregular, and nebulous streaks and patches. These, then, are different kinds of things in themselves, not merely different to us. There are such things as nebulae side by side with stars and with clusters of stars. Nebulous matter resolvable occurs close to nebulous matter irresolvable."

This argument from coexistence in nearly the same region of space, reiterated and reinforced with others by Mr. Spencer, was urged with his accustomed force and freshness by Mr. Proctor. It is unanswerable. There is no maintaining nebulae to be simply remote worlds of stars in the face of an agglomeration like the Nubecula Major, containing in its (certainly capacious) bosom both stars and nebulae. Add the facts that a considerable proportion of these perplexing objects are gaseous, and that an intimate relation obviously subsists between the mode of their scattering and the lie of the Milky Way, and it becomes impossible to resist the conclusion that both nebular and stellar systems are parts of a single scheme.[1614]

As to the stars themselves, the presumption of their approximate uniformity in size and brightness has been effectually dissipated. Differences of distance can no longer be invoked to account for dissimilarity in lustre. Minute orbs, altogether invisible without optical aid, are found to be indefinitely nearer to us than such radiant objects as Canopus, Betelgeux, or Rigel. Moreover, intensity of light is perceived to be a very imperfect index to real magnitude. Brilliant suns are swayed from their course by the attractive power of massive yet faintly luminous companions, and suffer eclipse from obscure interpositions. Besides, effective lustre is now known to depend no less upon the qualities of the investing atmosphere than upon the extent and radiative power of the stellar surface. Red stars must be far larger in proportion to the light diffused by them than white or yellow stars.[1615] There can be no doubt that our sun would at least double its brightness were the absorption suffered by its rays to be reduced to the Sirian standard; and, on the other hand, that it would lose half its present efficiency as a light-source if the atmosphere partially veiling its splendours were rendered as dense as that of Aldebaran.

Thus, variety of all kinds is seen to abound in the heavens; and it must be admitted that the consequent insecurity of all hypotheses as to the relative distances of individual stars singularly complicates the question of their allocation in space. Nevertheless, something has been learnt even on that point; and the tendency of modern research is, on the whole, strongly confirmatory of the views expressed by Herschel in 1802. He then no longer regarded the Milky Way as the mere visual effect of an enormously extended stratum of stars, but as an actual aggregation, highly irregular in structure, made up of stellar clouds and groups and nodosities. All the facts since ascertained fit in with this conception, to which Proctor added arguments favouring the view, since adopted by Barnard[1616] and Easton,[1617] that the stars forming the galactic stream are not only situated more closely together, but are also really, as well as apparently, of smaller dimensions than the lucid orbs studding our skies. By the laborious process of isographically charting the whole of Argelander's 324,000 stars, he brought out in 1871[1618] signs of relationship between the distribution of the brighter stars and the complex branchings of the Milky Way, which has been stamped as authentic by Newcomb's recent statistical inquiries.[1619] There is, besides, a marked condensation of stars, especially in the southern hemisphere, towards a great circle inclined some twenty degrees to the galactic plane; and these were supposed by Gould to form with the sun a subordinate cluster, of which the components are seen projected upon the sky as a zone of stellar brilliants.[1620] The zone has, however, galactic rather than solar affinities, and represents, perhaps, not a group, but a stream.

The idea is gaining ground that the Milky Way is designed, in its main outlines, on a spiral pattern, and that its various branches and sections are consequently situated at very different distances from ourselves. Proctor gave a preliminarily interpretation of their complexities on this principle, and Easton of Rotterdam[1621] has renewed the attempt with better success.

A most suggestive delineation of the Milky Way, completed in 1889, after five years of labour, by Dr. Otto Boedicker, Lord Rosse's astronomer at Parsonstown, was published by lithography in 1892. It showed a curiously intricate structure, composed of dimly luminous streams, and shreds, and patches, intermixed with dark gaps and channels. Ramifications from the main trunk ran out towards the Andromeda nebula and the "Bee-hive" cluster in Cancer, involved the Pleiades and Hyades, and, winding round the constellation of Orion, just attained the Sword-handle nebula. The last delicate touches had scarcely been put to the picture, when the laborious eye-and-hand method was, in this quarter, as already in so many others, superseded by a more expeditious process. Professor Barnard took the first photographs ever secured of the true Milky Way, July 28, August 1 and 2, 1889, at the Lick Observatory. Special conditions were required for success; above all, a wide field and a strong light-grasp, both complied with through the use of a 6-inch portrait-lens. Even thus, the sensitive plate needed some hours to pick out the exceedingly faint stars collected in the galactic clouds. These cannot be photographed under the nebulous aspect they wear to the eye; the camera takes note of their real nature, and registers their constituent stars rank by rank. Hence the difficulty of disclosing them. "In the photographs made with the 6-inch portrait-lens," Professor Barnard wrote, "besides myriads of stars, there are shown, for the first time, the vast and wonderful cloud-forms, with all their remarkable structure of lanes, holes, and black gaps, and sprays of stars. They present to us these forms in all their delicacy and beauty, as no eye or telescope can ever hope to see them."[1622] In Plate VI. one of these strange galactic landscapes is reproduced. It occurs in the Bow of Sagittarius, not far from the Trifid nebula, where the aggregations of the Milky Way are more than usually varied and characteristic. One of their distinctive features comes out with particular prominence. It will be noticed that the bright mass near the centre of the plate is tunnelled with dark holes and furrowed by dusky lanes. Such interruptions recur perpetually in the Milky Way. They are exemplified on the largest scale in the great rift dividing it into two branches all the way from Cygnus to Crux; and they are reproduced in miniature in many clusters.

PLATE VI.



Mr. H. C. Russell, at Sydney in 1890, successfully imitated Professor Barnard's example.[1623] His photographs of the southern Milky Way have many points of interest. They show the great rift, black to the eye, yet densely star-strewn to the perception of the chemical retina; while the "Coal-sack" appears absolutely dark only in its northern portion. His most remarkable discovery, however, was that of the spiral character of the two Nubeculae. With an effective exposure of four and a half hours, the Greater Cloud came out as "a complex spiral, with two centres"; while the similar conformation of its minor companion developed only after eight hours of persistent actinic action. The revelation is full of significance.

Scarcely less so, although after a different fashion, is the disclosure on plates exposed by Dr. Max Wolf, with a 5-inch lens, in June, 1891, of a vastly extended nebula, bringing some of the leading stars in Cygnus into apparently organic connection with the piles of galactic star-dust likewise involved by it.[1624] Barnard has similarly found great tracts of the Milky Way to be photographically nebulous, and the conclusion seems inevitable that we see in it a prodigious mixed system, resembling that of the Pleiades in point of composition, though differing widely from it in plan of structure. Of corroborative testimony, moreover, is the discovery independently resulting from Gill's and Pickering's photographic reviews, that stars of the first type of spectrum largely prevail in the galactic zone of the heavens.[1625] With approach to that zone, Kapteyn noticed a steady growth of actinic intensity relative to visual brightness in the stars depicted on the Cape Durchmusterung plates.[1626] In other words, stellar light is, in the Milky Way, bluer than elsewhere. And the reality of the primitive character hence to be inferred for the entire structure was, in a manner, certified by Mr. McClean's observation that Helium stars—the supposed immediate products of nebulous matter—crowd towards its medial plane.

The first step towards the unravelment of the tangled web of stellar movements was taken when Herschel established the reality, and indicated the direction of the sun's journey. But the gradual shifting backward of the whole of the celestial scenery amid which we advance accounts for only a part of the observed displacements. The stars have motions of their own besides those reflected upon them from ours. All attempts, however, to grasp the general scheme of these motions have hitherto failed. Yet they have not remained wholly fruitless. The community of slow movement in Taurus, upon which Maedler based his famous theory, has proved to be a fact, and one of very extended significance.

In 1870 Mr. Proctor undertook to chart down the directions and proportionate amounts of about 1,600 proper motions, as determined by Messrs. Stone and Main, with the result of bringing to light the remarkable phenomenon termed by him "star-drift."[1627] Quite unmistakably, large groups of stars, otherwise apparently disconnected, were seen to be in progress together, in the same direction and at the same rate, across the sky. An example of this kind of unanimity was alleged by him in the five intermediate stars of the Plough; and that the agreement in thwartwise motion is no casual one is practically demonstrated by the concordant radial velocities determined at Potsdam for four out of the five objects in question. All of these approach the earth at the rate of about eighteen miles a second; and the fifth and faintest, Delta Ursae, though not yet measured, may be held to share their advance. One of them, moreover, Zeta Ursae, alias Mizar, carries with it three other stars—Alcor, the Arab "Rider" of the horse, visible to the naked eye, besides a telescopic and a spectroscopic attendant. So that the group may be regarded as octuple. It is of vast compass. Dr. Hoeffler assigned to it in 1897[1628]—although on grounds more or less hypothetical—a mean parallax corresponding to a light-journey of 192 years, which would give to the marching squadron a total extent of at least fourteen times the distance from the sun to Alpha Centauri, while implying for its brightest member—Eta Ursae Majoris—the lustre of six hundred suns. The organising principle of this grand scheme must long remain mysterious.

It is no solitary example. Particular association, indeed—as was surmised by Michell far back in the eighteenth century—appears to be the rule rather than an exception in the sidereal system. Stars are bound together by twos, by threes, by dozens, by hundreds. Our own sun is, perhaps, not exempt from this gregarious tendency. Yet the search for its companions has, up to the present, been unavailing. Gould's cluster[1629] seems remote and intangible; Kapteyn's collection of solar stars proved to have been a creation of erroneous data, and was abolished by his unrelenting industry. Rather, we appear to have secured a compartment to ourselves for our long journey through space. A practical certainty has, at any rate, been gained that whatever aggregation holds the sun as a constituent is of a far looser build than the Pleiades or Praesepe. Of all such majestic communities the laws and revolutions remain, as yet, inaccessible to inquiry; centuries may elapse before even a rudimentary acquaintance with them begins to develop; while the economy of the higher order of association, which we must reasonably believe that they unite to compose, will possibly continue to stimulate and baffle human curiosity to the end of time.

FOOTNOTES:

[Footnote 1369: Report Brit. Assoc., 1868, p. 166. Rutherfurd gave a rudimentary sketch of a classification of the kind in December, 1862, but based on imperfect observation. See Am. Jour. of Sc., vol. xxxv., p. 77.]

[Footnote 1370: Publicationem, Potsdam, No. 14, 1884, p. 31.]

[Footnote 1371: Von Konkoly once derived from a slow-moving meteor a hydro-carbon spectrum. A. S. Herschel, Nature, vol. xxiv., p. 507.]

[Footnote 1372: Phil. Trans., vol. cliv., p. 429.]

[Footnote 1373: Am. Jour. of Sc., vol. xix., p. 467.]

[Footnote 1374: Photom. Unters., p. 243.]

[Footnote 1375: Spectre Solaire, p. 38.]

[Footnote 1376: Mr. J. Birmingham, in the Introduction to his Catalogue of Red Stars, adduced sundry instances of colour-change in a direction the opposite to that assumed by Zoellner to be the inevitable result of time. Trans. R. Irish Acad., vol. xxvi., p. 251. A learned discussion by Dr. T. J. J. See, moreover, enforces the belief that Sirius was absolutely red eighteen hundred years ago. Astr. and Astroph., vol. xi., p. 269.]

[Footnote 1377: Phil. Trans., vol. clxiv., p. 492.]

[Footnote 1378: Astr. Nach., No. 2,000.]

[Footnote 1379: Proc. Roy. Soc., vols. xvi., p. 31; xvii., p. 48.]

[Footnote 1380: Annalen der Physik, Bd. xx., p. 155.]

[Footnote 1381: Ibid., p. 153.]

[Footnote 1382: Knowledge, vol. xiv., p. 101.]

[Footnote 1383: Meteoritic Hypothesis, p. 380.]

[Footnote 1384: Phil. Trans., vol. cxci. A., p. 128; Spectra of Southern Stars, p. 3.]

[Footnote 1385: See the author's System of the Stars, p. 84.]

[Footnote 1386: A designation applied by Sir Norman Lockyer to third-type stars.]

[Footnote 1387: See ante, p. 198.]

[Footnote 1388: Bothkamp Beobachtungen, Heft ii., p. 146.]

[Footnote 1389: Astr. Nach., No. 2,539.]

[Footnote 1390: Ibid., No. 2,548; Observatory, vol. vi., p. 332.]

[Footnote 1391: Month. Not., vol. xlvii., p. 92.]

[Footnote 1392: Publ. Astr. Pac. Soc., vol. i., p. 80; Observatory, vol. xiii., p. 46.]

[Footnote 1393: Lockyer, Proc. Roy. Soc., vol. lvii., p. 173.]

[Footnote 1394: Astr. Nach., No. 3,129.]

[Footnote 1395: Month. Not., vol. lix., p. 505.]

[Footnote 1396: Astr. Nach., No. 2,581.]

[Footnote 1397: Ibid., Nos. 2,651-2.]

[Footnote 1398: Ibid., No. 3,051; Astr. and Astrophysics, vol. xi., p. 25; Belopolsky, Astr. Nach., No. 3,129.]

[Footnote 1399: Comptes Rendus, t. lxv., p. 292.]

[Footnote 1400: Copernicus, vol. iii., p. 207.]

[Footnote 1401: System of the Stars, p. 70; Harvard Annals, vol. xxviii., pt. ii., p. 243 (Miss Cannon).]

[Footnote 1402: Potsdam Publ., No. 14, p. 17.]

[Footnote 1403: Proc. Roy. Soc., vol. xlix., p. 33.]

[Footnote 1404: Miss A. J. Cannon, Harvard Annals, vol. xxviii., pt. ii., p. 141.]

[Footnote 1405: Astr. and Astroph., vol. xiii., p. 448.]

[Footnote 1406: Potsdam Publ., No. 2.]

[Footnote 1407: The results of Von Konkoly's extension of Vogel's work to 15 deg. of south declination were published in O Gyalla Beobachtungen, Bd. viii., Th. ii., 1887.]

[Footnote 1408: Astroph. Jour., vols. viii., p. 237; ix., p. 271.]

[Footnote 1409: Ibid., vol. ix., p. 119.]

[Footnote 1410: Phil. Trans., vol. cliv., p. 413. Some preliminary results were embodied in a "note" communicated to the Royal Society, February 19, 1863 (Proc. Roy. Soc., vol. xii., p. 444).]

[Footnote 1411: Bothkamp Beob., Heft i., p. 25.]

[Footnote 1412: Astroph. Jour., vol. vi., p. 423.]

[Footnote 1413: Phil. Trans., vol. cliv., p. 429, note.]

[Footnote 1414: Month. Not., vol. xxiii., p. 180.]

[Footnote 1415: Proc. Roy. Soc., vol. xxv., p. 446.]

[Footnote 1416: Phil. Trans., vol. clxxi., p. 669; Atlas of Stellar Spectra, p. 22.]

[Footnote 1417: Astr. Nach., No. 2,301; Monatsb., Berlin, 1879, p. 119; 1880, p. 192.]

[Footnote 1418: Jour. de Physique, t. v., p. 98.]

[Footnote 1419: System of the Stars, p. 39.]

[Footnote 1420: See ante, p. 198.]

[Footnote 1421: Proc. Roy. Soc., vol. xlviii., p. 314.]

[Footnote 1422: Harvard Circulars, Nos. 12, 18; Astroph. Jour., vol. v., p. 92.]

[Footnote 1423: Astroph. Jour., vol. vi., p. 233.]

[Footnote 1424: McClean, Phil. Trans., vol. cxci. A., p. 129.]

[Footnote 1425: Proc. Roy. Soc., vol. lxii., p. 417.]

[Footnote 1426: Ibid., April 27, 1899; Astroph. Jour., vol. x., p. 272.]

[Footnote 1427: Astr. Nach., No. 3,565.]

[Footnote 1428: Ibid., No. 3,583.]

[Footnote 1429: Lunt, Astroph. Jour., vol. xi., p. 262; Proc. Roy. Soc., vol. lxvi., p. 44; Lockyer, ibid., November 23, 1899; Nature, vol. lxi., p. 263.]

[Footnote 1430: Die Spectralanalyse, p. 314.]

[Footnote 1431: Henry Draper Memorial, First Ann. Report, 1887.]

[Footnote 1432: Mem. Amer. Acad., vol. xi., p. 215.]

[Footnote 1433: Harvard Annals, vol. xxvii.]

[Footnote 1434: Harvard Annals, vol. xxviii., parts i. and ii.]

[Footnote 1435: See ante, p. 201.]

[Footnote 1436: Phil. Trans., vol. clviii., p. 529.]

[Footnote 1437: Schellen, Die Spectralanalyse, Bd. ii., p. 326 (ed. 1883).]

[Footnote 1438: Proc. Roy. Soc., vol. xx., p. 386.]

[Footnote 1439: System of the Stars, p. 199.]

[Footnote 1440: Pickering, Am. Jour. of Sc., vol. xxxix., p. 46; Vogel, Astr. Nach. No. 3,017.]

[Footnote 1441: Sitzungsberichte, Berlin, May 2, 1901; Astroph. Jour., vol. xiii., p. 324.]

[Footnote 1442: The "relative orbit" of a double star is that described by one round the other as a fixed point. Micrometrical measures are always thus executed. But in reality both stars move in opposite directions, and at rates inversely as their masses round their common centre of gravity.]

[Footnote 1443: Vogel, Astr. Nach., Nos. 3,017, 3,039.]

[Footnote 1444: Huggins, Pres. Address, 1891; Cornu, Sur la Methode Doppler-Fizeau p. D. 38.]

[Footnote 1445: Sitzungsb., Berlin, 1890, p. 401; Astr. Nach., No. 2,995.]

[Footnote 1446: Ibid.]

[Footnote 1447: Astroph. Jour., vol. v., p. 1; Newall, Month. Not., vol. lvii., p. 575.]

[Footnote 1448: Bull. de l'Acad. de St. Petersb., tt. vi., viii.]

[Footnote 1449: Astroph. Jour., vol. x., p. 177; Month. Not., vol. lx., p. 418; Vogel, Sitzungsb., Berlin, April 19, 1900.]

[Footnote 1450: Month. Not., vol. lx., p. 595.]

[Footnote 1451: Hussey, Astr. Jour., No. 484.]

[Footnote 1452: Astroph. Jour., vols. x, p. 180; xiv., p. 140; Lick Bulletin, No. 4; Belopolsky, Astr. Nach., No. 3,637.]

[Footnote 1453: The significance of the name "El Ghoul" leaves little doubt that the Arab astronomers took note of this star's variability. E. M. Clerke, Observatory, vol. xv., p. 271.]

[Footnote 1454: Phil. Trans., vol. lxxiii., p. 484.]

[Footnote 1455: Proc. Amer. Acad., vol. xvi., p. 17; Observatory, vol. iv., p. 116. For a preliminary essay by T. S. Aldis, see Phil. Mag., vol. xxxix., p. 363, 1870.]

[Footnote 1456: Astr. Nach., No. 2,947.]

[Footnote 1457: Astr. Jour., Nos. 165-6, 255-6, 509. See also Knowledge, vol. xv., p. 186.]

[Footnote 1458: Bauschinger, V. J. S. Astr. Ges., Jahrg. xxix.; but cf. Searle, Harvard Annals, vol. xxix., p. 223; Boss, Astr. Jour., No. 343.]

[Footnote 1459: Comptes Rendus, t. cxx., p. 125.]

[Footnote 1460: Myers, Astroph. Jour., vol. vii., p. 1; A. W. Roberts, Ibid., vol. xiii., p. 181.]

[Footnote 1461: Proc. R. Irish Ac., July, 1884.]

[Footnote 1462: Ibid., vol. i., p. 97.]

[Footnote 1463: Astr. Jour., Nos. 179, 180.]

[Footnote 1464: Ibid., Nos. 300, 379.]

[Footnote 1465: Astr. Jour., Nos. 491-2.]

[Footnote 1466: System of the Stars, p. 125.]

[Footnote 1467: Proc. Roy. Soc., vol. xv., p. 146.]

[Footnote 1468: Weiss, Astr. Nach., No. 1,590; Espin, Ibid., No. 3,200.]

[Footnote 1469: Comptes Rendus, t. lxxxiii., p. 1172.]

[Footnote 1470: Monatsb., Berlin, 1877, pp. 241, 826.]

[Footnote 1471: Copernicus, vol. ii., p. 101.]

[Footnote 1472: Burnham, Month. Not., vol. lii., p. 457.]

[Footnote 1473: Astr. Nach., No. 2,682.]

[Footnote 1474: A. Hall, Am. Jour. of Sc., vol. xxxi., p. 301.]

[Footnote 1475: Young, Sid. Messenger, vol. iv., p. 282; Hasselberg, Astr. Nach., No. 2,690.]

[Footnote 1476: Report Brit. Assoc., 1885, p. 935.]

[Footnote 1477: Month. Not., vol. xlvii., p. 54.]

[Footnote 1478: Nature, vol. xxxii., p. 522.]

[Footnote 1479: Astr. Nach., Nos. 1,267, 2,715.]

[Footnote 1480: Month. Not., vol. xxi., p. 32.]

[Footnote 1481: Observatory, vol. viii., p. 335.]

[Footnote 1482: Astr. Nach., No. 3,118; Astr. and Astroph., vol. xi., p. 907.]

[Footnote 1483: Cape Results, p. 137.]

[Footnote 1484: Trans. R. Soc. of Edinburgh, vol. xxvii., p. 51; Astr. and Astroph., August, 1892, p. 593.]

[Footnote 1485: Vogel, Astr. Nach., No. 3,079.]

[Footnote 1486: Observatory, vol. xv., p. 287; Seeliger, Astr. Nach., No. 3,118; Astr. and Astroph., vol. xi., p. 906.]

[Footnote 1487: Ranyard, Knowledge, vol. xv., p. 110.]

[Footnote 1488: Proc. Roy. Soc., vol. li., p. 492.]

[Footnote 1489: Burnham, Month. Not., vol. liii., p. 58.]

[Footnote 1490: Astr. Nach., Nos. 3,118, 3,143.]

[Footnote 1491: Renz, Ibid., Nos. 3,119, 3,238; Huggins, Astr. and Astroph., vol. xiii., p. 314.]

[Footnote 1492: Astr. Nach., No. 3,111.]

[Footnote 1493: Belopolsky, Astr. Nach., No. 3,120.]

[Footnote 1494: Nature, September 15, 1892.]

[Footnote 1495: Astr. Nach., Nos. 3,122, 3,129.]

[Footnote 1496: Ibid., No. 3,133; Astr. and Astroph., vol. xi., p. 715.]

[Footnote 1497: Publ. Astr. Pac. Soc., vol. iv., p. 244.]

[Footnote 1498: Barnard, Astroph. Jour., vol. xiv., p. 152; Campbell, Observatory, vol. xxiv., p. 360.]

[Footnote 1499: Pop. Astr., March, 1895, p. 307.]

[Footnote 1500: Harvard Circular, No. 4, December 20, 1895. The first Nova Persei was spectrographically recorded in 1887.]

[Footnote 1501: Vogel, Sitzungsb., Berlin, April 19, 1900, p. 389.]

[Footnote 1502: Sidgreaves, Observatory, vol. xxiv., p. 191.]

[Footnote 1503: Ibid., Knowledge, vol. xxv., p. 10.]

[Footnote 1504: Lick Bulletin, No. 8.]

[Footnote 1505: Astr. Nach., No. 3,736.]

[Footnote 1506: Astroph. Jour., vol. xiv., p. 167.]

[Footnote 1507: Lick Bulletin, No. 10.]

[Footnote 1508: Astroph. Jour., vols. xiv., p. 293; xv., p. 129.]

[Footnote 1509: Cf. the theories on the subject of M. Wolf, Astr. Nach., Nos. 3,752, 3,753; Kapteyn, Ibid., No. 3,756; F. W. Very, Ibid., No. 3,771; and W. E. Wilson, Proc. Roy. Dublin Soc., No. 45, p. 556.]

[Footnote 1510: Phil. Trans., vol. cliv., p. 437.]

[Footnote 1511: Phil. Trans., vol. clviii., p. 540. The true proportion seems to be about one-tenth (Harvard Annals, vol. xxvi., pt. ii., p. 205), the Tulse Hill working-list having been formed of specially selected objects.]

[Footnote 1512: Scheiner, Astr. Nach., No. 3,476; Astroph. Jour., vol. vii., p. 231; Campbell, Ibid., vols. ix., p. 312; x., p. 22.]

[Footnote 1513: Proc. Roy. Soc., vols. xlvi., p. 40; xlviii., p. 202.]

[Footnote 1514: Publ. Astr. Pac. Soc., vol. ii., p. 265; Proc. Roy. Soc., vol. xlix., p. 399.]

[Footnote 1515: Astr. Nach., No 3,549.]

[Footnote 1516: Atlas of Stellar Spectra, p. 125.]

[Footnote 1517: Knowledge, vol. xix., p. 39.]

[Footnote 1518: Astr. Nach., Nos. 1,366, 1,391, 1,689; Chambers, Descriptive Astr. (3rd ed.), p. 543; Flammarion, L'Univers Sideral, p. 818.]

[Footnote 1519: Month. Not., vol. li., p. 94.]

[Footnote 1520: Ibid., vol. lix., p. 372.]

[Footnote 1521: Ibid., vol. lx., p. 424.]

[Footnote 1522: Dreyer, Ibid., vol. lii., p. 100.]

[Footnote 1523: Wash. Obs., vol. xxv., App. 1.]

[Footnote 1524: Am. Jour. of Sc., vol. xiv., p. 433; C. Dreyer, Month. Not., vol. xlvii., p. 419.]

[Footnote 1525: Ibid., vol. li., p. 496.]

[Footnote 1526: Reproduced in Knowledge, April, 1893.]

[Footnote 1527: Unless an exception be found in the Pleiades nebulae, which may be assumed to share the small apparent movement of the stars they adhere to.]

[Footnote 1528: Abhandl. Akad. der Wiss., Leipzig, 1857, Bd. iii., p. 295.]

[Footnote 1529: Month. Not., vol. lii., p. 31.]

[Footnote 1530: Proc. Roy. Soc., 1874, p. 251.]

[Footnote 1531: Publ. Astr. Pac. Soc., vol. ii., p. 278.]

[Footnote 1532: System of the Stars, p. 257.]

[Footnote 1533: Proc. Roy. Soc., vol. xlix., p. 399.]

[Footnote 1534: Potsdam Publ., Bd. vii., Th. i.]

[Footnote 1535: Astr. Nach., No. 2,714; Schoenfeld, V. J. S. Astr. Ges., Jahrg. xxi., p. 58.]

[Footnote 1536: Astroph. Journ., vol. xiii., p. 80.]

[Footnote 1537: Proc. Roy. Soc., vol. xxxiii., p. 425; Report Brit. Assoc., 1882, p. 444. An impression of the four lower lines in the same spectrum was almost simultaneously obtained by Dr. Draper. Comptes Rendus, t. xciv., p. 1243.]

[Footnote 1538: Proc. Roy. Soc., vol. xlviii., p. 213.]

[Footnote 1539: Month. Not., vol. xlviii., p. 360.]

[Footnote 1540: Proc. Roy. Soc., vol. xlvi., p. 40; System of the Stars, p. 79.]

[Footnote 1541: Sitzungsb., Berlin, February 13, 1890.]

[Footnote 1542: Wash. Obs., vol. xxv., App. i., p. 226.]

[Footnote 1543: Comptes Rendus, t. xcii., p. 261.]

[Footnote 1544: Month. Not., vol. xliii., p. 255.]

[Footnote 1545: Harvard Annals, vol. xviii., p. 116.]

[Footnote 1546: Sid. Mess., vol. ix., p. 1.]

[Footnote 1547: Knowledge, vol. xv., p. 191.]

[Footnote 1548: Month. Not., vol. xlix., p. 65.]

[Footnote 1549: System of the Stars, p. 269.]

[Footnote 1550: Astr. Nach., Nos. 2,749, 2,754.]

[Footnote 1551: Vogel, Astr. Nach., 2,854.]

[Footnote 1552: Nature, vol. xliii., p. 419.]

[Footnote 1553: L'Astronomie, t. xl., p. 171.]

[Footnote 1554: Astr. Nach., Baende xlvii., p. 1; xlviii., p. 1; xlix., p. 81. Pickering, Mem. Am. Ac., vol. xi., p. 180.]

[Footnote 1555: Gould on Celestial Photography, Observatory, vol. ii., p. 16.]

[Footnote 1556: Annals N. Y. Acad. of Sciences, vol. vi., p. 239, 1892; Elkin, Publ. Astr. Pac. Soc., vol. iv., p. 134.]

[Footnote 1557: Trans. Yale Observatory, vol. i., pt. i.]

[Footnote 1558: Astroph. Jour., vol. xiii., p. 56.]

[Footnote 1559: Astr. Nach., No. 2,719.]

[Footnote 1560: Ibid., No. 2,726.]

[Footnote 1561: Ibid., No. 2,730.]

[Footnote 1562: Month. Not., vol. xlvii., p. 24.]

[Footnote 1563: Les Mondes, t. iii., p. 529.]

[Footnote 1564: Mouchez, Comptes Rendus, t. cvi., p. 912.]

[Footnote 1565: Astr. Nach., No. 3,422.]

[Footnote 1566: Ibid., No. 3,441.]

[Footnote 1567: Ibid., Nos. 3,018, 3,032.]

[Footnote 1568: Journ. Brit. Astr. Assoc., vol. ix., p. 133.]

[Footnote 1569: Astr. Nach., No. 3,253.]

[Footnote 1570: Observatory, vol. xxi., pp. 351, 386.]

[Footnote 1571: Reproduced in Astroph. Journ., vol. xi., p. 324.]

[Footnote 1572: Ibid., p. 347.]

[Footnote 1573: Astr. Nach., No. 3,704.]

[Footnote 1574: Sitzungsb. Bayer. Akad., March 23, 1901.]

[Footnote 1575: Annals of the Cape Observatory, vols. iii., iv., v.]

[Footnote 1576: Month. Not., vol. lx., p. 381.]

[Footnote 1577: D. Klumpke, Observatory, vol. xv., p. 305.]

[Footnote 1578: Gilbert, Sid. Mess., vol. i., p. 288.]

[Footnote 1579: Astr. Papers for the Amer. Ephemeris, vol. viii., pt. ii.]

[Footnote 1580: Nature, vol. xxiv., p. 91; Dunsink Observations, pt. v., 1884.]

[Footnote 1581: Elkin, Report for 1891-92, p. 25; Newcomb, The Stars, p. 151.]

[Footnote 1582: Annals of the Cape Observatory, vol. viii., pt. ii. Some of the measures were made by Messrs. Finlay and de Sitter.]

[Footnote 1583: Astr. Nach., No. 3,483; Observatory, vol. xxi., p. 180.]

[Footnote 1584: Annalen der Sternwarte in Leiden, Bd. vii.]

[Footnote 1585: Report of Harvard Conference in 1898 (Snyder).]

[Footnote 1586: Researches in Stellar Parallax, pt. ii., 1892.]

[Footnote 1587: V. J. S. Astr. Ges., Jahrg., xxviii., p. 117.]

[Footnote 1588: Bulletin de la Carte du Ciel, No. 1, p. 262.]

[Footnote 1589: Publ. of the Astr. Laboratory at Groningen, No. 1.]

[Footnote 1590: Nature, vol. xxvi., p. 177.]

[Footnote 1591: Proc. R. Irish Acad., vol. i., p. 571, ser. iii.]

[Footnote 1592: Mem. R. A. S., vol. xlvii., p. 178.]

[Footnote 1593: Astr. Nach., No. 3,142.]

[Footnote 1594: Publ. Astr. Pac. Soc., No. 76.]

[Footnote 1595: Campbell, Lick Bulletin, No. 4.]

[Footnote 1596: Publ. Yerkes Observatory, vol. i., 1900.]

[Footnote 1597: Annals Cape Observatory, vol. ii., pt. ii.]

[Footnote 1598: Astr. Jour., Nos. 431-2.]

[Footnote 1599: W. J. Hussey, Publ. Astr. Pac. Soc., No. 74.]

[Footnote 1600: Astr. Jour., No. 258.]

[Footnote 1601: Sitzungsberichte, Berlin, October 26, 1893.]

[Footnote 1602: Annales de l'Obs. de Nice, t. ii.]

[Footnote 1603: Washington Observations, 1888, App. i.]

[Footnote 1604: Publ. Lick Observatory, vol. v., 1901.]

[Footnote 1605: T. Lewis, Observatory, vol. xvi., p. 312.]

[Footnote 1606: Harvard Annals, vol. xiv., pt. i., 1884.]

[Footnote 1607: Observatory, vol. viii., p. 309.]

[Footnote 1608: Month. Not., vol. xlvi., p. 277.]

[Footnote 1609: Harvard Annals, vol. xxxiv.]

[Footnote 1610: Ibid., vol. xlv.]

[Footnote 1611: Carte Phot. du Ciel. Reunion du Comite Permanent, Paris, 1891, p. 100.]

[Footnote 1612: Essays (2nd ser.), The Nebular Hypothesis.]

[Footnote 1613: On the Plurality of Worlds, p. 214 (2nd ed.).]

[Footnote 1614: Proctor, Month. Not., vol. xxix., p. 342.]

[Footnote 1615: This remark was first made by J. Michell, Phil. Trans., vol. lvii., p. 25 (1767).]

[Footnote 1616: Pop. Astr., No. 45.]

[Footnote 1617: Astroph. Jour., vol. i., p. 220.]

[Footnote 1618: Month. Not., vols. xxxi., p. 175; xxxii., p. 1.]

[Footnote 1619: The Stars, p. 273.]

[Footnote 1620: System of the Stars, p. 384; Old and New Astronomy, p. 749 (Ranyard).]

[Footnote 1621: Astroph. Jour., vol. xii., p. 156.]

[Footnote 1622: Publ. Astr. Pac. Soc., vol. ii., p. 242.]

[Footnote 1623: Month. Not., vol. li., pp. 40, 97. For reproductions of some of the photographs in question, see Knowledge, vol. xiv., p. 50.]

[Footnote 1624: Astr. Nach., No. 3,048; Observatory, vol. xiv., p. 301.]

[Footnote 1625: Proc. Roy. Inst., May 29, 1891 (Gill).]

[Footnote 1626: Annals Cape Obs., iii., Introduction, p. 22.]

[Footnote 1627: Proc. Roy. Soc., vol. xviii., p. 169.]

[Footnote 1628: Astr. Nach., No. 3,456; Observatory, vol. xxi., p. 65; Newcomb, The Stars, p. 80.]

[Footnote 1629: Month. Not., vol. xl., p. 249.]



CHAPTER XIII

METHODS OF RESEARCH

Comparing the methods now available for astronomical inquiries with those in use forty years ago, we are at once struck with the fact that they have multiplied. The telescope has been supplemented by the spectroscope and the photographic camera. Now, this really involves a whole world of change. It means that astronomy has left the place where she dwelt apart in rapt union with mathematics, indifferent to all things on earth save only to those mechanical improvements which should aid her to penetrate further into the heavens, and has descended into the forum of human knowledge, at once a suppliant and a patron, alternately invoking help from and promising it to each of the sciences, and patiently waiting upon the advances of all. The science of the heavenly bodies has, in a word, become a branch of terrestrial physics, or rather a higher kind of integration of all their results. It has, however, this leading peculiarity, that the materials for the whole of its inquiries are telescopically furnished. They are such as come very imperfectly, or not at all, within the cognisance of the unarmed eye.

Spectroscopic and photographic apparatus are simply additions to the telescope. They do not supersede or render it of less importance. On the contrary, the efficacy of their action depends primarily upon the optical qualities of the instruments they are attached to. Hence the development, to their fullest extent, of the powers of the telescope is of vital moment to the progress of modern physical astronomy, while the older mathematical astronomy could afford to remain comparatively indifferent to it.

The colossal Rosse reflector still marks, as to size, the ne plus ultra of performance in that line. A mirror four feet in diameter was, however, sent out to Melbourne by the late Thomas Grubb of Dublin in 1870. This is mounted in the Cassegrainian manner, so that the observer looks straight through it towards the object viewed, of which he really sees a twice-reflected image. The dust-laden atmosphere of Melbourne is said to impede very seriously the usefulness of this originally fine instrument.

It may be doubted whether so large a spectrum will ever again be constructed. A new material for the mirrors of reflecting telescopes, proposed by Steinheil in 1856, and independently by Foucault in 1857,[1630] has in a great measure superseded the use of a metallic alloy. This is glass upon which a thin film of silver has been deposited by a chemical process originally invented by Liebig. It gives a peculiarly brilliant reflective surface, throwing back more light than a metallic mirror of the same area, in the proportion of about sixteen to nine. Resilvering, too, involves much less risk and trouble than repolishing a speculum. The first use of this plan on a large scale was in an instrument of thirty-six inches aperture, finished by Calver for Dr. Common in 1879. To its excellent qualities turned to account with rare skill, his triumphs in celestial photography are mainly due. A more daring experiment was the construction and mounting, by Dr. Common himself, of a 5-foot reflector. But the first glass disc ordered from France for the purpose proved radically defective. When figured, polished, and silvered, towards the close of 1888, it gave elliptical instead of circular star-images.[1631] A new one had to be procured, and was ready for astronomical use in 1891. The satisfactory nature of its performance is vouched for by the observations made with it upon Jupiter's new satellite in December, 1892. This instrument, to which a Newtonian form has been given, had no rival in respect of light-concentration at the time when it was built. It has now two—the Paris 50-inch refractor and the Yerkes 5-foot reflector.

It is, however, in the construction of refracting telescopes that the most conspicuous advances have recently been made. The Harvard College 15-inch achromatic was mounted and ready for work in June, 1847. A similar instrument had already for some years been in its place at Pulkowa. It was long before the possibility of surpassing these masterpieces of German skill presented itself to any optician. For fifteen years it seemed as if a line had been drawn just there. It was first transgressed in America. A portrait-painter of Cambridgeport, Massachusetts, named Alvan Clark, had for some time amused his leisure with grinding lenses, the singular excellence of which was discovered in England by Mr. Dawes in 1853.[1632] Seven years passed, and then an order came from the University of Mississippi for an object-glass of the unexampled size of eighteen inches. An experimental glance through it to test its definition resulted, as we have seen, in the detection of the companion of Sirius, January 31, 1862. It never reached its destination in the South. War troubles supervened, and it was eventually sent to Chicago, where it served Professor Hough in his investigations of Jupiter, and Mr. Burnham in his scrutiny of double stars.

The next step was an even longer one, and it was again taken by a self-taught optician, Thomas Cooke, the son of a shoemaker at Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead ordered from him in 1863 a 25-inch object-glass. It was finished early in 1868, but at the cost of shortening the life of its maker, who died October 19, 1868, before the giant refractor he had toiled at for five years was completely mounted. This instrument, the fine qualities of which had long been neutralized by an unfavourable situation, was presented by Mr. Newall to the University of Cambridge, a few weeks before his death, April 21, 1889. Under the care of his son, Mr. Frank Newall, it has proved highly efficient in the delicate work of measuring stellar radial motions.

Close upon its construction followed that of the Washington 26-inch, for which twenty thousand dollars were paid to Alvan Clark. The most illustrious point in its career, entered upon in 1873, has been the discovery of the satellites of Mars. Once known to be there, these were, indeed, found to be perceptible with very moderate optical means (Mr. Wentworth Erck saw Deimos with a 7-inch Clark); but the first detection of such minute objects is a feat of a very different order from their subsequent observation. Dr. See's perception with this instrument, in 1899, of Neptune's cloud-belts, and his refined series of micrometrical measures of the various planets, attest the unimpaired excellence of its optical qualities.

It held the primacy for more than eight years. Then, in December, 1880, the place of honour had to be yielded to a 27-inch achromatic, built by Howard Grubb (son and successor of Thomas Grubb) for the Vienna Observatory. This, in its turn, was surpassed by two of respectively 29-1/2 and 30 inches, sent by Gautier of Paris to Nice, and by Alvan Clark to Pulkowa; and an object-glass, three feet in diameter, was in 1886 successfully turned out by the latter firm for the Lick Observatory in California. The difficulties, however, encountered in procuring discs of glass of the size and purity required for this last venture seemed to indicate that a term to progress in this direction was not far off. The flint was, indeed, cast with comparative ease in the workshops of M. Feil at Paris. The flawless mass weighed 170 kilogrammes, was over 38 inches across, and cost 10,000 dollars. But with the crown part of the designed achromatic combination things went less smoothly. The production of a perfect disc was only achieved after nineteen failures, involving a delay of more than two years; and the glass for a third lens, designed to render the telescope available at pleasure for photographic purposes, proved to be strained, and consequently went to pieces in the process of grinding. It has been replaced by one of 33 inches, with which a series of admirable lunar and other photographs have been taken.

Nor is the difficulty in obtaining suitable material the only obstacle to increasing the size of refractors. The "secondary spectrum," as it is called, also interposes a barrier troublesome to surmount. True achromatism cannot be obtained with ordinary flint and crown-glass; and although in lenses of "Jena glass," outstanding colour is reduced to about one-sixth its usual amount, their term of service is fatally abridged by rapid deterioration. Nevertheless, a 13-inch objective of the new variety was mounted at Koenigsberg in 1898; and discs of Jena crown and flint, 23 inches across, were purchased by Brashear at the Chicago Exhibition of 1893. An achromatic combination of three kinds of glass, devised by Mr. A. Taylor[1633] for Messrs. Cooke of York, has less serious drawbacks, but has not yet come into extensive use. Meanwhile, in giant telescopes affected to the full extent by chromatic aberration, such as the Lick and Yerkes refractors, the differences of focal length for the various colours are counted by inches,[1634] and this not through any lack of skill in the makers, but by the necessity of the case. Embarrassing consequences follow. Only a small part of the spectrum of a heavenly body, for instance, can be distinctly seen at one time; and a focal adjustment of half an inch is required in passing from the observation of a planetary nebula to that of its stellar nucleus. A refracting telescope loses, besides, one of its chief advantages over a reflector when its size is increased beyond a certain limit. That advantage is the greater luminosity of the images given by it. Considerably more light is transmitted through a glass lens than is reflected from an equal metallic surface. But only so long as both are of moderate dimensions. For the glass necessarily grows in thickness as its area augments, and consequently stops a larger percentage of the rays it refracts. So that a point at length arrives—fixed by the late Dr. Robinson at a diameter a little short of 3 feet[1635]—where the glass and the metal are, in this respect, on an equality; while above it the metal has the advantage. And since silvered glass gives back considerably more light than speculum metal, the stage of equalisation with lenses is reached proportionately sooner where this material is employed.[1636]

The most distinctive faculty of reflectors, however, is that of bringing rays of all refrangibilities to a focus together. They are naturally achromatic. None of the beams they collect are thrown away in colour-fringes, obnoxious both in themselves and as a waste of the chief object of astrophysicists' greed—light. Reflectors, then, are in this respect specially adapted to photographic and spectrographic use. But they have a countervailing drawback. The penalties imposed by bigness are for them peculiarly heavy. Perfect definition becomes with increasing size, more and more difficult to attain; once attained, it becomes more and more difficult to keep. For the huge masses of material employed to form great object-glasses or mirrors tend with every movement to become deformed by their own weight. Now, the slightest bending of a mirror is fatal to its performance, the effect being doubled by reflection; while in a lens alteration of figure is compensated by the equal and contrary flexures of the opposing surfaces, so that the emergent beams pursue much the same paths as if the curves of the refracting medium had remained theoretically perfect. For this reason work of precision must remain the province of refracting telescopes, although great reflectors retain the primacy in the portraiture of the heavenly bodies, as well as in certain branches of spectroscopy. Professor Hale, accordingly, summarised a valuable discussion on the subject by asserting[1637] "that the astrophysicist may properly consider the reflector to be an even more important part of his instrumental equipment than the refractor." A new era in its employment west of the Atlantic opened with the transfer from Halifax to Mount Hamilton of the Crossley reflector. Its prerogatives in nebular photography were splendidly indicated in 1899 by Professor Keeler's exquisite and searching portrayals of the cloud-worlds of space, and those obtained two years later, with a similar, though smaller, instrument, by Professor Ritchey of the Yerkes Observatory, were fully comparable with them. The performances of the Yerkes 5-foot reflector still belong to the future.

Ambition as regards telescopic power is by no means yet satisfied. Nor ought it to be. The advance of astrophysical researches of all kinds depends largely upon light-grasp. For the spectroscopic examination of stars, for the measurement of their motions in the line of sight, for the discovery and study of nebulae, for stellar and nebular photography, the cry continually is "more light." There is no enterprising head of an observatory but must feel cramped in his designs if he can command no more than 14 or 15 inches of aperture, and he aspires to greater instrumental capacity, not merely with a view to the chances of discovery, but for the steady prosecution of some legitimate line of inquiry. Thus projects of telescope-building on a large scale are rife, and some obtain realisation year by year. Sir Howard Grubb finished in 1893 a 28-inch achromatic for the Royal Observatory, Greenwich; the Thompson equatoreal, mounted at the same establishment in 1897, carries on a single axis a 26-inch photographic refractor and a 30-inch silvered-glass reflector; the Victoria telescope, inaugurated at the Cape in 1901, comprises a powerful spectrographic apparatus, together with a chemically corrected 24-inch refractor, the whole being the munificent gift of Mr. Frank McClean; at Potsdam, at Meudon, at Paris, at Alleghany, engines for light-concentration have been, or shortly will be, erected of dimensions which, two generations back, would have seemed extravagant and impossible.

Perhaps the finest, though not absolutely the greatest, among them, marked the summit and end of the performances of Alvan G. Clark, the last survivor of the Cambridgeport firm.

In October, 1892, Mr. Yerkes of Chicago offered an unlimited sum for the provision of the University of that city with a "superlative" telescope. And it happened, fortunately, that a pair of glass discs, nearly 42 inches in diameter, and of perfect quality, were ready at hand. They had been cast by Mantois for the University of Southern California, when the erection of a great observatory on Wilson's Peak was under consideration. In the Clark workshop they were combined into a superb objective, brought to perfection by trials and delicate touches extending over nearly five years. Then the maker accompanied it to its destination, by the shore of a far Western Lake Geneva, and died immediately after his return, June 9, 1897. Nor has the implement of celestial research he just lived to complete been allowed to "rust unburnished." Manipulated by Hale, Burnham, and Barnard, it has done work that would have been impracticable with less efficient optical aid. Its construction thus marks a noticeable enlargement of astronomical possibilities, exemplified—to cite one among many performances—by Barnard's success in keeping track of cluster-variables when below the common limit of visual perception.

With the Lick telescope results have also been achieved testifying to its unsurpassed excellence. Holden's and Schaeberle's views of planetary nebulae, Burnham's and Hussey's hair's-breadth star-splitting operations, Keeler's measurements of nebular radial motion, Barnard's detections and prolonged pursuit of faint comets, his discovery of Jupiter's tiny moon, Campbell's spectroscopic determinations—all this could only have been accomplished, even by an exceptionally able and energetic staff, with the aid of an instrument of high power and quality. But there was another condition which should not be overlooked.

The best telescope may be crippled by a bad situation. The larger it is, indeed, the more helpless is it to cope with atmospheric troubles. These are the worst plagues of all those that afflict the astronomer. No mechanical skill avails to neutralise or alleviate them. They augment with each increase of aperture; they grow with the magnifying powers applied. The rays from the heavenly bodies, when they can penetrate the cloud-veils that too often bar their path, reach us in an enfeebled, scattered, and disturbed condition. Hence the twinkling of stars, the "boiling" effects at the edges of sun, moon, and planets; hence distortions of bright, effacements of feeble telescopic images; hence, too, the paucity of the results obtained with many powerful light-gathering machines.

No sooner had the Parsonstown telescope been built than it became obvious that the limit of profitable augmentation of size had, under climatic conditions at all nearly resembling those prevailing there, been reached, if not overpassed; and Lord Rosse himself was foremost to discern the need of pausing to look round the world for a clearer and stiller air than was to be found within the bounds of the United Kingdom. With this express object Mr. Lassell transported his 2-foot Newtonian to Malta in 1852, and mounted there, in 1860, a similar instrument of fourfold capacity, with which, in the course of about two years, 600 new nebulae were discovered. Professor Piazzi Smyth's experiences during a trip to the Peak of Teneriffe in 1856 in search of astronomical opportunities[1638] gave countenance to the most sanguine hopes of deliverance, at suitable elevated stations, from some of the oppressive conditions of low-level star-gazing; yet for a number of years nothing effectual was done for their realisation. Now, at last, however, mountain observatories are not only an admitted necessity but an accomplished fact; and Newton's long forecast of a time when astronomers would be compelled, by the developed powers of their telescopes, to mount high above the "grosser clouds" in order to use them,[1639] had been justified by the event.

James Lick, the millionaire of San Francisco, had already chosen, when he died, October 1, 1876, a site for the new observatory, to the building and endowment of which he had devoted a part of his large fortune. The situation of the establishment is exceptional and splendid. Planted on one of the three peaks of Mount Hamilton, a crowning summit of the Californian Coast Range, at an elevation of 4,200 feet above the sea, in a climate scarce rivalled throughout the world, it commands views both celestial and terrestrial which the lover of nature and astronomy may alike rejoice in. Impediments to observation are there found to be most materially reduced. Professor Holden, who was appointed, in 1885, president of the University of California and director of the new observatory affiliated to it, stated that during six or seven months of the year an unbroken serenity prevails, and that half the remaining nights are clear.[1640] The power of continuous work thus afforded is of itself an inestimable advantage; and the high visual excellences testified to by Mr. Burnham's discovery, during a two months' trip to Mount Hamilton in the autumn of 1879, of forty-two new double stars with a 6-inch achromatic, gave hopes since fully realised of a brilliant future for the Lick establishment. Its advantages are shared, as Professor Holden desired them to be, by the whole astronomical world.[1641] A sort of appellate jurisdiction was at once accorded to the great equatoreal, and more than one disputed point has been satisfactorily settled by recourse to it.

Its performances, considered both as to quality and kind, are unlikely to be improved upon by merely outbidding it in size, unless the care expended upon the selection of its site be imitated. Professor Pickering thus showed his customary prudence in reserving his efforts to procure a great telescope until Harvard College owned a dependent observatory where it could be employed to advantage. This was found by Mr. W. H. Pickering, after many experiments in Colorado, California, and Peru, at Arequipa, on a slope of the Andes, 8,000 feet above the sea-level. Here the post provided for by the "Boyden Fund" was established in 1891, under ideal meteorological conditions. Temperature preserves a "golden mean"; the barometer is almost absolutely steady; the yearly rainfall amounts to no more than three or four inches. No wonder, then, that the "seeing" there is of the extraordinary excellence attested by Mr. Pickering's observations. In the absence of bright moonlight, he tells us,[1642] eleven Pleiades can always be counted; the Andromeda nebula appears to the naked eye conspicuously bright, and larger than the full moon; third magnitude stars have been followed to their disappearance at the true horizon; the zodiacal light spans the heavens as a complete arch, the "Gegenschein" forming a regular part of the scenery of the heavens. Corresponding telescopic facilities are enjoyed. The chief instrument at the station, a 13-inch equatoreal by Clark, shows the fainter parts of the Orion nebula, photographed at Harvard College in 1887, by which the dimensions given to it in Bond's drawing are doubled; stars are at times seen encircled by half a dozen immovable diffraction rings, up to twelve of which have been counted round Alpha Centauri; while on many occasions no available increase of magnifying power availed to bring out any wavering in the limbs of the planets. Moreover, the series of fine nights is nearly unbroken from March to November.

The facilities thus offered for continuous photographic research rendered feasible Professor Bailey's amazing discovery of variable star-clusters. They belong exclusively to the "globular" class, and the peculiarity is most strikingly apparent in the groups known as Omega Centauri, and Messier 3, 5, and 15. A large number of their minute components run through perfectly definite cycles of change in periods usually of a few hours.[1643] Altogether, about 500 "cluster-variables" have been recorded since 1895. It should be mentioned that Mr. David Packer and Dr. Common discerned, about 1890, some premonitory symptoms of light-fluctuation among the crowded stars of Messier 5.[1644] With the Bruce telescope, a photographic doublet 24 inches in diameter, a store of 5,686 negatives was collected at Arequipa between 1896 and 1901. Some were exposed directly, others with the intervention of a prism; and all are available for important purposes of detection or investigation.

Vapours and air-currents do not alone embarrass the use of giant telescopes. Mechanical difficulties also oppose a formidable barrier to much further growth in size. But what seems a barrier often proves to be only a fresh starting-point; and signs are not wanting that it may be found so in this case. It is possible that the monumental domes and huge movable tubes of our present observatories will, in a few decades, be as much things of the past as Huygens's "aerial" telescopes. It is certain that the thin edge of the wedge of innovation has been driven into the old plan of equatoreal mounting.

M. Loewy, the present director of the Paris Observatory, proposed to Delaunay in 1871 the direction of a telescope on a novel system. The design seemed feasible, and was adopted; but the death of Delaunay and the other untoward circumstances of the time interrupted its execution. Its resumption, after some years, was rendered possible by M. Bischoffsheim's gift of 25,000 francs for expenses, and the coude or "bent" equatoreal has been, since 1882, one of the leading instruments at the Paris establishment.

Its principle is briefly this: The telescope is, as it were, its own polar axis. The anterior part of the tube is supported at both ends, and is thus fixed in a direction pointing towards the pole, with only the power of twisting axially. The posterior section is joined on to it at right angles, and presents the object-glass, accordingly, to the celestial equator, in the plane of which it revolves. Stars in any other part of the heavens have their beams reflected upon the object-glass by means of a plane rotating mirror placed in front of it. The observer, meanwhile, is looking steadfastly down the bent tube towards the invisible southern pole. He would naturally see nothing whatever were it not that a second plane mirror is fixed at the "elbow" of the instrument, so as to send the rays which have traversed the object-glass to his eye. He never needs to move from his place. He watches the stars, seated in an arm-chair in a warm room, with as perfect convenience as if he were examining the seeds of a fungus with a microscope. Nor is this a mere gain of personal ease. The abolition of hardship includes a vast accession of power.[1645]

Among other advantages of this method of construction are, first, that of added stability, the motion given to the ordinary equatoreal being transferred, in part, to an auxiliary mirror. Next, that of increased focal length. The fixed part of the tube can be made almost indefinitely long without inconvenience, and with enormous advantage to the optical qualities of a large instrument. Finally, the costly and unmanageable cupola is got rid of, a mere shed serving all purposes of protection required for the "coude."

The desirability of some such change as that which M. Loewy has realised has been felt by others. Professor Pickering sketched, in 1881, a plan for fixing large refractors in a permanently horizontal position, and reflecting into them, by means of a shifting mirror, the objects desired to be observed.[1646] The observations for his photometric catalogues are, in fact, made with a "broken transit," in which the line of sight remains permanently horizontal, whatever the altitude of the star examined. Sir Howard Grubb, moreover, set up, in 1882, a kind of siderostat at the Crawford Observatory, Cork. In a paper read before the Royal Society, January 21, 1884, he proposed to carry out the principle on a more extended scale;[1647] and shortly afterwards undertook its application to a telescope 18 inches in aperture for the Armagh Observatory.[1648] The chief honours, however, remain to the Paris inventor. None of the prognosticated causes of failure have proved effective. The loss of light from the double reflection is insignificant. The menaced deformation of images is, through the exquisite skill of the MM. Henry in producing plane mirrors of all but absolute perfection, quite imperceptible. The definition was admitted to be singularly good. Sir David Gill stated in 1884 that he had never measured a double star so easily as he did Gamma Leonis by its means.[1649] Sir Norman Lockyer pronounced it to be "one of the instruments of the future"; and the principle of its construction was immediately adopted by the directors of the Besancon and Algiers Observatories, as well as for a 17-inch telescope destined for a new observatory at Buenos Ayres. At Paris, it has since been carried out on a larger scale. A "coude," of 23-1/2 inches aperture and 62 feet focal length was in 1890 installed at the National Observatory, and has served M. Loewy for his ingenious studies on refraction and aberration—above all, for taking the magnificent plates of his lunar atlas. The "bent" form is capable of being, but has not yet been, adapted to reflectors.[1650]

The "coelostat," in the form given to it by Professor Turner, has proved an invaluable adjunct to eclipse-equipments. It consists essentially of a mirror rotating in forty-eight hours on an axis in its own plane, and parallel to the earth's axis. In the field of a telescope kept rigidly pointed towards such a mirror, stars appear immovably fixed. The employment of long-focus lenses for coronal photography is thus facilitated, and the size of the image is proportional to the length of the focus. Professor Barnard, accordingly, depicted the totality of 1900 with a horizontal telescope 61-1/2 feet long, fed by a mirror 18 inches across, the diameter of the moon on his plates being 7 inches. The largest siderostat in the world is the Paris 50-inch refractor, which formed the chief attraction of the Palais d'Optique at the Exhibition of 1900. It has a focal length of nearly 200 feet, and can be used either for photographic or for visual purposes.

Celestial photography has not reached its grand climacteric; yet its earliest beginnings already seem centuries behind its present performances. The details of its gradual yet rapid improvement are of too technical a nature to find a place in these pages. Suffice it to say that the "dry-plate" process, with which such wonderful results have been obtained, appears to have been first made available by Sir William Huggins in photographing the spectrum of Vega in 1876, and was then successively adopted by Common, Draper, and Janssen. Nor should Captain Abney's remarkable extension of the powers of the camera be left unnoticed. He began his experiments on the chemical action of red and infra-red rays in 1874, and at length succeeded in obtaining a substance—the "blue" bromide of silver—highly sensitive to these slower vibrations of light. With its aid he explored a vast, unknown, and for ever invisible region of the solar spectrum, presenting to the Royal Society, December 5, 1879,[1651] a detailed map of its infra-red portion (wave-lengths 7,600 to 10,750), from which valuable inferences may yet be derived as to the condition of the various kinds of matter ignited in the solar atmosphere. Upon plates rendered "orthochromatic" by staining with alizarine, or other dye-stuffs, the whole visible spectrum can now be photographed; but those with their maximum of sensitiveness near G are found preferable, except where the results of light-analysis are sought to be completely recorded. And since photographic refractors are corrected for the blue rays, exposures with them of orthochromatic surfaces would be entirely futile.

The chemical plate has two advantages over the human retina:[1652] First, it is sensitive to rays which are utterly powerless to produce any visual effect; next, it can accumulate impression almost indefinitely, while from the retina they fade after one-tenth part of a second, leaving it a continually renewed tabula rasa.

It is, accordingly, quite possible to photograph objects so faint as to be altogether beyond the power of any telescope to reveal—witness the chemical disclosure of the invisible nebula encircling Nova Persei—and we may thus eventually learn whether a blank space in the sky truly represents the end of the stellar universe in that direction, or whether farther and farther worlds roll and shine beyond, veiled in the obscurity of immeasurable distance.

Of many ingenious improvements in spectroscopic appliances the most fundamentally important relate to what are known as "gratings." These are very finely striated surfaces, by which light-waves are brought to interfere, and are thus sifted out, strictly according to their different lengths, into "normal" spectra. Since no universally valid measures can be made in any others, their production is quite indispensable to spectroscopic science. Fraunhofer, who initiated the study of the diffraction spectrum, used a real grating of very fine wires: but rulings on glass were adopted by his successors, and were by Nobert executed with such consummate skill that a single square inch of surface was made to contain 100,000 hand-drawn lines. Such rare and costly triumphs of art, however, found their way into very few hands, and practical availability was first given to this kind of instrument by the inventiveness and mechanical dexterity of two American investigators. Both Rutherfurd's and Rowland's gratings are machine-ruled, and reflect instead of transmitting the rays they analyse; but Rowland's present to them a very much larger diffractive surface, and consequently possess a higher resolving power. The first preliminary to his improvements was the production, in 1882, of a faultless screw, those previously in use having been the inevitable source of periodical errors in striation, giving, in their turn, ghost-lines as subjects of spectroscopic study.[1653] Their abolition was not one of Rowland's least achievements. With his perfected machine a metallic area of 6-1/4 by 4-1/4 inches can be ruled with exquisite accuracy to almost any degree of fineness; he considered, however, 43,000 lines to the inch to be the limit of usefulness.[1654] The ruled surface is, moreover, concave, and hence brings the spectrum to a focus without a telescope. A slit and an eye-piece are alone needed to view it, and absorption of light by glass lenses is obviated—an advantage especially sensible in dealing with the ultra- or infra-visible rays.

The high qualities of Rowland's great photographic map of the solar spectrum were thus based upon his previous improvement of the instrumental means used in its execution. The amount of detail shown in it is illustrated by the appearance on the negatives of 150 lines between H and K; and many lines depict themselves as double which, until examined with a concave grating, had passed for one and indivisible. A corresponding hand-drawing, for which M. Thollon received in 1886 the Lalande Prize, exhibits, not the diffractive, but the prismatic spectrum as obtained with bisulphide of carbon prisms of large dispersive power. About one-third of the visible gamut of the solar radiations (A to b) is covered by it; it includes 3,200 lines, and is over ten metres long.[1655] The grating is an expensive tool in the way of light. Where there is none to spare, its advantages must be foregone. They could not, accordingly, be turned to account in stellar spectroscopy until the Lick telescope was at hand to supply more abundant material for research. By the use thus made possible of Rowland's gratings, Professor Keeler was able to apply enormous dispersion to the rays of stars and nebulae, and so to attain a previously unheard-of degree of accuracy in their measurement. His memorable detection of nebular movement in line of sight ensued as a consequence. Professor Campbell, his successor, has since obtained, by the same means, the first satisfactory photographs of stellar diffraction-spectra.

The means at the disposal of astronomers have not multiplied faster than the tasks imposed upon them. Looking back to the year 1800, we cannot fail to be astonished at the change. The comparatively simple and serene science of the heavenly bodies known to our predecessors, almost perfect so far as it went, incurious of what lay beyond its grasp, has developed into a body of manifold powers and parts, each with its separate mode and means of growth, full of strong vitality, but animated by a restless and unsatisfied spirit, haunted by the sense of problems unsolved, and tormented by conscious impotence to sound the immensities it perpetually confronts.

Knowledge might be said, when the Mecanique Celeste issued from the press, to be bounded by the solar system; but even the solar system presented itself under an aspect strangely different from what it now wears. It consisted of the sun, seven planets, and twice as many satellites, all circling harmoniously in obedience to a universal law, by the compensating action of which the indefinite stability of their mutual relations was secured. The occasional incursion of a comet, or the periodical presence of a single such wanderer chained down from escape to outer space by planetary attraction, availed nothing to impair the symmetry of the majestic spectacle.

Now, not alone the ascertained limits of the system have been widened by a thousand millions of miles, with the addition of one more giant planet and seven satellites to the ancient classes of its members, but a complexity has been given to its constitution baffling description or thought. Five hundred circulating planetary bodies bridge the gap between Jupiter and Mars, the complete investigation of the movements of any one of which would overtask the energies of a lifetime. Meteorites, strangers, apparently, to the fundamental ordering of the solar household, swarm, nevertheless, by millions in every cranny of its space, returning at regular intervals like the comets so singularly associated with them, or sweeping across it with hyperbolic velocities, brought, perhaps, from some distant star. And each of these cosmical grains of dust has a theory far more complex than that of Jupiter; it bears within it the secret of its origin, and fulfils a function in the universe. The sun itself is no longer a semi-fabulous, fire-girt globe, but the vast scene of the play of forces as yet imperfectly known to us, offering a boundless field for the most arduous and inspiring researches. Among the planets the widest variety in physical habitudes is seen to prevail, and each is recognised as a world apart, inviting inquiries which, to be effective, must necessarily be special and detailed. Even our own moon threatens to break loose from the trammels of calculation, and commits "errors" which sap the very foundations of the lunar theory, and suggest the formidable necessity for its complete revision. Nay, the steadfast earth has forfeited the implicit confidence placed in it as a time-keeper, and questions relating to the stability of the earth's axis and the constancy of the earth's rate of rotation are among those which it behoves the future to answer. Everywhere there is multiformity and change, stimulating a curiosity which the rapid development of methods of research offers the possibility of at least partially gratifying.

Outside the solar system, the problems which demand a practical solution are virtually infinite in number and extent. And these have all arisen and crowded upon our thoughts within less than a hundred years. For sidereal science became a recognised branch of astronomy only through Herschel's discovery of the revolutions of double stars in 1802. Yet already it may be, and has been called, "the astronomy of the future," so rapidly has the development of a keen and universal interest attended and stimulated the growth of power to investigate this sublime subject. What has been done is little—is scarcely a beginning; yet it is much in comparison with the total blank of a century past. And our knowledge will, we are easily persuaded, appear in turn the merest ignorance to those who come after us. Yet it is not to be despised, since by it we reach up groping fingers to touch the hem of the garment of the Most High.

FOOTNOTES:

[Footnote 1630: Comptes Rendus, t. xliv., p. 339.]

[Footnote 1631: A. A. Common, Memoirs R. Astr. Soc., vol. i., p. 118.]

[Footnote 1632: Newcomb, Pop. Astr., p. 137.]

[Footnote 1633: Month. Not., vol. liv., p. 67.]

[Footnote 1634: Keeler, Publ. Astr. Pac. Soc., vol. ii., p. 160.]

[Footnote 1635: H. Grubb, Trans. Roy. Dub. Soc., vol. i. (new ser.), p. 2.]

[Footnote 1636: Hale, nevertheless (Astroph. Jour., vol. v., p. 128), considers that refractors preserve their superiority of visual light-grasp over Newtonian reflectors up to an aperture of 52-1/2, while equalisation is reached for the photographic rays at 34 inches.]

[Footnote 1637: Astroph. Jour., vol. v., p. 130.]

[Footnote 1638: Phil. Trans., vol. cxlviii., p. 465.]

[Footnote 1639: Optics, p. 107 (2nd ed., 1719).]

[Footnote 1640: Observatory, vol. viii., p. 85.]

[Footnote 1641: Holden on Celestial Photography, Overland Monthly, Nov., 1886.]

[Footnote 1642: Observatory, vol. xv., p. 283.]

[Footnote 1643: Bailey, Astroph. Jour., vol. x., p. 255.]

[Footnote 1644: Harvard Circulars, Nos. 2, 18, 24, 33;]

[Footnote 1645: Loewy, Bull. Astr., t. i., p. 286; Nature, vol. xxix., p. 36.]

[Footnote 1646: Nature, vol. xxiv., p. 389.]

[Footnote 1647: Ibid., vol. xxix., p. 470.]