Chapter XII: Stars and Nebulæ (3)

It is tolerably safe to predict that no work of its kind and for its purpose will ever again be undertaken. In a small part of one night stars can now be got to register themselves more numerously and more accurately than by the eye and hand of the most skilled observer in the course of a year. Fundamental catalogues, constructed by the old, time-honoured method, will continue to furnish indispensable starting-points for measurement; and one of especial excellence was published by Professor Newcomb in 1899;[1579] but the relative places of the small crowded stars--the sidereal [Greek: hoi pholloi]--will henceforth be derived from their autographic statements on the sensitive plate. Even the secondary purpose--that of asteroidal discovery--served by detailed stellar enumeration, is more surely attained by photography than by laborious visual comparison. For planetary movement betrays itself in a comparatively short time by turning the imprinted image of the object affected by it from a dot into a trail.

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. Brünnow detected in several stars, and in others remeasured with a care which inspired just confidence. His parallax for Alpha Lyræ (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 Ursæ 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 nebulæ, 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 Zöllner'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°.[1610] A photometric zone, 20° wide, has for some time been in course of observation at Potsdam by MM. Müller and Kempf. The instrument employed by them is constructed on the polarising principle as adapted by Zöllner.

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 formulæ, 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 nebulæ.

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 nebulæ 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 Nubeculæ, as Whewell insisted,[1613] "there coexist, in a limited compass and in indiscriminate position, stars, clusters of stars, nebulæ, 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 nebulæ 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 nebulæ 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 nebulæ. 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.

Photographed by Professor E. E. Barnard.]

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 Nubeculæ. 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 Mädler 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 Ursæ, though not yet measured, may be held to share their advance. One of them, moreover, Zeta Ursæ, 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. Höffler 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 Ursæ 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 Præsepe. 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 Zöllner 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; Bélopolsky, _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° 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 Méthode 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. Pétersb._, 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; Bélopolsky, _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: Bélopolsky, _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 Sidéral_, 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 nebulæ, 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; Schönfeld, _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._, Bände 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. Réunion du Comité 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.]