Chapter 38 of 38 · 26082 words · ~130 min read

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telescopes, proposed by Steinheil in 1856, and independently by Foucault in 1857,[1630] has in a great measure superseded the use of a metallic alloy.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 Königsberg 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 nebulæ, 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 nebulæ, 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 nebulæ 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 _coudé_ 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 "coudé."

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 Besançon 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 "coudé," 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 nebulæ, 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 _Mécanique Céleste_ 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.]

[Footnote 1648: _Trans. R. Dublin Soc._, vol. iii., p. 61.]

[Footnote 1649: _Observatory_, vol. vii., p. 167.]

[Footnote 1650: Loewy, _Bull. Astr._, t. i., p. 265.]

[Footnote 1651: _Phil. Trans._, vol. clxxi., p. 653.]

[Footnote 1652: Janssen, _L'Astronomie_, t. ii., p. 121.]

[Footnote 1653: Rev. A. L. Cortie, _Astr. and Astrophysics_, vol. xi., p. 400.]

[Footnote 1654: _Phil. Mag._, vol. xiii., 1882, p. 469.]

[Footnote 1655: _Bull. Astr._, t. iii., p. 331.]

APPENDIX

TABLE I

CHRONOLOGY, 1774-1893

1774, March 4 Herschel's first observation. Subject, the Orion Nebula. 1774 Sun-spots geometrically proved to be depressions by Wilson. 1774 First experimental determination of the earth's mean density by Maskelyne. 1781, March 13 Discovery of Uranus. 1782 Herschel's first Catalogue of Double Stars. 1783 Herschel's first investigation of the sun's movement in space. 1783 Goodricke's discovery of Algol's law of variation. 1784 Analogy between Mars and the Earth pointed out by Herschel. 1784 Construction of the Heavens investigated by Herschel's method of star-gauging. "Cloven-disc" plan of the Milky Way. 1784 Discovery of binary stars anticipated by Michell. 1786 Herschel's first Catalogue of Nebulæ. 1787, Jan. 11 Discovery by Herschel of two Uranian moons (Oberon and Titania). 1787, Nov. 19 Acceleration of the moon explained by Laplace. 1789 Herschel's second Catalogue of Nebulæ, and classification by age of these objects. 1789 Completion of Herschel's forty-foot reflector. 1789, Aug. 28 His discovery with it of the two inner Saturnian and Sept. 17 satellites. 1789 Repeating-circle invented by Borda. 1789 Five-foot circle constructed by Ramsden for Piazzi. 1790 Maskelyne's Catalogue of thirty-six fundamental stars. 1791 Herschel propounds the hypothesis of a fluid constitution for nebulæ. 1792 Atmospheric refraction in Venus announced by Schröter. 1794 Rotation-period of Saturn fixed by Herschel at 10h. 16m. 1795 Herschel's theory of the solar constitution. 1796 Herschel's first measures of comparative stellar brightness. 1796 Laplace's Nebular Hypothesis published in _Exposition du Système du Monde_. 1797 Publication of Olbers's method of computing cometary orbits. 1798 Retrograde motions of Uranian satellites detected by Herschel. 1799 Publication of first two volumes of _Mécanique Céleste_. 1799, May 7 Transit of Mercury observed by Schröter. 1799, Nov. 12 Star-shower observed by Humboldt at Cumana. 1800 _Monatliche Correspondenz_ started by Von Zach. 1800 Invisible heat-rays detected in the solar spectrum by Herschel. 1801, Jan. 1 Discovery of Ceres by Piazzi. 1801 Publication of Lalande's _Histoire Céleste_. 1801 Investigation by Herschel of solar emissive variability in connection with spot-development. 1802, March 28 Discovery of Pallas by Olbers. 1802 Herschel's third Catalogue of Nebulæ. 1802 Herschel's discovery of binary stars. 1802 Marks of clustering in the Milky Way noted by Herschel. 1802 Wollaston records seven dark lines in the solar spectrum. 1802, Nov. 9 Transit of Mercury observed by Herschel. 1804, Sept. 2 Discovery of Juno by Harding. 1804 Foundation of Optical Institute at Munich. 1805 Herschel's second determination of the solar apex. 1807, March 29 Discovery of Vesta by Olbers. 1811 Herschel's theory of the development of stars from nebulæ. 1811, Feb. 9 Death of Maskelyne. Pond appointed to succeed him as Astronomer-Royal. 1811, Sept. 12 Perihelion passage of great comet. 1812 Theory of electrical repulsion in comets originated by Olbers. 1812, Sept. 15 Perihelion passage of Pons's comet. 1814 Herschel demonstrates the irregular distribution of stars in space. 1815 Fraunhofer maps 324 dark lines in the solar spectrum. 1818 Publication of Bessel's _Fundamenta Astronomiæ_. 1819 Recognition by Encke of the first short-period comet. 1819, June 26 Passage of the earth through the tail of a comet. 1820 Foundation of the Royal Astronomical Society. 1821 Foundation of Paramatta Observatory. 1821, September First number of _Astronomische Nachrichten_. 1822, May 24 First calculated return of Encke's comet. 1822, August 25 Death of Herschel. 1823 Bessel introduces the correction of observations for personal equation. 1823 Fraunhofer examines the spectra of fixed stars. 1824 Distance of the sun concluded by Encke to be 95-1/4 million miles. 1824 Publication of Lohrmann's Lunar Chart. 1824 Dorpat refractor mounted equatoreally. 1826 Commencement of Schwabe's observations of sun-spots. 1826, Feb. 27 Biela's discovery of a comet. 1827 Orbit of a binary star calculated by Savary. 1829 Completion of the Royal Observatory at the Cape of Good Hope. 1829 The Königsberg heliometer mounted. 1830 Publication of Bessel's _Tabulæ Regiomontanæ_. 1832 Discovery by Brewster of "atmospheric lines" in the solar spectrum. 1833 Magnetic observatory established at Göttingen. 1833, Nov. 12,13 Star-shower visible in North America. 1833 Completion of Sir J. Herschel's survey of the northern heavens. 1834, Jan. 16 Sir J. Herschel's landing at the Cape. 1835, September Airy appointed Astronomer-Royal in succession to Pond. 1835, Nov. 16 Perihelion passage of Halley's comet. 1837 Solar movement determined by Argelander. 1837 Bessel's application of the heliometer to measurements of stellar parallax. 1837 Publication of Beer and Mädler's _Der Mond_. 1837 Publication of Struve's _Mensuræ Micrometricæ_. 1837, Dec. 16 Outburst of Eta Carinæ observed by Sir J. Herschel. 1837 Thermal power of the sun measured by Herschel and Pouillet. 1838 Parallax of 61 Cygni determined by Bessel. 1839, Jan. 9 Parallax of Alpha Centauri announced by Henderson. 1839 Completion of Pulkowa Observatory. 1839 Solidity of the earth concluded by Hopkins. 1840, March 2 Death of Olbers. 1840 First attempt to photograph the moon by J. W. Draper. 1842 Doppler enounces principle of colour-change by motion. 1842 Conclusion of Baily's experiments in weighing the Earth. 1842, July 8 Total solar eclipse. Corona and prominences observed by Airy, Baily, Arago, and Struve. 1843, Feb. 27 Perihelion-passage of great comet. 1845, February Completion of Parsonstown reflector. 1845, April Discovery with it of spiral nebulæ. 1845, April 2 Daguerreotype of the sun taken by Foucault and Fizeau. 1845, Oct. 21 Place of Neptune assigned by Adams. 1845, Dec. 8 Discovery of Astræa by Hencke. 1845, Dec. 29 Duplication of Biela's comet observed at Yale College. 1846 Melloni's detection of heating effects from moonlight. 1846, March 17 Death of Bessel. 1846, Sept. 23 Discovery of Neptune by Galle. 1846, Oct. 10 Neptune's satellite discovered by Lassell. 1847 Publication of Sir J. Herschel's _Results of Observations at the Cape of Good Hope_. 1847 Cyclonic theory of sun-spots stated by him. 1848 J. R. Mayer's meteoric hypothesis of solar conservation. 1848 Motion-displacements of Fraunhofer lines adverted to by Fizeau. 1848, April 27 New Star in Ophiuchus observed by Hind. 1848, Sept. 19 Simultaneous discovery of Hyperion by Bond and Lassell. 1849 First experimental determination of the velocity of light (Fizeau). 1850, July 17 Vega photographed at Harvard College. 1850, Nov. 15 Discovery by Bond of Saturn's dusky ring. 1851 O. Struve's first measurements of Saturn's ring-system 1851, July 28 Total solar eclipse observed in Sweden. 1851, Oct. 24 Discovery by Lassell of two inner Uranian satellites. 1851 Schwabe's discovery of sun-spot periodicity published by Humboldt. 1852, May 6 Coincidence of magnetic and sun-spot periods announced by Sabine. 1852, Oct. 11 Variable nebula in Taurus discovered by Hind. 1852 Lassell's two-foot reflector transported to Malta. 1853 Adams shows Laplace's explanation of the moon's acceleration to be incomplete. 1854 Hansen infers from lunar theory a reduced value for the distance of the sun. 1854 Helmholtz's "gravitation theory" of solar energy. 1856 Piazzi Smyth's observations on the Peak of Teneriffe. 1857 Saturn's rings shown by Clerk Maxwell to be of meteoric formation. 1857, April 27 Double-star photography initiated at Harvard College. 1858 Solar photography begun at Kew. 1858, Sept. 30 Perihelion of Donati's comet. 1859 Spectrum analysis established by Kirchhoff and Bunsen. 1859 Carrington's discovery of the compound nature of the sun's rotation. 1859, Sept. 1 Luminous solar outburst and magnetic storm. 1859, Oct. 19 Merope nebula discovered by Tempel. 1859, Dec. 15 Chemical constitution of the sun described by Kirchhoff. 1860, Feb. 27 Discovery by Liais of a "double comet." 1860, May 21 New star in Scorpio detected by Auwers. 1860, July 18 Total solar eclipse observed in Spain. Prominences shown by photography to be solar appendages. 1861, June 30 The earth involved in the tail of a great comet. 1861-1862 Kirchhoff's map of the solar spectrum. 1862 Solar hydrogen-absorption recognised by Ångström. 1862, Jan. 31 Discovery by Alvan G. Clark of the companion of Sirius. 1862 Foucault determines the sun's distance by the velocity of light. 1862 Opposition of Mars. Determination of solar parallax. 1862 Completion of _Bonner Durchmusterung_. 1863 Secchi's classification of stellar spectra. 1863 Foundation of the German Astronomical Society. 1864, March 5 Rotation period of Mars determined by Kaiser. 1864 Huggins's first results in stellar spectrum analysis. 1864, Aug. 5 Spectroscopic examination of Tempel's comet by Donati shows it to be composed of glowing gas. 1864, Aug. 29 Discovery by Huggins of gaseous nebulæ. 1864 Value of 91,000,000 miles adopted for the sun's distance. 1864 Croll's explanation of glacial epochs. 1864, Nov. 23 Death of Struve. 1865, Jan. 4 Spectroscopic observation by Huggins of the occultation of Eta Piscium. 1865, Jan. 16 Faye's theory of the solar constitution. 1865 Kew results published. 1865 Zöllner argues for a high temperature in the great planets. 1866 Identity of the orbits of the August meteors and of comet 1862 iii. demonstrated by Schiaparelli. 1866 Delaunay explains lunar acceleration by a lengthening of the day through tidal friction. 1866, March 4 Spectroscopic study of the sun's surface by Lockyer. 1866, March 12 New star in Corona Borealis detected by Birmingham. 1866, October Schmidt announces the disappearance of the lunar crater Linné. 1866, Nov. 13 Meteoric shower visible in Europe. 1867 Period of November meteors determined by Adams. 1867, Aug. 29 Total solar eclipse. Minimum sun-spot type of corona observed by Grosch at Santiago. 1867 Discovery of gaseous stars in Cygnus by Wolf and Rayet. 1868, February Principle of daylight spectroscopic visibility of prominences started by Huggins. 1868, Aug. 18 Great Indian eclipse. Spectrum of prominences observed. 1868, Aug. 19 Janssen's first daylight view of a prominence. 1868, Oct. 26 Lockyer and Janssen independently announce their discovery of the spectroscopic method. 1868 Doppler's principle applied by Huggins to measure stellar radial movements. 1868 Publication of Ångström's map of the normal solar spectrum. 1868 Spectrum of Winnecke's comet found by Huggins to agree with that of olefiant gas. 1869, Feb. 11 Tenuity of chromospheric gases inferred by Lockyer and Frankland. 1869, Feb. 13 Huggins observes a prominence with an "open slit." 1869, Aug. 7 American eclipse. Detection of bright-line coronal spectrum. 1870 Mounting of Newall's 25-inch achromatic at Gateshead. 1870 Proctor indicates the prevalence of drifting movements among the stars. 1870 A solar prominence photographed by Young. 1870, Dec. 22 Sicilian eclipse. Young discovers reversing layer. 1871, May 11 Death of Sir J. Herschel. 1871, June 9 Line-displacements due to solar rotation detected by Vogel. 1871, Dec. 12 Total eclipse visible in India. Janssen observes reflected Fraunhofer lines in spectrum of corona. 1872 Conclusion of a three years' series of observations on lunar heat by Lord Rosse. 1872 Spectrum of Vega photographed by H. Draper. 1872 Faye's cyclonic hypothesis of sun-spots. 1872 Young's solar-spectroscopic observations at Mount Sherman. 1872 Cornu's experiments on the velocity of light. 1872, Nov. 27 Meteoric shower connected with Biela's comet. 1873 Determination of mean density of the earth by Cornu and Baille. 1873 Solar photographic work begun at Greenwich. 1873 Erection of 26-inch Washington refractor. 1874 Light-equation redetermined by Glasenapp. 1874 Vogel's classification of stellar spectra. 1874, Dec. 8 Transit of Venus. 1876 Publication of Neison's _The Moon_. 1876, Nov. 24 New star in Cygnus discovered by Schmidt. 1876 Spectrum of Vega photographed by Huggins. First use of dry gelatine plates in celestial photography. 1877, May 19 Klein observes a supposed new lunar crater (Hyginus N.). 1877 Measurement by Vogel of selective absorption in solar atmosphere. 1877, Aug. 16-17 Discovery of two satellites of Mars by Hall at Washington. 1877, Sept. 23 Death of Leverrier. 1877 Canals of Mars discovered by Schiaparelli. 1877 Opposition of Mars observed by Gill at Ascension. Solar parallax deduced = 8.78". 1878, January Stationary meteor-radiants described by Denning. 1878 Publication of Schmidt's _Charte der Gebirge des Mondes_. 1878 First observations of Great Red Spot on Jupiter. 1878 Conclusion of Newcomb's researches on the lunar theory. 1878, May 6 Transit of Mercury. 1878 Foundation of Selenographical Society. 1878, July 29 Total eclipse visible in America. Vast equatoreal extension of the corona. 1878, October Completion of Potsdam Astrophysical Observatory. 1878, Dec. 12 Lockyer's theory of celestial dissociation communicated to the Royal Society. 1879 Michelson's experiments on the velocity of light. 1879 Publication of Gould's _Uranometria Argentina_. 1879, November Observations of the spectra of sun-spots begun at South Kensington. 1879, Dec. 5 Abney's map of the infra-red solar spectrum presented to the Royal Society. 1879, Dec. 18 Ultra-violet spectra of white stars described by Huggins. 1879, Dec. 18 Communication of G. H. Darwin's researches into the early history of the moon. 1880, Jan. 31 Discovery at Cordoba of a great southern comet. 1880 Conditions of Algol's eclipses determined by Pickering. 1880 Pickering computes mass-brightness of binary stars. 1880, Sept. 30 Draper's photograph of the Orion nebula. 1880 The bolometer invented by Langley. 1881, Jan. 20 Communication of G. H. Darwin's researches into the effects of tidal friction on the evolution of the solar system. 1881 Langley's observations of atmospheric absorption on Mount Whitney. 1881, June 16 Perihelion of Tebbutt's comet. 1881, June 24 Its spectrum photographed by Huggins. 1881, June Photographs of Tebbutt's comet by Janssen and Draper. 1881, Aug. 15 Retirement of Sir George Airy. Succeeded by Christie. 1881, Aug. 22 Perihelion of Schaeberle's comet. 1881 Publication of Stone's Cape Catalogue for 1880. 1882 Struve's second measures of Saturn's ring-system. 1882 Newcomb's determination of the velocity of light. Resulting solar parallax = 8·79". 1882 Correction by Nyrén of Struve's constant of aberration. 1882, March 7 Spectrum of Orion nebula photographed by Huggins. 1882, May 17 Total solar eclipse observed at Sohag in Egypt. 1882, May 27 Sodium-rays observed at Dunecht in spectrum of Comet Wells. 1882, June 10 Perihelion of Comet Wells. 1882, Sept. 17 Perihelion of Great Comet. Daylight detection by Common. Transit observed at the Cape. 1882, Sept. 18 Iron lines identified in spectrum by Copeland and J. G. Lohse. 1882, September Photographs of comet taken at the Cape Observatory, showing a background crowded with stars. 1882, Dec. 6 Transit of Venus. 1882 Duplication of Martian canals observed by Schiaparelli. 1882 Completion by Loewy at Paris of first equatoreal Coudé. 1882 Rigidity of the earth concluded from tidal observations by G. H. Darwin. 1882 Experiments by Huggins on photographing the corona without an eclipse. 1882 Publication of Holden's _Monograph of the Orion Nebula_. 1883, Jan. 30 Orion Nebula photographed by Common. 1883, May 6 Caroline Island eclipse. 1883, June 1 Great comet of 1882 observed from Cordoba at a distance from the earth of 470 million miles. 1883 Parallaxes of nine southern stars measured by Gill and Elkin. 1883 Catalogue of the spectra of 4,051 stars by Vogel. 1884, Jan. 25 Return to perihelion of Pons's comet. 1884 Photometric Catalogue by Pickering of 4,260 stars. 1884 Publication of Gore's Catalogue of Variable Stars. 1884 Publication of Faye's _Origine du Monde_. 1884, Oct. 4 Eclipse of the moon. Heat-phases measured by Boeddicker at Parsonstown. 1884 Dunér's Catalogue of Stars with Banded Spectra. 1884 Backlund's researches into the movements of Encke's comet. 1885, February Langley measures the lunar heat-spectrum. 1885 Publication of _Uranometria Nova Oxoniensis_. 1885, Aug. 17 New star in Andromeda nebula discerned by Gully. 1885, Sept. 5 Thollon's drawing of the solar spectrum presented to the Paris Academy. 1885, Sept. 9 Solar eclipse visible in New Zealand. 1885, Nov. 16 Photographic discovery by Paul and Prosper Henry of a nebula in the Pleiades. 1885, Nov. 27 Shower of Biela meteors. 1885 Thirty-inch achromatic mounted at Pulkowa. 1885 Publication of Rowland's photographic map of the normal solar spectrum. 1885 Bakhuyzen's determination of the rotation period of Mars. 1885 Stellar photographs by Paul and Prosper Henry. 1886, Jan. 26 Spectra of forty Pleiades simultaneously photographed at Harvard College. 1886, Feb. 5 First visual observation of the Maia nebula with the Pulkowa 30-inch refractor. 1886, March Photographs by the Henrys of the Pleiades, showing 2,326 stars with nebulæ intermixed. 1886, May Photographic investigations of stellar parallax undertaken by Pritchard. 1886, May 6 Periodical changes in spectra of sun-spots announced by Lockyer. 1886, June 4 An international Photographic Congress proposed by Gill. 1886, Aug. 29 Total eclipse of the sun observed at Grenada. 1886, Oct. 1 Roberts's photograph showing annular structure of the Andromeda nebula. 1886, Dec. 8 Roberts's photograph of the Pleiades nebulosities. 1886 Solar heat-spectrum extended by Langley to below five microns. 1886, Dec. 28 Detection by Copeland of helium-ray in spectrum of the Orion nebula. 1886 Thirty-inch refractor mounted at Nice. 1886 Publication of Argentine General Catalogue. 1886 Completion of Auwers's reduction of Bradley's observations. 1886 Draper Memorial photographic work begun at Harvard College. 1886 Photographic detection at Harvard College of bright hydrogen lines in spectra of variables (Mira Ceti and U Orionis). 1887, Jan. 18 Discovery by Thome at Cordoba of a great comet belonging to the same group as the comet of 1882. 1887 Publication of Lockyer's _Chemistry of the Sun_. 1887, April 16 Meeting at Paris of the International Astrophotographic Congress. 1887 Heliometric triangulation of the Pleiades by Elkin. 1887 L. Struve's investigation of the sun's motion, and redetermination of the constant of precession. 1887 Von Konkoly's extension to 15° S. Dec. of Vogel's spectroscopic Catalogue. 1887 Auwers's investigation of the solar diameter. 1887 Publication of Schiaparelli's Measures of Double Stars (1875-85). 1887, April 8 Death of Thollon at Nice. 1887, Aug. 19 Total eclipse of the sun. Shadow-path crossed Russia. Observations marred by bad weather. 1887, November Langley's researches on the temperature of the moon. 1887, Nov. 17 Lockyer's _Researches on Meteorites_ communicated to the Royal Society. 1887 Completion of 36-inch Lick refractor. 1888 Küstner's detection of variations in the latitude of Berlin brought before the International Geodetic Association. 1888 Chandler's first Catalogue of Variable Stars. 1888 Mean parallax of northern first magnitude stars determined by Elkin. 1888 Publication of Dreyer's _New General Catalogue_ of 7,844 nebulæ. 1888 Vogel's first spectrographic determinations of stellar radial motion. 1888 Carbon absorption recognised in solar spectrum by Trowbridge and Hutchins. 1888, Jan. 28 Total eclipse of the moon. Heat-phases measured at Parsonstown. 1888, Feb. 5 Remarkable photograph of the Orion nebula spectrum taken at Tulse Hill. 1888, June 1 Activity of the Lick Observatory begun. 1888 Completion of Dr. Common's 5-foot reflector. 1888 Heliometric measures of Iris for solar parallax at the Cape, Newhaven (U.S.A.), and Leipsic. 1888 Loewy describes a comparative method of determining constant of aberration. 1888 Presentation of the Dunecht instrumental outfit to the nation by Lord Crawford. Copeland succeeds Piazzi Smyth as Astronomer-Royal for Scotland. 1888, Sept. 12 Death of R. A. Proctor. 1889 Photograph of the Orion nebula taken by W. H. Pickering, showing it to be the nucleus of a vast spiral. 1889 Discovery at a Harvard College of the first-known spectroscopic doubles, Zeta Ursæ Majoris and Beta Aurigæ. 1889 Eclipses of Algol demonstrated spectrographically by Vogel. 1889 Completion of photographic work for the Southern Durchmusterung. 1889 Boeddicker's drawing of the Milky Way. 1889 Draper Memorial photographs of southern star-spectra taken in Peru. 1889 Pernter's experiments on scintillation from the Sonnblick. 1889 H. Struve's researches on Saturn's satellites. 1889 Harkness's investigation of the masses of Mercury, Venus, and the Earth. 1889 Heliometric measures of Victoria and Sappho at the Cape. 1889, Jan. 1 Total solar eclipse visible in California. 1889, Feb. 7 Foundation of the Astronomical Society of the Pacific. 1889, March Investigation by Sir William and Lady Huggins of the spectrum of the Orion nebula. 1889, July-Aug. First photographs of the Milky Way taken by Barnard. 1889, August 2 Observation by Barnard of four companions to Brooks's comet. 1889, Nov. 1 Passage of Japetus behind Saturn's dusky ring observed by Barnard. 1889, December Schiaparelli announces synchronous rotation and revolution of Mercury. 1889, Dec. 22 Total eclipse of the sun visible in Guiana. Death of Father Perry, December 27. 1889 Spectrum of Uranus investigated visually by Keeler, photographically by Huggins. 1890 Long-exposure photographs of ring-nebula in Lyra. 1890 Determinations of the solar translation by L. Boss and O. Stumpe. 1890 Schiaparelli finds for Venus an identical period of rotation and revolution. 1890 Publication of Thollon's map of the solar spectrum. 1890 Bigelow's mathematical theory of coronal structures. 1890 Foundation of the British Astronomical Association. 1890 Measurements by Keeler at Lick of nebular radial movements. 1890 Janssen's ascent of Mont Blanc, by which he ascertained the purely terrestrial origin of the oxygen-absorption in the solar spectrum. 1890 Newcomb's discussion of the transits of Venus of 1761 and 1769. 1890 Spiral structure of Magellanic Clouds displayed in photographs taken by H. C. Russell of Sydney. 1890 Publication of the Draper Catalogue of Stellar Spectra. 1890, April 24 Spica announced by Vogel to be a spectroscopic binary. 1890, June Gore's Catalogue of computed Binaries. 1890, November Study by Sir William and Lady Huggins of the spectra of Wolf and Rayet's stars in Cygnus. 1890, November Discovery by Barnard of a close nebulous companion to Merope in the Pleiades. 1890, November McClean Spectrographs of the High and Low Sun. 1891 Capture-theory of comets developed by Callandreau, Tisserand, and Newton. 1891 Dunér's spectroscopic researches on the sun's rotation. 1891 Preponderance of Sirian stars in the Milky Way concluded by Pickering, Gill, and Kapteyn. 1891 Detection by Mrs. Fleming of spectral variations corresponding to light-changes in Beta Lyræ. 1891 Establishment of the Harvard College Station at Arequipa in Peru (height 8,000 feet). 1891 Variations of latitude investigated by Chandler. 1891 Prominence-photography set on foot by Hale at Chicago and Deslandres at Paris. 1891 Schmidt's Theory of Refraction in the Sun. 1891, April Meeting at Paris of the Permanent Committee for the Photographic Charting of the Heavens. 1891, May 9 Transit of Mercury. 1891, Aug. 19 Presidential Address by Huggins at the Cardiff Meeting of the British Association. 1891, Dec. 10 Nova Aurigæ photographed at Harvard College. 1891, Dec. 20 Photographic maximum of Nova Aurigæ. 1891, Dec. 22 First photographic discovery of a minor planet by Max Wolf at Heidelberg. 1892 Commencement of international photographic charting work. 1892 Photographic determination by Scheiner of 833 stars in the Hercules Cluster (M 13). 1892 Publication of Vogel's spectrographic determinations for fifty-one stars. 1892 Publication of Pritchard's photographic parallaxes. 1892, Jan. 2 Death of Sir George Airy. 1892, Jan. 21 Death of Professor Adams. 1892, Feb. 1 Announcement by Anderson of the outburst of a new star in Auriga. 1892, Feb. 5 Appearance of the largest sun-spot ever photographed at Greenwich. 1892, March Photograph of Argo nebula taken by Gill in twelve hours. 1892, March 6 Discovery of a bright comet by Swift. 1892, June 29 Death of Admiral Mouchez. Succeeded by Tisserand as director of the National Observatory, Paris. 1892, Aug. 4 Favourable Opposition of Mars. 1892, Aug. 17 Rediscovery at Lick of Nova Aurigæ. 1892, Sept. 9 Discovery by Barnard of Jupiter's inner satellite. 1892, Oct. 12 First photographic discovery of a comet by Barnard. 1892, Nov. 6 Discovery of Holmes's comet. 1892, Nov. 23 Shower of Andromede meteors visible in America. 1892 Poynting's Determination of the Earth's Mean Density. 1892 Dunér's Investigation of the System of upsilon Cygni. 1892 Photographic investigation by Deslandres of the spectra of prominences. 1892 Photographs of the sun with faculæ and chromospheric surroundings taken by Hale with a single exposure. 1892 Investigation by T. J. J. See of the ancient colour of Sirius. 1892 Publication of T. J. J. See's Thesis on the Evolution of Binary Systems. 1892 Chandler's theory of Algol's inequalities. 1892 Nebula in Cygnus photographically discovered by Max Wolf. 1893, Jan. 28 Kapteyn's investigation of the structure of the stellar universe. 1893, March 10 Gill announces his results from the Opposition of Victoria, among them a solar parallax = 8.809". 1893, April 16 Total solar eclipse observed in South America and West Africa. 1893 Publication of Kruger's _Catalog der Farbigen Sterne_. 1893 Conclusion of Boys's series of Experiments on the Density of the Earth. 1893 Publication of _Cordoba Durchmusterung_, vol. i. 1893 Fabry shows comets to be dependents of the Solar System. 1893 Publication of Easton's _Voie Lactée_. 1893 Campbell detects bright H Alpha in Gamma Argûs and Alcyone. 1893 Nova Normæ photographed July 10; discovered on plates, October 26. 1893, May 28 Death of Professor Pritchard. 1893, July 27 Installation of 28-inch refractor at the Royal Observatory, Greenwich. 1893, December Exterior nebulosities of Pleiades photographed by Barnard. 1893, Dec. 6 Death of Rudolf Wolf. 1894, January Sun-spot maximum. 1894 Publication of Potsdam _Photometric Durchmusterung_,

## part i.

1894 Publication of Roberts's _Celestial Photographs_, vol. i. 1894 Wilson and Gray's determination of the sun's temperature. 1894 Barnard's micrometric measures of asteroids. 1894 McClean's gift of an astrophysical outfit to the Cape Observatory. 1894 Establishment of the Lowell Observatory at Flagstaff, Arizona. 1894 Taylor's triple achromatic objective described. 1894, April 3 Discovery of Gale's Comet. 1894 Sampson's investigation of the sun's rotation. 1894, Oct. 20 Favourable opposition of Mars. 1894, Nov. 11 Transit of Mercury. 1894, December Howlett impugns the Wilsonian theory of sun-spots. 1894, Dec. 14 Death of A. Cowper Ranyard. 1895 Publication of Newcomb's _Astronomical Constants_. 1895 Bailey's Photometric Catalogue of 7,922 Southern Stars. 1895 Bailey's photographic discovery of variable star clusters. 1895 Publication of E. W. Brown's _Lunar Theory_. 1895 Tisserand's theory of the inequalities of Algol. 1895 Stratonoff's determination of the sun's rotation from photographs of faculæ. 1895 Binary character of Eta Aquilæ spectroscopically recognised by Bélopolsky. 1895 Presentation of the Crossley reflector to the Lick Observatory. 1895, March 23 Great nebula in Ophiuchus discovered photographically by Barnard. 1895, March 25 Ramsay's capture of Helium. 1895, April Constitution of Saturn's rings spectrographically demonstrated by Keeler. 1895 Binary character of Delta Cephei spectroscopically detected by Bélopolsky. 1895, June 11 Death of Daniel Kirkwood. 1895, July 7 Death of F. W. G. Spörer. 1895, October Nova Carinæ spectrographically discovered by Mrs. Fleming. 1895, Dec. 12 Nova Centauri spectrographically discovered by Mrs. Fleming. 1895, Dec. 28 Death of John Russell Hind. 1896 Gill's Report on the Geodetic Survey of South Africa. 1896 Appearance of Loewy's Photographic Atlas of the Moon,

## part i.

1896, January Fessenden's electrostatic theory of comets. 1896 Chandler's Third Catalogue of Variable Stars. 1896 Publication of Lick Observatory Photographic Atlas of the Moon, part i. 1896, February Effects of pressure on wave-length described by Humphreys and Mohler. 1896, April 5 Opening of new Scottish Royal Observatory on Blackford Hill, Edinburgh. 1896, April Pickering's photometric determinations of light curves of variable stars. 1896 One of the stars of Castor spectroscopically resolved into two by Bélopolsky. 1896, May Third Astrographic Chart Conference at Paris. 1896, Aug. 9 Total eclipse of the sun visible in Novaya Zemlya. Reversing layer photographed by Shackleton. 1896, Aug. 30 Death of Hubert A. Newton. 1896, Sept. 18 Death of Hippolyte Fizeau. 1896, Oct. 20 Death of F. Tisserand. Succeeded by Maurice Loewy. 1896, Nov. 13 Detection by Schaeberle of Procyon's missing satellite. 1896, Nov. 26 Death of Benjamin Apthorp Gould. 1896, November Second series of hydrogen-lines discovered by Pickering in stellar spectra. 1896, December Zeeman's discovery of spectral modifications through magnetic influence. 1896, December Oxygen-absorption identified in the sun by Runge and Paschen. 1896 Study of lunar formations by Loewy and Puiseux. 1896 Mounting of the Mills spectrograph at the Lick Observatory. 1897 Installation at Greenwich of the Thompson 26-inch photographic refractor. 1897 Publication of Miss Maury's Discussion of the Photographed Spectra of 681 Stars. 1897 Callandreau's researches on cometary disaggregation. 1897 Braun's determination of the earth's mean density. 1897 Tenuity of calcium vapour in chromosphere demonstrated spectroscopically by Sir William and Lady Huggins. 1897 Completion at the Cape Observatory of McClean's spectrographic survey of the heavens. 1897 Twenty-one Wolf-Rayet stars found by Mrs. Fleming in Magellanic Cloud. 1897 Percival Lowell's _New Observations on the Planet Mercury_ presented to the American Academy. 1897, April 8 McClean recognises oxygen-absorption in helium stars. 1897, May 9 Death of E. J. Stone, Radcliffe Observer. 1897, June 10 Death of Alvan G. Clark. 1897, June 18 Spectrum of a meteor photographed at Arequipa. 1897, Oct. 21 Inauguration of the Yerkes Observatory. 1897 Rabourdin's photographs of nebulæ with the Meudon reflector. 1897 Dr. See's discoveries of Southern double stars with the Lowell 24-inch refractor. 1898, Jan. 22 Total eclipse of the sun visible in India. 1898, February Binary character of Zeta Geminorum ascertained spectroscopically by Bélopolsky. 1898 Star with proper motion of nearly 9" discovered by Innes and Kapteyn from the Cape Durchmusterung plates. 1898, March 8 Nova Sagittarii photographed on Draper Memorial plates. 1898, June 20 Opening of Grand-ducal Observatory at Königsstuhl, Heidelberg. 1898 Keeler succeeds Holden as Director of the Lick Observatory. 1898 Bruno Peter's results in stellar parallax. 1898 Lewis Swift's discoveries of nebulæ at Echo Mountain, California. 1898 Hale's photographic investigation of carbon stars. 1898, Aug. 14 Discovery of Eros by Witt. 1898 Flint's investigations of stellar parallax by meridian differences. 1898 Easton's spiral theory of the Milky Way. 1898 Seeliger's research on star distribution. 1898, October Multiple hydrogen-bands observed by Campbell in Mira Ceti. 1898, November Orbit of a Leonid meteor photographically determined by Elkin. 1899 Publication of Potsdam _Photometric Durchmusterung_,

## part ii.

1899 Innes's _Reference Catalogue of Southern Double Stars_. 1899 Keeler's photographs of nebulæ with the Crossley reflector and generalization of their spiral character. 1899, January Spectrum of Andromeda nebula photographed by Scheiner. 1899, April Photographic discovery of Nova Aquilæ by Mrs. Fleming. 1899, Aug. 26 Installation of 31-inch photographic refractor at Potsdam. 1899 Campbell's detection of Polaris as spectroscopically triple. 1899, October Duplicate discovery by Campbell and Newall of Capella as a spectroscopic binary. 1899, Nov. 15 Failure of the Leonids. Deflection of the stream predicted by Johnstone Stoney and Downing. 1899, December Publication of Sir William and Lady Huggins's _Atlas of Representative Stellar Spectra_. 1899 Thirty-two-inch photographic refractor mounted at Meudon. 1899 Issue of first volume of Potsdam measures of international catalogue plates. 1900, Jan. 27 Kapteyn's determination of the apex of solar motion. 1900 Chase's measures for parallax of swiftly-moving stars. 1900 Publication of Gill's _Researches on Stellar Parallax_. 1900 Kapteyn proposes a method for a stellar parallax Durchmusterung, and gives specimen results for 248 stars. 1900 Burnham's general catalogue of 1,290 double stars. 1900 Publication of the concluding volume of the _Cape Photographic Durchmusterung_. 1900, May 28 Spanish-American total eclipse of the sun. 1900, July International Conference at Paris. Co-operation arranged of fifty-eight observatories in measures of Eros for solar parallax. 1900 Horizontal refractor, of 50 inches aperture, 197 feet focus, installed in Paris Exhibition. 1900, Aug. 12 Death of Professor Keeler. Succeeded by Campbell in direction of Lick Observatory. 1900, November Opposition of Eros. 1900 Publication of Roberts's _Celestial Photographs_, vol. ii. 1900 Complete publication of Langley's researches on the infra-red spectrum. 1900 Printing begun of Paris section of International Photographic Catalogue. 1901, Feb. 22 Nova Persei discovered by Anderson. 1901, February Variability of Eros announced by Oppolzer. 1901, April 23 Apparition of a great comet at the Cape. 1901 Publication of Pickering's _Photometric Durchmusterung_. 1901 Miss Cannon's discussion of the spectra of 1,122 Southern stars. 1901 Kapteyn's investigation of mean stellar parallax. 1901 Campbell's determination of the sun's velocity. 1901 Porter's research on the solar motion in space. 1901 Bigelow's magnetic theory of the solar corona. 1901 Hussey's measurements of the Pulkowa double stars. 1901 Radial velocities of the components of Delta Equulei measured at Lick. 1901, April 16 Death of Henry A. Rowland. 1901, June Nebular spectrum derived from Nova Persei. 1901, Aug. 23 Nebula near Nova Persei photographed by Max Wolf. 1901, Sept. 20 The same exhibited in spiral form on a plate taken by Ritchey at the Yerkes Observatory. 1901, Nov. 8 Photograph taken by Perrine with the Crossley reflector showed nebula in course of rapid change. 1901, Sept. 19 Unveiling of the McClean "Victoria" telescope at the Royal Observatory, Cape of Good Hope. 1901 Sun-spot minimum.

TABLE II.

CHEMICAL ELEMENTS IN THE SUN (ROWLAND, 1891).

Arranged according to the number of their representative Lines in the Solar Spectrum.

Iron (2000+). Neodymium. Cadmium. Nickel. Lanthanum. Rhodium. Titanium. Yttrium. Erbium. Manganese. Niobium. Zinc. Chromium. Molybdenum. Copper (2). Cobalt. Palladium. Silver (2). Carbon (200+). Magnesium (20+). Glucinum (2). Vanadium. Sodium (11). Germanium. Zirconium. Silicon. Tin. Cerium. Strontium. Lead (1). Calcium (75+). Barium. Potassium (1). Scandium. Aluminium (4).

_Doubtful Elements._--Iridium, osmium, platinum, ruthenium, tantalum, thorium, tungsten, uranium.

_Not in Solar Spectrum._--Antimony, arsenic, bismuth, boron, nitrogen (vacuum tube), cæsium, gold, iridium, mercury, phosphorus, rubidium, selenium, sulphur, thallium, praseodymium.

Oxygen was added to the solar ingredients by Runge and Paschen in 1896, gallium by Hartley and Ramage in 1899. Lithium may be admitted provisionally, and the chromospheric constituent helium takes rank, since 1895, as a chemical element.

TABLE III.

EPOCHS OF SUN-SPOT MAXIMUM AND MINIMUM FROM 1610 TO 1901.

+----------+----------++----------+----------+----------+----------+ | Minima. | Maxima. || Minima. | Maxima. | Minima. | Maxima. | +----------+----------++----------+----------+----------+----------+ | 1610.8 | 1615.5 || 1712.0 | 1718.2 | 1810.6 | 1816.4 | | 1619.0 | 1626.0 || 1723.5 | 1727.5 | 1823.3 | 1829.9 | | 1634.0 | 1639.5 || 1734.0 | 1738.7 | 1833.9 | 1837.2 | | 1645.0 | 1649.0 || 1745.0 | 1750.3 | 1843.5 | 1848.1 | | 1655.0 | 1660.0 || 1755.2 | 1761.5 | 1856.0 | 1860.1 | | 1666.0 | 1675.0 || 1766.5 | 1769.7 | 1867.2 | 1870.6 | | 1679.5 | 1685.0 || 1775.5 | 1778.4 | 1878.9 | 1884.0 | | 1689.5 | 1693.0 || 1784.7 | 1788.1 | 1890.2 | 1894.0 | | 1698.9 | 1705.5 || 1798.3 | 1804.2 | 1901.9 | | +----------+----------++----------+----------+----------+----------+

TABLE IV.

MOVEMENTS OF SUN AND STARS.

1. Translation of Solar System.

+--------------------+-------------------------+-------+ | Apex of Movement. | Authority. | Date. | +--------------------+-------------------------+-------+ | R. A. Dec. | | | | | | | | 277° 30' + 35° | Newcomb | 1898 | | 273° 36' + 29° 30' | Kapteyn | 1901 | | 279° + 46° | Porter | 1901 | | 275° + 45° | Boss | 1901 | | 277° 30' + 20° | Campbell (from stellar | 1902 | | | spectroscopic measures) | | +--------------------+-------------------------+-------+ | Velocity = 12·4 miles per second (Campbell). | +------------------------------------------------------+

2. Stellar Velocities.

TABLE V.

LIST OF GREAT TELESCOPES.

TABLE VI.

List of Observatories employed in the Construction of the Photographic Chart and Catalogue of the Heavens.

All are provided with 13-inch photographic, coupled with 11-inch visual refractors:

---------------------------------------------------------- | Name of Observatory. | Constructors of Instruments.| | |-------------------------------| | | Optical Part.|Mechanical Part.| |------------------------|--------------|----------------| |Paris | Henrys | Gautier | |Algiers | " | " | |Bordeaux | " | " | |Toulouse | " | " | |San Fernando (Spain) | " | " | |Vatican | " | " | |Cordoba | " | " | |Montevideo | " | " | |Perth, Western Australia| " | " | |Helsingfors | " | Repsold | |Potsdam | Steinheil | " | |Catania | " | Salmoiraghi | |Greenwich | Sir H. Grubb | Sir H. Grubb | |Oxford | " | " | |The Cape | " | " | |Melbourne | " | " | |Sydney | " | " | |Tacubaya (Mexico) | " | " | ----------------------------------------------------------

INDEX

Abbe, Cleveland, corona of 1878, 176, 177

Aberdour, Lord, solar chromosphere, 68

Aberration, discovered by Bradley, 3, 15; cause of, 31, 231; investigations of, 241, 438

Abney, daylight coronal photographs, 179; infra-red photography, 210, 223, 438

Absorption, terrestrial atmospheric, 134, 211, 214-216, 225; solar, 134-136, 172, 213, 221, 222, 225, 277; correlative with emission, 135, 136, 140

Adams, discovery of Neptune, 79-82; lunar acceleration, 271; orbit of November meteors, 331

Aerolites, falls of, 339, 340

Airy, solar translation, 39; observations during eclipses, 62, 64, 70; Astronomer-Royal, 79; search for Neptune, 80, 81; corona of 1851, 175; solar parallax, 227, 236; transit of Venus, 233; Mercurian halo, 235; lunar atmosphere, 264

Aitken, double star discoveries, 419

Albedo, of Mercury, 246; of Venus, 255; of Mars, 283; of minor planets, 288; of Jupiter, 290; of Saturn, 303; of Uranus, 304

Alexander, spiral nebulæ, 118; observation during eclipse, 245

Algol, variability of light, 10, 390; eclipses, 390; nature of system, 391

Altitude and azimuth instrument, 120 _note_, 121

Amici, comet of 1843, 103

Anderson, discovery of Nova Aurigæ, 396; of Nova Persei, 400

Andrews, conditions of liquefaction, 151

Ångström, C. J., _Optical Researches_, 138; spark spectrum, 139; nature of photosphere, 152; solar spectroscopy, 210, 212; hydrogen in sun, 211; temperature of stars, 375

Ångström, K., infra-red solar spectrum, 210; solar constant, 225

Arago, eclipse of 1842, 62, 64, 65; prominences, 69; polarization in comets, 103; magnetic relations of auroræ, 130; nature of photosphere, 151; meteor-systems, 329

Arai, photographs of corona of 1887, 185

Arcturus, spectrum, 373, 383; radial movement, 387

Argelander, Bonn Durchmusterung, 32, 423; solar motion, 39; centre of Milky Way, 40; comet of 1811, 100

Aristotle, description of a comet, 350

Arrhenius, light-pressure theory of comets, 348

Asten, movements of Encke's comet, 94

Asteroids, so designated by Herschel, 75

Astronomical circles, 121, 122

Astronomical physics, 7, 141, 142

Astronomical Society founded, 6; Herschel its first President, 14

Astronomy, classification, 1; popularity and progress, 5; in United States, 6; in Germany, 28; practical reform, 32; of the invisible, 42; physical, 141

Atmosphere, solar, 94, 182, 192, 221, 225; of Venus, 236, 239, 253, 254; of Mercury, 246-248; of the moon, 263, 264; of Mars, 276; of minor planets, 288

Auroræ, periodicity, 129, 162; excited by meteors, 335

Auwers, reduction of Bradley's observations, 39; system of Procyon, 42; opposition of Victoria, 238; solar parallax, 240; new star in Scorpio, 395

Babinet, nebular hypothesis, 314

Backlund, movements of Encke's comet, 94, 360

Baden-Powell, Sir George, eclipse expedition, 188

Bailey, nebulosity round Pleiades, 411; stellar photometric observations, 421; discovery of variable clusters, 436

Baily, early life and career, 59-61; observations of eclipses, 61-64; density of the earth, 60, 261

Baily's Beads, 61, 62

Bakhuyzen, rotation of Mars, 275

Ball, Sir Robert, parallaxes of stars, 36 _note_, 416; contacts in transits, 239

Balmer's Law, 198, 383

Barnard, micrometrical measures of Neptune, 84; of minor planets, 288; of Saturn's rings, 301; photographs of solar corona, 186, 190; transit of Mercury, 245; halo round Venus, 254; surface of Mars, 280; ellipticity of Jupiter's first satellite, 292; of Uranus, 304; discovery of inner Jovian satellite, 293, 434; red spot on Jupiter, 296; eclipse of Japetus, 300; attendants on comet of 1882, 363; on Brooks's comet, 366, 367; Swift's comet, 368; photographic discovery of a comet, 369; observations of Nova Aurigæ, 398, 399; Hind's variable nebula, 403; exterior Pleiades nebulosities, 411; galactic stars, 423; photographs of Milky Way, 424, 425; cluster variables, 433; horizontal telescope, 438

Bartlett, photograph of a partial eclipse, 166

Basic lines, 206, 207

Baxendell, meteors of 1866, 331

Becker, drawings of solar spectrum, 211

Beckett, Sir E. (Lord Grimthorpe), value of solar parallax, 232

Beer and Mädler, surveys of lunar surface, 265, 267; studies of Mars, 275

Bélopolsky, coronal photographs, 185; theory of corona, 191; rotation of Venus, 252; of Jupiter, 297; spectroscopic determinations of Saturn's rings, 300; spectrum of Gamma Cassiopeiæ, 378; system of Castor, 389, 391; detection of variable stars as spectroscopic binaries, 391

Berberich, mass of asteroids, 287; orbit of Holmes's comet, 337

Berkowski, daguerrotype of eclipsed sun, 166

Bessel, biographical sketch, 28-30; reduction of Bradley's observations, 32; parallax of 61 Cygni, 36; disturbed motion of Sirius and Procyon, 41; trans-Uranian planet, 79; Halley's comet, 102; theory of instrumental errors, 122; personal equation, 123; rotation of Mercury, 246; lunar atmosphere, 263; cometary emanations, 325, 345; multiple tails, 347; comet of 1807, 352

Betelgeux, remoteness, 37, 417; spectrum, 373, 381, 383, 384; radial movement, 387

Bianchini, rotation of Venus, 250

Biela, discovery of a comet, 95

Bigelow, magnetic and solar disturbances, 161; theory of corona, 191

Bigourdan, eclipse of 1893, 187; velocity of comet of 1882, 364

Bird's quadrants, 4, 112, 121

Birmingham, colours of stars, 375 _note_; discovery of T Coronæ, 393

Birt, rotation of a sun-spot, 144; Selenographical Society, 266

Bischoffsheim, Coudé telescope, 436

Black Ligament, 235

Bode, popular writings, 5; solar constitution, 57; missing planet, 72, 73

Bode's Law, 71, 83, 286

Boeddicker, heat-phases during lunar eclipses, 269, 270; drawings of Jupiter, 296; of the Milky Way, 424

Boehm, solar observations, 146, 148

Boguslawski, centre of sidereal revolutions, 41; observation of Halley's comet, 102

Bolometer, principle of construction, 222

Bond, G. P., his father's successor, 86; light of Jupiter, 289; Saturn's rings, 298; Donati's comet, 324, 325; Andromeda nebula, 409; double-star photography, 409

Bond, W. C., observation of Neptune's satellite, 84; discovery of Hyperion, 85; of Saturn's dusky ring, 86; resolution of nebulæ, 119; celestial photography, 153, 409; satellite-transit on Jupiter, 291

Borda, repeating circle, 121

Boss, solar translation, 40; observations on comets, 352, 356

Bossert, proper motions of stars, 415

Bouguer, solar atmospheric absorption, 221

Boulliaud, period of Mira, 10

Bouvard, tables of Uranus, 78; Encke's comet, 90

Boys, radio-micrometer, 220; density of the earth, 261

Bradley, discoveries of aberration and nutation, 3; solar translation, 10; star-distances, 10, 16; observation on Castor, 17; instruments, 28, 120; observations reduced by Bessel and Auwers, 32, 39

Brahe, Tycho, star of 1572, 24

Brandes, observations of meteors, 327, 334;

Braun, prominence photography, 197; density of the earth, 261

Brayley, meteoric origin of planets, 311

Brédikhine, theory of cometary appendages, 100, 348; repulsive forces, 346, 347; chemical differences, 347, 348; formative types, 351, 352, 355, 363, 369; structure of chromosphere, 199; red spot on Jupiter, 294; Andromede meteors, 337; stationary radiants, 341; spectrum of Coggia's comet, 343

Bremiker, star maps, 81

Brenner, rotation of Venus, 252

Brester, _Théorie du Soleil_, 152

Brewster, diffraction theory of corona, 67; telluric lines in solar spectrum, 134; absorption spectra, 136

Brinkley, ostensible stellar parallaxes, 33

Brisbane, establishment of Paramatta Observatory, 6, 90

Brooks, fragment of 1882 comet, 363; cometary discoveries, 365, 366

Brünnow, stellar parallaxes, 113, 416

Bruno, Giordano, motion of stars, 9

Buffham, rotation of Uranus, 303

Buffon, internal heat of Jupiter, 289

Bunsen, discovery of spectrum analysis, 132

Burchell, magnitude of Eta Carinæ, 48

Burnham, stellar orbits, 46; coronal photographs, 186; measures of Nova Aurigæ, 399; of planetary nebulæ, 404; discoveries of double stars, 418, 430, 433, 435; catalogue, 419; system of 61 Cygni, 419

Burton, canals of Mars, 279; rotation of Jupiter's satellites, 292

Calandrelli, stellar parallaxes, 33

Callandreau, capture theory of comets, 98

Campani, Saturn's dusky ring, 86

Campbell, Lieutenant, polarisation of corona, 170

Campbell, Professor, stellar radial velocities, 39, 406, 434; flash spectrum, 189; spectroscopic observations of Saturn's rings, 300; Wolf-Rayet stars, 380; spectroscopic binaries, 389; Nova Aurigæ, 398; translation of solar system, 406; stellar diffraction-spectra, 440

Canals of Mars, 278-280

Cannon, Miss A. J., spectrographic researches, 386

Canopus, remoteness, 37, 417; spectrum, 416

Capella, spectrum, 373, 383, 384; a spectroscopic binary, 389

Carbon, material of photosphere, 152; absorption by, in sun, 212; in stars, 374

Carbonelle, origin of meteorites, 340

Carinæ, Eta, light variation, 48, 49; spectrum, 379

Carrington, astronomical career, 144, 145; sun-spot observations, 146; solar rotation, 147; spot-distribution, 148; luminous outburst on sun, 159, 160; Jovian and sun-spot periods, 163; origin of comets, 370

Cassini, Domenico, discoveries of Saturnian satellites, 84; of division in ring, 85; solar rotation period, 146; solar parallax, 228; rotation of Venus, 250; of Mars, 274; of Jupiter, 290, 295; satellite of Venus, 256; satellite-transit on Jupiter, 291

Cassini, J. J., stellar proper motions, 10; sun-spots on limb, 54; theory of corona, 66; rotation of Venus, 250; structure of Saturn's rings, 299

Castor, system of, 18, 389

Cavendish experiment, 60, 261

Ceres, discovery, 73, 74; diameter, 75, 288

Chacornac, observation of sun-spot, 156; star-maps, 284, 413; variable nebula, 403

Challis, search for Neptune, 81, 82; duplication of Biela's comet, 96

Charlois, discoveries of minor planets, 283

Charroppin, coronal photographs, 186

Chase, photographic discovery of a comet, 338; stellar parallaxes, 416

Chladni, origin of meteors, 327, 332

Christie, Mercurian halo, 245

Chromosphere, early indications, 68; distinct recognition, 69, 70, 167; depth, 174, 175, 200; metallic injections, 195; eruptive character, 199; spectrum, 200

Clark, Alvan, large refractors, 114, 429, 430, 436

Clark, Alvan G., discovery of Sirian companion, 42, 430; 40-inch refractor, 433

Clarke, Colonel, figure of the earth, 262

Clarke, F. W., celestial dissociation, 206

Clausen, period of 1843 comet, 105; cometary systems, 362

Clerihew, secondary tail of 1843 comet, 103

Clusters, variable stars in, 436

Coggia, discovery of a comet, 343

Comet, Halley's, return in 1759, 4, 88; orbit computed by Bessel, 29; capture by Neptune, 98, 365; return in 1835, 101-103, 345; type of tail, 346, 352; of 1843, 7, 103-105; type of tail, 346, 352; relationships, 349-351; Newton's, 88, 364; Encke's, 90; changes of volume, 92; of brightness, 95; acceleration, 93, 94; capture by Mercury, 99; Winnecke's, 94, 342; Biela's, 95-97, 333; Brorsen's, 97; Vico's, 97, 367; Faye's, 98; of 1811, 99-101, 346; of 1807, 100, 347, 352; of 1819, 101, 103; Lexell's, 106, 367; Tewfik, 178, 358, 362, 369; Donati's, 323-325, 347, 348; of 1861, 326, 327, 346; Perseid, 327, 332; Leonid, 327, 332, 333, 343; Klinkerfues's, 335; Holmes's, 337, 343, 369; Coggia's, 343, 346, 347; of 1901, 343; of 1880, 349, 351; Aristotle's, 350; Tebbutt's, 352-355; Schaeberle's, 355, 356; Wells's, 356, 357; of September, 1882, 358-361, 362-364; Thome's, 361; Pons-Brooks, 365, 366; Sawerthal's, 366; Brooks's, of 1889, 366, 367; Swift's, 368

Cometary tails, repulsive action upon, 100, 103, 104, 346-348; coruscations in, 105; three types, 346-348, 355, 363; multiple, 347, 348, 351, 352, 355, 363, 368

Comets, subject to gravitation, 88; of short period, 92, 93; translucency, 95, 105, 106, 353; small masses, 96, 106; capture by planets, 98, 306, 367; changes of volume, 102, 365, 369; polarisation of light, 103, 354, 355; refractive inertness, 106, 353; relations to meteor-systems, 327, 332-336; disintegration, 333, 339, 362, 363; spectra, 342-344, 354, 355, 362-364; luminous by electricity, 344, 355, 357; systems, 353, 355, 357, 362, 365; origin, 369-371

Common, reflectors for eclipse photography, 187; Jupiter's inner satellite, 293; detection of great comet near the sun, 358; its five nuclei, 362; photographs of Andromeda nebula, 395; of Orion nebula and Jupiter, 407, 408; great reflectors, 412, 429; cluster variables, 436

Common, Miss, drawing of eclipsed sun, 187

Comstock, lunar atmosphere, 264

Comte, celestial chemistry, 140; astronomy, 142

Cooke, 25-inch refractor, 430

Copeland, comets of 1843 and 1880, 349; spectrum of comet of 1882, 364; of Gamma Cassiopeiæ, 378; of Nova Andromedæ, 395; of Orion nebula, 407; discoveries of gaseous stars, 379; Nova Aurigæ, 396, 398

Copernicus, stellar parallax, 16

Cornu, telluric lines in solar spectrum, 202; velocities in prominences, 205; ultra-violet solar spectrum, 210, 215; velocity of light, 232 _note_, 241; spectrum of hydrogen, 383; of Nova Cygni, 393

Cornu and Bailie, density of the earth, 261

Corona of 1842, 62-64, 67; early records and theories, 65-67; photographs, 166, 173, 178, 181, 185-190; spectrum, 170, 173, 178, 188, 190, 193; varying types, 174-176, 193; of 1877, 175-177; of 1882, 177; of 1869, 183; of 1886, 185; of 1889, 185-187; of 1893, 188; of 1898, 189; of 1900, 189; of 1901, 190; daylight photography of, 178-180; glare theory, 182; mechanical theory, 191; electro-magnetic theories, 191, 192

Coronium, 171, 174, 193

Cortie, movements in sun-spots, 157; their spectral changes, 208

Cotes, corona of 1715, 176

Croll, secular changes of climate, 259, 260; derivation of solar energy, 313

Crookes, chemical elements, 210

Crova, solar constant, 225

Cruls, comet of 1882, 358, 364

Cusa, solar constitution, 57

Cysatus, Orion nebula, 21; comet of 1618, 362

Damoiseau, theory of Halley's comet, 101

D'Arrest, orbits of minor planets, 285; Andromede meteors, 334; ages of stars, 375; variable nebulæ, 403; measures of nebulæ, 404

Darwin, G. H., rigidity of the earth, 258; Saturn's ring system, 301; origin of the moon, 316-318; development of solar system, 318, 319, 322; solar tidal friction, 319

Daubrée, falls of aerolites, 339

Davidson, satellite-transit on Jupiter, 292

Davis, stellar parallaxes, 417

Dawes, prominences in 1851, 70; Saturn's dusky ring, 86; a star behind a comet, 106; solar observations, 143, 164; observations and drawings of Mars, 276, 278, 280; satellite-transits on Jupiter, 291, 292

De Ball, markings on Mercury, 248

Delambre, Greenwich observations, 3; solar rotation, 146; light-equation, 231

De la Roche, Newton's law of cooling, 217

De la Rue, celestial photography, 152, 153, 268; solar investigations, 154; expedition to Spain, 166, 167

De la Tour, experiments on liquefaction, 151

Delaunay, tidal friction, 271, 272; Coudé telescope, 436

Delisle, diffraction theory of corona, 67; transits of Venus, 233, 239

Dembowski, double star measurements, 418

Denning, observations of Mercury, 246, 247; mountain on Venus, 253; rotation of Jupiter, 290; red spot, 295; periodicity of markings, 297; rotation of Saturn, 302; meteors of 1885, 336; of 1892, 338; stationary radiants, 341

Denza, meteors of 1872, 334

Derham, theory of sun-spots, 53; ashen light on Venus, 255

Deslandres, eclipse expedition, 187; rotation of corona, 188; prominence photography, 198; hydrogen spectrum in prominences, 198, 383; photographs of Jupiter, 297; radial movements of Saturn's rings, 300; helium absorption in stars, 376; stellar radial velocities, 406

Diffraction, corona explained by, 67, 70, 181; spectrum, 139, 210, 223, 439

Dissociation in the sun, 152, 206-210; in space, 312

Doberck, orbits of double stars, 38, 418

Dollond, discovery of achromatic telescope, 4, 112

Donati, discovery of comet, 323; spectra of comets, 342; of stars, 372

Doppler, effect of motion on light, 200

Douglass, observations of Jupiter's satellites, 292

Downing, perturbations of the Leonids, 338

Draper, H., ultra-violet spectrum, 210; oxygen in sun, 213; photographs of the moon, 268; of Jupiter's spectrum, 291; of Tebbutt's comet, 354; of spectrum of Vega, 382; of Orion nebula 407

Draper, J. W., lunar photographs, 152; distribution of energy in spectrum, 223 _note_

Draper Memorial, 384-386

Dreyer, Catalogue of Nebulæ, 50

Dulong and Petit, law of radiation, 217, 219

Dunér, spectra of sun-spots, 156; spectroscopic measurement of solar rotation, 203; spectroscopic star catalogue, 381

Dunkin, solar translation, 39

Duponchel, sun-spot period, 163

Durchmusterung, Bonn, 33, 412; Cape photographic, 412; parallax, 418; photometric, 421

Dyson, coronal photographs, 190

Earth, mean density, 60, 261; knowledge regarding, 257; rigidity, 257, 259; variation of latitude, 258, 259; figure, 261, 262; effects of tidal friction, 271-273; bodily tides, 316; primitive disruption, 317

Easton, structure of Milky Way, 423, 424

Ebert, coronoidal discharges, 192

Eclipse, solar, of 1836, 61; of 1842, 62-65, 67, 69; of 1851, 69, 70, 166; of 1860, 166, 167; of 1868, 167-170; of 1869, 170; of 1870, 171; of 1871, 173; of 1878, 174-177; of 1882, 177, 178; of 1883, 180, 181; of 1885, 183; of 1886, 184; of 1887, 185; of 1889, 185-187; of 1893, 187, 188; of 1896, 188; of 1898, 189; of 1900, 189, 190; of 1901, 190

Eclipses, lunar, heat-phases during, 269, 270

Eclipses, solar, importance, 59; ancient, 60, 273; classification, 61; results, 192, 193

Eddie, comet of 1880, 349; of 1882, 363

Edison, tasimeter, 177

Egoroff, telluric lines in solar spectrum, 211, 214

Elements, chemical, dissociation in sun, 206, 209, 210

Elkin, star parallaxes, 37, 416, 417; photography of meteors, 338; transit of great comet, 358, 360; secondary tail, 363; triangulation of the Pleiades, 410

Elliot, opinions regarding the sun, 57

Elvins, red spot on Jupiter, 296

Encke, star maps, 78; calculation of short-period comet, 90; resisting medium, 93; distance of the sun, 230, 232; period of Pons's comet, 365

Engelmann, rotation of Jupiter's satellites, 292

Ericsson, solar temperature, 218

Erman, meteoric rings, 330

Eros, measures of, for solar parallax, 238; discovery, 284; variability, 285

Ertborn, mountain in Venus, 253

Espin, spectra of variable stars, 379; stars with banded spectra, 381; Nova Aurigæ, 397, 398

Euler, resisting medium, 93

Evershed, eclipse photographs, 189, 200

Evolution, of solar system, 308, 309, 313-316, 322; of earth-moon system, 316-318; of stellar systems, 420

Fabricius, David, discovery of Mira Ceti, 10

Fabricius, John, detection of sun-spots, 52

Faculæ, relation to spots, 53, 155, 158; solar rotation from, 155; photographed, 197, 198, 377

Faye, nature of prominences, 70, 166; discovery of a comet, 98; cyclonic theory of sun-spots, 144, 157; solar constitution, 150-152; maximum of 1883, 163; velocities in prominences, 205; distance of the sun, 240; planetary evolution, 314, 315, 321

Feilitsch, solar appendages, 70

Fényi, solar observations, 184, 204

Ferrel, tidal friction, 272

Ferrer, nature of corona, 67; prominences, 69

Fessenden, electrical theory of comets, 348

Finlay, transit of great comet, 358, 360

Fizeau, daguerrotype of the sun, 153; Doppler's principle, 201; velocity of light, 232

Flammarion, canals of Mars, 280; trans-Neptunian planet, 306

Flamsteed, solar constitution, 57; distance, 228

Flaugergues, detection of 1811 comet, 99; transit of Mercury, 244

Fleming, Mrs., spectrum of Beta Lyræ, 379; preparation of Draper Catalogue, 386; discoveries of new stars, 399

Flint, star-parallaxes, 417

Fontana, mountains of Venus, 252; satellite, 256; spots on Mars, 274

Forbes, George, trans-Neptunian planets, 306, 307

Forbes, James D., spectrum of annularly eclipsed sun, 134; solar constant, 225

Foucault, spectrum of voltaic arc, 137; photograph of the sun, 153; velocity of light, 232, 240; silvered glass reflectors, 429

Fraunhofer, early accident, 33; improvement of refractors, 34; clockwork motion, 121; spectra of flames, 131; of sun and stars, 133, 134, 372; objective prism, 385; diffraction gratings, 439

Fraunhofer lines, mapped, 133, 136; origin, 135-137, 171, 172; reflected in coronal spectrum, 170, 173, 181; in cometary spectra, 354, 357; shifted by radial motion, 201

Freycinet, distribution of minor planets, 287

Fritz, auroral periodicity, 162

Frost, solar heat radiation, 222

Galileo, descriptive astronomy, 2; double-star method of parallaxes, 16; discovery of sun-spots, 52; solar rotation, 146; planets and sun-spots, 163; darkening at sun's edge, 221

Galle, discovery of Neptune, 81, 82; Saturn's dusky ring, 86; distance of the sun, 237; path of Andromede meteors, 334

Galloway, solar translation, 39

Gambart, discovery of comet, 95

Gauss, orbits of minor planets, 74; _Theoria Motus_, 77; magnetic observations, 126, 127; cometary orbits, 370

Gautier, sun-spot and magnetic periods, 126, 128; sun-spots and weather, 129

German Astronomical Society, 6, 414

Gill, star-parallaxes, 37, 42, 416, 417; expedition to Ascension, 237; distance of the sun, 237, 238, 240; constant of aberration, 241; arc measurements, 261, 262; comet of 1882, 359, 412; oxygen-absorption in stars, 384; photograph of Argo nebula, 404; Cape Durchmusterung, 412; photographic celestial survey, 413;

## actinic intensity of galactic stars, 425;

Coudé telescope, 438

Gladstone, J. H., spectrum analysis, 134, 136

Glaisher, occultation by Halley's comet, 106

Glasenapp, coronal photographs, 185; light equation, 231, 241; double star measures, 419

Glass, optical, excise duty on, 112, 115; Guinand's, 113, 114; Jena, 431

Gledhill, spot on Jupiter, 294

Goldschmidt, nebulæ in the Pleiades, 411

Goodricke, periodicity of Algol, 390

Gore, catalogue of variable stars, 391; of computed binaries, 418

Gothard, bright-line stellar spectra, 378, 379; spectrum of Nova Aurigæ, 398; photographs of nebulæ, 409

Gould, variation of latitude, 258; photograph of Mars, 281; comets of 1807 and 1881, 349, 352; luminous instability of stars, 392; photographic measures of the Pleiades, 410; _Uranometria Argentina_, 415; solar cluster, 423, 426

Graham, discovery of Metis, 77

Grant, solar envelope, 70, 167; transit phenomena, 254

Green, observation of Mars, 280

Greenwich observations, 3, 27, 32

Gregory, David, achromatic lenses, 112 _note_

Gregory, James, double star method of parallaxes, 16; reflecting telescopes, 109

Groombridge, star catalogue, 31

Grosch, corona of 1867, 176

Grubb, Sir Howard, photographic reflector, 409; great refractors, 430, 433; siderostat, 437

Grubb, Thomas, Melbourne reflector, 110 _note_, 428

Gruithuisen, snow-caps of Venus, 255; lunar inhabitants, 265

Gully, detection of Nova Andromedæ, 394

Guthrie, nebulous glow round Venus, 253

Hadley, Saturn's dusky ring, 86; reflecting telescope, 109

Haerdtl, Winnecke's comet, 94

Hale, luminous outburst on sun, 161; daylight coronal photography, 179; spectrum of prominences, 195, 198; prominence photography, 197, 198; photographs of faculæ, 198, 377; carbon in chromosphere, 200; bright lines in fourth-type stars, 381; reflectors and refractors, 432

Hall, Asaph, parallax of the sun, 241; discovery of Martian satellites, 282; rotation of Saturn, 302; double star measurements, 419

Hall, Chester More, invention of achromatic telescope, 112

Hall, Maxwell, rotation of Neptune, 305

Halley, stellar proper motions, 9; composition of nebulæ, 22; observation of Eta Carinæ, 48; eclipse of 1715, 66, 68; predicted return of comet, 88; magnetic theory of auroræ, 130; transits of Venus, 233; lunar acceleration, 271; origin of meteors, 327

Halm, magnetic relations of latitude variation, 259

Hansen, solar parallax from lunar theory, 230

Hansky, coronal photographs, 188, 189

Harding, discovery of Juno, 75; celestial atlas, 77

Harkness, spectrum of corona, 170; corona of 1878, 175; shadow of the moon in solar eclipses, 182; light equation, 231; distance of the sun, 237, 240, 241, 242

Harriot, observations on Halley's comet, 29

Hartley, gallium in the sun, 200, 213

Hartwig, Nova Andromedæ, 394

Hasselberg, metallic spectra, 211; spectra of comets, 342, 357; of Nova Andromedæ, 395

Hastings, composition of photosphere, 152; observations at Caroline Island, 181; Saturn's dusky ring, 299

Hegel, number of the planets, 73

Heis, radiant of Andromedes, 334

Heliometer, 34, 234, 237, 238, 240

Helium, a constituent of prominences, 194, 195, 199; no absorption by, in solar spectrum, 213; absorptive action in first-type stars, 376; bright in gaseous stars, 377, 378, 380; in Orion nebula, 407

Helmholtz, gravitational theory of sun-heat, 311-313

Hencke, discoveries of minor planets, 76

Henderson, parallax of Alpha Centauri, 36, 416; observation of chromosphere, 68

Henry, Paul and Prosper, lunar twilight, 264; markings on Uranus, 303; photograph of Saturn, 408; photographs of nebulæ in the Pleiades, 410, 411; stellar photography, 413; plane mirrors, 438

Herrick and Bradley, duplication of Biela's comet, 96

Herschel, Alexander S., cometary and meteoric orbits, 332

Herschel, Caroline, her brother's assistant, 12; observation of Encke's comet, 90

Herschel, Colonel, spectrum of prominences, 168; of reversing layer, 172; of corona, 174

Herschel, Sir John, life and work, 45-50; Magellanic clouds, 47, 422; sun-spots, 58, 59, 144; solar flames, 68; anticipation of Neptune's discovery, 81; status of Hyperion, 85; Biela's comet, 95; Halley's, 102; comet of 1843, 103; sixth star in "trapezium," 113; grinding of specula, 116; spectrum analysis, 136; solar photography, 145, 154; solar constitution, 151; shadow round eclipsed sun, 182;

## actinometrical experiments, 216;

solar heat, 217; climate and eccentricity, 259; lunar atmosphere, 263; surface of Mars, 276; Andromeda nebula, 396; observations of nebulæ, 404; double nebulæ, 412

Herschel, Sir William, discovery of Uranus, 5; founder of sidereal astronomy, 9; biographical sketch, 11-14; sun's motion in space, 15, 39, 425; revolutions of double stars, 18, 442; structure of Milky Way, 19-21, 423; nature of nebulæ, 21-26, 401; results of his observations, 25; centre of sidereal system, 40; theory of the sun, 54-56, 70; asteroids, 75; discoveries of Saturnian and Uranian satellites, 84, 87, 110; comet of 1811, 99; reflecting telescopes, 109-111; sun-spots and weather, 129; transit of Mercury, 244; refraction in Venus, 252; lunar volcanoes, 266; terrestrial affinity of Mars, 274; Jovian trade-winds, 289; rotation of Jupiter's satellites, 292; ring of Saturn, 298; rotation of Saturn, 302; origin of comets, 369; stellar photometry, 420

Herz, comets' tails, 348

Hevelius, "Mira" Ceti, 10; contraction of comets, 92; granular structure of comet, 362

Higgs, photographs of solar spectrum, 211, 214

Hind, solar flames, 70; Iris and Flora discovered by, 77; distortion of Biela's comet, 96; transit of a comet, 101; earth in a comet's tail, 326; comets of 1843 and 1880, 349; Schmidt's comet, 363; new star, 392; variable nebula, 403

Hirn, solar temperature, 220; resistance in space, 348

Hodgson, outburst on the sun, 160

Hoeffler, star-drift in Ursa Major, 426

Hoek, cometary systems, 362

Holden, Uranian satellites, 87; eclipse expedition, 180; coronal extensions, 186; solar rotation, 203; transit of Mercury, 245; intra-Mercurian planets, 250; drawing of Venus, 252; lunar photographs, 268; canals on Mars, 279; surface of Mars, 281; transits of Jupiter's satellites, 292; markings on Uranus, 304; disintegration of comet, 362; colours of double stars, 374; Nova Aurigæ, 398; Orion and Trifid nebulæ, 403, 404; director of Lick Observatory, 435

Holden and Schaeberle, observations of nebulæ, 433

Holmes, discovery of a comet, 337

Homann, solar translation, 406

Hooke, solar translation, 10; stellar parallax, 16; repulsive action on comets, 102 _note_; automatic movement of telescopes, 120; spots on Mars, 274, 275

Hopkins, solidity of the earth, 257

Horrebow, sun-spot periodicity, 125; satellite of Venus, 256

Hough, G. W., red spot on Jupiter, 295, 430; observations of double stars, 419

Houzeau, solar parallax, 240

Howlett, sun-spot observations, 155

Hubbard, period of comet of 1843, 105, 351

Huggins, Sir William, spectroscopic observations of prominences, 170, 195; hydrogen spectrum in stars, 178, 198; daylight coronal photography, 178, 179; repulsive action in corona, 191; stellar motions in line of sight, 201, 386, 387; transit of Mercury, 245; occultation of a star, 263; snowcaps on Mars, 276; spectrum of Mars, 277; of Jupiter, 290; Jovian markings and sun-spots, 297; spectrum of Uranus, 304; of comets, 342, 343; photographs, 354, 357; stellar spectroscopy, 373; colours of stars, 374; classification of star spectra, 376; photographs, 382, 383, 438; stellar chemistry, 381, 382; spectra of new stars, 393, 395; theory of Nova Aurigæ, 397; spectra of nebulæ, 401, 402, 407; nebular radial movement, 405

Huggins, Sir William and Lady, photograph of Uranian spectrum, 305; spectra of Wolf-Rayet stars, 380; ultra-violet spectrum of Sirius, 383; nitrogen in stars, 384; spectrum of Nova Aurigæ, 396-398; of Andromeda nebula, 403; of Orion nebula, 407

Humboldt, sun-spot period, 126; magnetic observations, 127; meteoric shower, 329

Hussey, T. J., search for Neptune, 79

Hussey, W. J., cloud effects on Mars, 281; cometary appendages, 369; period of Delta Equulei, 419; discoveries of double stars, 419, 433

Huygens, stellar parallax, 16; Orion nebula, 22; discovery of Titan, 84; Saturn's ring, 85, 301; spot on Mars, 275

Hydrogen, a constituent of prominences, 168, 195, 199; spectrum, 178, 198, 383, 384; absorption in stars, 198, 373, 381-383; in sun, 211; theoretical material of comets' tails, 347; emissions in stars, 377-380, 384, 393, 397; in nebulæ, 402, 407

Innes, Southern double stars, 419

Jacoby, measurement of Rutherfurd's plates, 410; Pritchard's parallax work, 417

Janssen, photographs of the sun, 165; spectroscopic observations of prominences, 168, 169; escape from Paris in a balloon, 171; coronal spectrum, 173, 181; coronal photographs, 181; rarefaction of chromospheric gases, 182; oxygen absorption in solar spectrum, 214; transit of 1874, 234; spectrum of Venus, 254; of Saturn, 303; photographs of Tebbutt's comet, 353, 354; of Orion nebula, 407

Japetus, eclipse of, 300; variability in light, 302

Jewell, solar spectroscopy, 200, 211

Joule, heat and motion, 309

Jupiter, mass corrected, 77, 92; conjectured influence on sun-spot development, 163; physical condition, 289, 290; spectrum, 290, 291; satellite-transits, 291, 292; discovery of inner satellite, 293; red spot, 293-296; photographs, 297, 408; periodicity of markings, 297

Kaiser, rotation of Mars, 275; map of Mars, 278

Kammermann, observation of Maia nebula, 410

Kant, status of nebulæ, 14; Sirius the central sun, 40; planetary intervals, 71; tidal friction, 272; condition of Jupiter, 289; cosmogony, 308

Kapteyn, solar translation, 40; Cape Durchmusterung, 412; stellar parallaxes, 417, 418;

## actinic intensity of galactic stars, 425;

solar cluster, 426

Kayser and Runge, spectroscopic investigations, 211, 213

Keeler, red spot on Jupiter, 296; spectroscopic determination of movements in Saturn's rings, 300; spectrum of Uranus, 304; of third type stars, 382; of nebulæ, 402; photographs of nebulæ, 403, 411, 412, 432; nebular radial movements, 405, 434, 440; grating spectroscope, 440

Kepler, star of 1604, 25; solar corona, 66; missing planets, 71; cometary decay, 91, 339; comet of 1618, 96; physical astronomy, 141

Kiaer, comets' tails, 348

Kirchhoff, foundation of spectrum analysis, 132, 135-137, 372; map of solar spectrum, 137; solar constitution, 149, 151, 172

Kirkwood, distribution of minor planets, 286; grouped orbits, 287; divisions in Saturn's rings, 301, 302; origin of planets, 314; their mode of rotation, 321; comets and meteors, 333, 339

Kleiber, Perseid radiants, 341

Klein, Hyginus N., 267, 268

Klinkerfues, comet predicted by, 335, 339; apparitions of Southern comet, 350; tidal theory of new stars, 397

Knobel, cloud effects on Mars, 281

Konkoly, spectrum of Gamma Cassiopeiæ, 378; spectroscopic survey, 381 _note_

Kreil, lunar magnetic action, 130

Kreutz, period of 1843 comet, 105; orbit of 1861 comet, 327; period of great September comet, 361; cause of disintegration, 363; eclipse-comet of 1882, 362

Krüger, segmentation of great comet, 362

Küstner, variation of latitude, 258

Kunowsky, spots on Mars, 275

Lacaille, southern nebulæ, 22; Eta Carinæ, 48

Lagrange, theory of solar system, 2; planetary disruption, 76

Lahire, diffraction theory of corona, 67; distance of the sun, 228; mountains of Venus, 252

Lalande, popularisation of astronomy, 5; revolving stars, 18; _Histoire Céleste_, 31, 415; nature of sun-spots, 53; observations of Neptune, 83

Lambert, solar motion, 10; construction of the universe, 14, 40; missing planets, 71

Lamont, magnetic period, 127, 128

Lamp, ashen light on Venus, 256

Langdon, mountains of Venus, 253

Langley, solar granules, 165; corona of 1878, 176; spectroscopic effects of solar rotation, 202; infra-red spectrum, 210, 223, 224; experiments at Pittsburg, 221; bolometer, 222; distribution of energy in spectrum, 224, 225; atmospheric absorption, 224, 225, 276; solar constant, 225; lunar heat-spectrum, 269; temperature of lunar surface, 270; age of the sun, 312

Laplace, lunar acceleration, 2, 271; _Système du Monde_, 5; nebular hypothesis, 25, 308, 309, 313, 314, 322; stability of Saturn's rings, 85, 298; solar atmosphere, 94, 221; Lexell's comet, 106, 367; solar distance by lunar theory, 230; origin of meteors, 328; of comets, 370

Lassell, discovery of Neptune's satellite, 83; of Hyperion, 85; Saturn's dusky ring, 86; observations at Malta, 87, 434; reflectors, 114; equatoreal mounting, 121

Latitude, variation of, 258, 259

Laugier, period of 1843 comet, 105; solar rotation, 146

Le Chatelier, temperature of the sun, 219

Lescarbault, pseudo-discovery of Vulcan, 248; halo round Venus, 254

Lespiault, orbits of minor planets, 285

Le Sueur, spectrum of Jupiter, 291

Leverrier, discovery of Neptune, 80-82; Lexell's comet, 98, 367; distance of the sun, 230, 240; revolutions of Mercury, 248; supposed transits of Vulcan, 249; mass of asteroids, 287; orbit of November meteors, 332; Perseids and Leonids, 333

Lexell, comet of 1770, 98, 106, 367

Liais, supposed transit of Vulcan, 249; comet of 1861, 326; division of a comet, 339

Librations, of Mercury, 247; of Venus, 251; of the moon, 266

Lick, foundation of observatory, 434

Light, velocity, 38, 232, 241; extinction in space, 45; refrangibility changed by movement, 201

Light-equation, 231, 241

Ligondès, development of solar system, 316

Lindsay, Lord, expedition to Mauritius, 234

Line of sight, movements in, 201, 386; of solar limbs, 202, 203; in prominences, 204, 208; of stars, 201, 386, 387; binaries detected by, 387-391

Listing, dimensions of the globe, 262

Littrow, chromosphere, 70; sun-spot periodicity, 126

Liveing and Dewar, carbon in the sun, 212

Lockyer, solar spectroscopy, 156, 212; theory of sun-spots, 159, 163; daylight observations of prominences, 169, 194, 204, 205; eclipse of 1870, 171; slitless spectroscope, 173; corona of 1878, 175; glare theory of corona, 182; eclipse of 1886, 184; chromospheric spectrum, 195; classification of prominences, 196; their radial movements, 204; celestial dissociation, 206-210; chemistry of sun-spots, 207; spots on Mars, 275; meteoritic hypothesis, 376, 402; equatoreal Coudé, 438

Loewy, constant of aberration, 241, 438; lunar photographs, 268; director of Paris Observatory, 414; equatoreal Coudé, 436, 437

Lohrmann, lunar chart, 265; Linné, 267

Lohse, J. G., spectrum of great comet, 364

Lohse, O., daylight coronal photography, 178 _note_; spectral investigations, 211; twilight on Venus, 256; red spot on Jupiter, 294; periodicity of Jupiter's markings, 297; motion of Sirius, 386; spectrum of Nova Cygni, 393

Louville, nature of corona, 67; chromosphere, 68

Lowell, rotation of Mercury, 248; of Venus, 252; markings on Venus, 255; observations of Mars, 280, 281; satellites, 283

Lyman, atmosphere of Venus, 254

McClean, photographs of solar spectrum, 211, 215; helium stars, 377; oxygen stars, 384; equipment of Cape Observatory, 433

Macdonnell, luminous ring round Venus, 254

Maclaurin, eclipse of 1737, 65

Maclear, Admiral, observations during eclipses, 172, 182

Maclear, Sir Thomas, maximum of Eta Carinæ, 49; observation of Halley's comet, 102

Mädler, central sun, 41; observations of Venus, 253; lunar rills, 263; aspect of Linné, 267; common proper motions, 426

Magellanic clouds, 47, 422; spiral character, 425

Magnetism, terrestrial, international observations, 126; periodicity, 127, 128; solar relations, 128, 160, 161, 163, 205; lunar influence, 130

Mann, last observation of Donati's comet, 325

Maraldi, solar corona, 67; rotation of Mars, 274; satellite-transits on Jupiter, 291; spot on Jupiter, 295

Marius, Andromeda nebula, 21; sun-spots, 52

Mars, oppositions, 228; solar parallax from, 228, 237, 240; polar spots, 274, 276, 277, 280, 281; permanent markings, 274-276; rotation, 274, 275; atmosphere, 276, 277; climate, 277, 278; canals, 278-281; photographs, 281; satellites, 282, 283, 314, 320, 321

Marth, revolutions of Neptune's satellite, 305

Maskelyne, components of Castor, 18; Astronomer-Royal, 27; experiment at Schehallien, 261; comets and meteors, 332

Maunder, photographs of corona of 1886, 185; comparative massiveness of stars, 375; constitution of nebulæ, 403

Maunder, Mrs., coronal photographs, 189, 190

Maury, director of Naval Observatory, 7; duplication of Biela's comet, 96

Maury, Miss A. C., spectrographic investigations, 386; discoveries of spectroscopic binaries, 387, 388

Maxwell, J. Clerk, structure of Saturn's rings, 298, 300

Mayer, C., star satellites, 17

Mayer, Julius R., tidal friction, 272; meteoric sustentation of sun's heat, 310

Mayer, Tobias, stellar motions, 10; solar translation, 15; repeating circle, 122; solar distance, 230; satellite of Venus, 256; lunar surface, 263

Mazapil meteorite, 340

Meldrum, sun-spots and cyclones, 164

Melloni, lunar heat, 269

Melvill, spectra of flames, 131

Mercury, mass, 92; luminous phenomena during transits, 244, 245; spectrum, 245; mountainous conformation, 246, 247; rotation, 247, 248; theory of movements, 248, 250

Mersenne, reflecting telescope, 108

Messier, catalogue of nebulæ, 22

Meteoric hypothesis of solar sustentation, 310; of planetary formation, 311

Meteoritic hypothesis of cosmical constitution, 376, 402

Meteors, origin, 327, 328; relations to comets, 327, 332-334, 340; Leonids, 328-334, 338; Perseids, 329, 332, 333, 341; Andromedes, 334-338; stationary radiants, 341

Meunier, canals of Mars, 280

Meyer, divisions of Saturn's rings, 302; comet of 1880, 351; cometary refraction, 353; comet Tewfik, 362

Michell, double stars, 17; torsion balance, 261; star systems, 426

Michelson, velocity of light, 241

Milky Way, grindstone theory, 14; clustering power, 20, 26; structure, 20, 41, 45, 47, 423-425; centre of gravity, 40, 41; frequented by Wolf-Rayet, temporary, and helium stars, 380, 399, 425; by gaseous nebulæ, 402; drawings and photographs, 424, 425

Miller, W. A., spectrum analysis, 132, 136, 137; stellar chemistry, 373

Mira, light changes, 10; spectrum, 374, 379

Mitchel, lectures at Cincinnati, 6

Mitchell, photograph of reversing layer, 190

Möller, theory of Faye's comet, 98

Mohn, origin of comets, 370

Moll, transit of Mercury, 245

Monck, Perseid meteors, 341; new stars, 395

Moon, acceleration, 2, 271, 272; magnetic influence, 130; photographs, 152, 153, 268; solar parallax from disturbed motion, 230, 240; study of surface, 263; atmosphere, 263-265; charts, 265-267; librations, 266; superficial changes, 267, 268; thermal radiations, 269, 270; rotation, 272; tables, 272, 273; origin, 316-318

Morinus, celestial chemistry, 140

Morstadt, Andromede meteors, 332

Mouchez, photographic survey of the heavens, 413; death, 414

Müller, phases of Mercury, 246; of minor planets, 288; albedo of Mars, 283; of Jupiter, 290; of Saturn, 303; variability of Neptune, 305; of Pons's comet, 366; stellar photometry, 421

Munich, Optical Institute, 28, 34

Myer, solar eclipse, 183

Nasmyth, Lassell's reflector, 83; solar willow-leaves, 164; comparative lustre of Mercury and Venus, 255; condition of Jupiter, 289

Nasmyth and Carpenter, _The Moon_, 265

Nebula, Andromeda, early observations, 21; new star in, 394, 395; photographs, 395, 409; structure, 396; spectrum, 402, 403; visibility at Arequipa, 435

Nebula, Orion, observed by Herschel, 12; mentioned by Cysatus, 21; apparent resolvability, 119; suspected variability, 403; radial movement, 405; spectrum, 407; photographs, 407, 408, 436

Nebulæ, first discoveries, 22; catalogues, 22, 46, 50, 412; distribution, 23, 48, 422; composition, 24, 47, 401, 402; resolution, 47, 117, 119; double, 48, 412; spiral, 118, 410, 412; new stars in, 394-396, 399, 401; spectra, 401-403, 407; variability, 403, 404; radial movements, 405; photographs, 407-409, 425

Nebular hypothesis, Herschel's, 24, 25; Laplace's, 25, 308, 309, 322; objections, 313-315

Neison, atmosphere of Venus, 254; rills on the moon, 263; _The Moon_, 265

Neptune, discovery, 78-83; satellite, 83, 305; density, 84; comets captured by, 98, 306, 365; mode of rotation, 305, 313, 315, 322

Newall, F., duplicity of Capella, 389; stellar radial motions, 430

Newall, R. S., 25-inch refractor, 430

Newcomb, runaway stars, 39; solar translation, 40; origin of minor planets, 76; telescopic powers, 119; corona of 1878, 176; of 1869, 183; distance of the sun, 231-233; velocity of light, 241; variation of latitude, 259; lunar atmosphere, 263; lunar theory, 272, 273; disturbance of Neptune's satellite, 305; formation of planets, 314; star catalogue, 415; structure of Milky Way, 423

Newton, H. A., capture of comets by planets, 98; falls of aerolites, 311; November meteors, 330, 331; meteors of 1885, 336, 337; orbits of aerolites, 340

Newton, Sir Isaac, founder of theoretical astronomy, 1, 141; comets subject to gravitation, 88; first speculum, 109; solar radiations, 216; law of cooling, 217-219; telescopes and atmosphere, 434

Niesten, volume of asteroids, 287; red spot on Jupiter, 293

Nobert, diffraction gratings, 439

Noble, observations of Mercury, 246; secondary tail of comet, 355

Nolan, origin of the moon, 317; period of Phobos, 320

Norton, expulsion theory of solar appendages, 193 _note_; comets' tails, 345, 347

Nova Andromedæ, 394, 395

Nova Aurigæ, 396-399

Nova Cygni, 393, 394, 398

Nova Persei, 400, 401

Nutation, discovered by Bradley, 3, 15; a uranographical correction, 31

Nyrén, constant of aberration, 241

Observatory, Greenwich, 3, 27, 433; Cape of Good Hope, 6, 36, 433; Paramatta, 6, 90; Harvard College, 7, 85; Königsberg, 30; Dorpat, 43; Pulkowa, 44; Palermo, 72; Berlin, 90; Anclam, 149; Potsdam, 149; Kew, 153; Arequipa, 264, 435, 436; Yerkes, 433; Lick, 435

Occultations of stars by comets, 95, 105, 106; by the moon, 263; by Mars, 276; of Jupiter by the moon, 264

Olbers, Bessel's first patron, 29, 30; discoveries of minor planets, 74, 75; origin by explosion, 75, 76; career, 89, 90; Biela's comet, 95; comet of 1811, 99; electrical theory of comets, 100, 104, 324, 347; multiple tails, 100; comet of 1819, 101; cometary coruscations, 105; November meteors, 329

Olmsted, radiant of Leonids, 328; orbit, 329

Oppenheim, calculation of Schmidt's comet, 363

Oppolzer, E. von, theory of sun-spots, 159; variability of Eros, 285

Oppolzer, Th. von, Winnecke's comet, 94; comet of 1843, 350

Oxygen in sun, 213-215; telluric absorption, 214; in stars, 384

Packer, variable stars in cluster, 436

Palisa, search for Vulcan, 181, 250; discoveries of minor planets, 283

Pallas, discovery, 74; inclination of orbit, 75, 286; diameter, 75, 287, 288

Pape, Donati's comet, 345

Parallax, annual, of stars, 10, 16, 33, 36, 416-418; horizontal, of sun, 227; Encke's result, 230, 232; improved values from oppositions of Mars, 231, 237; from light velocity, 231, 232, 241; from recent transits, 236, 240; from observations of minor planets, 238, 239; general result, 242

Paris Catalogue of Stars, 415

Paschen, oxygen in sun, 215; solar temperature, 220

Pastorff, drawings of the sun, 101

Peirce, structure of Saturn's rings, 298

Perrine, eclipse photographs, 190; nature of corona, 191; observation of Holmes's comet, 369; nebula round Nova Persei, 401

Perrotin, rotation of Venus, 252; markings on, 255; canals of Mars, 279; clouds on Mars, 281; striation of Saturn's rings, 299; rotation and compression of Uranus, 303, 304; changes of Pons's comet, 366; Maia nebula, 410; measures of double stars, 419

Perry, eclipse of December, 1889, 187

Personal equation, 123, 235

Peter, star-parallaxes, 417

Peters, C. A. F., parallax of 61 Cygni, 36; disturbed motion of Sirius, 42

Peters, C. F. W., orbit of Leonid meteors and comet, 332

Peters, C. H. F., sun-spot observations, 147, 148; discoveries of minor planets, 283; star maps, 284, 415

Peytal, description of chromosphere, 69

Phobos, rapid revolution, 282, 283, 314; tidal relations, 320, 321

Photography, solar, 145, 153, 154, 165; of corona, 166, 173, 175, 178, 181, 185-190; without an eclipse, 178-180; of prominences, 167, 197, 198; of coronal spectrum, 171, 188, 190; of prominence-spectrum, 195, 198; of arc-spectrum, 206, 211; of solar spectrum, 210, 211, 215, 439, 440; of Uranian spectrum, 305; of cometary spectra, 354, 357; of stellar and nebular spectra, 382-384, 396, 398, 400, 407; lunar, 152, 153, 268; detection of comets by, 178, 338, 369; of asteroids, 284; of new stars, 399; use of, in transits of Venus, 234, 236, 240; Mars depicted by, 277, 281; Jupiter, 297, 408; comets, 353, 354, 368, 412; nebulæ, 395, 401, 407-409, 411, 425; Milky Way, 424, 425; star-charting by, 413, 414; star-parallaxes by, 417; rapid improvement, 438

Photometry, stellar, 49, 420, 421; of planetary phases, 245, 288; of Saturn's rings, 299; photographic, 421

Photosphere, named by Schröter, 55; structure, 151, 152, 164, 165

Piazzi, star catalogues, 31; parallaxes, 33; motion of 61 Cygni, 35; birth and training, 72; 5-foot circle, 72, 121; discovery of Ceres, 73, 74

Picard, Saturn's dark ring, 86; sun's distance, 228

Pickering, E. C., photometric measures of Martian satellites, 282; of minor planets, 287; variability of Japetus, 302; of Neptune, 305; meteoric photography, 339; gaseous stars, 379; hydrogen spectrum in stars 383; spectrographic results, 385; eclipses of Algol, 390; photographic celestial surveys, 399; star density in Pleiades, 411; photometric catalogues, 420, 421; photographic photometry, 421; white stars in Milky Way, 425; climate of Arequipa, 435; horizontal telescope, 437

Pickering, W. H., corona of 1886, 185; coronal photographs, January 1, 1889, 186; lunar twilight, 264; lunar volcanic action, 267; melting of snow on Mars, 277; Martian snowfall, 281; Jupiter's satellites, 292; photographs of comets, 368; of Orion nebula, 408; observatory at Arequipa, 435

Pingré, phenomena of comets, 92, 96

Planets, influence on sun-spots, 163; periods and distances, 228; intra-Mercurian, 248-250; inferior and superior, 288; trans-Neptunian, 306, 307; origin, 309, 313; relative ages, 314, 315

Planets, minor, existence inferred, 71, 72; discoveries, 73-75, 77, 283, 284; solar parallax from, 237-239; distribution of orbits, 286, 287; collective volume, 287; atmospheres, 288

Plantade, halo round Mercury, 244

Pleiades, community of movement near, 41; photographed spectra, 385; measurements, 410; photographs, 410, 411; nebulæ, 410, 411

Plücker, hydrogen in sun, 212

Plummer, solar translation, 39; Encke's comet, 99

Plutarch, solar corona, 65

Pogson, prominence spectrum, 168; reversing layer, 172; discovery of a comet, 335, 339; new star in cluster, 395

Pond, errors of Greenwich quadrant, 28; controversy with Brinkley, 33

Pons, discoveries of comets, 90, 94, 365

Pontécoulant, return of Halley's comet, 101

Poor, C. Lane, calculation of Lexell's comet, 367

Porter, solar translation, 40

Pouillet, solar constant, 216, 225; temperature of the sun, 217; of space, 270

Poynting, mean density of the earth, 261

Prince, glow round Venus, 253

Pritchard, parallax of Beta Aurigæ, 388; photographic determinations of stellar parallax, 417; photometric catalogue, 420

Pritchett, corona of January, 1889, 186; red spot on Jupiter, 294

Proctor, glare theory of corona, 182; speed of ejections from sun, 205; transit of Venus, 233; distance of sun, 236; atmosphere of Venus, 254; rotation of Mars, 275; map and canals of Mars, 278, 279; condition of great planets, 289; Nova Andromedæ, 403; status of nebulæ, 422, 423; structure of Milky Way, 424; star drift, 426

Procyon, satellite, 42; parallax, 417

Prominences, observed in 1842, 63, 64, 68; described by Vassenius, 68; observed in 1851, 70; photographed during eclipse, 167, 188, 190; without eclipse, 197, 198; spectrum, 168, 178, 194, 195, 198, 199; spectroscopic method of observing, 168-170, 194-196; white, 183, 184; chemistry, 195, 199; classification, 196; distribution, 199; movements in, 204-206; heat of development, 220

Quetelet, periodicity of August meteors, 329

Ranyard, drawing of sun-spot, 101; coronal types, 175, 185; lunar atmosphere, 265; Jupiter's markings, 297; meteors from fixed radiants, 341; cometary trains, 348; tenuity of nebulæ, 409

Rayet, spectrum of prominences, 168, 170

Red spot on Jupiter, 293, 296

Reduction of observations, 31; Bessel's improvements, 32, 122; Baily's, 60

Refraction, atmospheric, 31; effects looked for in comets, 106, 353; Cytherean, 235, 253, 254; lunar 263, 264

Reichenbach, foundation of Optical Institute, 28, 34, 122

Repsold, astronomical circles, 41, 122; Cape heliometer, 416

Resisting medium, 93, 94, 360

Respighi, slitless spectroscope, 173; prominences and chromosphere, 194, 196, 199; solar uprushes, 205; spectrum of Gamma Argûs, 380

Reversing layer, detected, 171, 172; photographed, 172, 189; depth, 173

Riccioli, secondary light of Venus, 255

Riccò, trials with coronagraph, 180; distribution of prominences, 199; spectrum of Venus, 254; spot on Jupiter, 294; spectrum of great comet, 364

Richer, distance of the sun, 228

Ristenpart, solar translation, 40

Ritchey, nebula round Nova Persei, 401; photographs of nebulæ, 432

Ritter, development of stars, 375

Roberts, A. W., southern variables, 392

Roberts, Isaac, search for ultra-Neptunian planet, 306; photographs of Orion nebula, 408; of Andromeda nebula, 409; of the Pleiades, 411

Roberval, structure of Saturn's rings, 299

Robinson, reflectors and refractors, 431

Roche, inner limit of satellite-formation, 301; modification of nebular hypothesis, 321

Römer, star places, 10; invention of equatoreal and transit instrument, 120; of altazimuth, 121; velocity of light, 231; satellite transit on Jupiter, 291

Rosenberger, return of Halley's comet, 101

Rosetti, temperature of the sun, 219

Rosse, third Earl of, biographical sketch, 114; great specula, 115-117; discovery of spiral nebulæ, 118; resolution of nebulæ, 119; climate and telescopes, 434

Rosse, fourth Earl of, experiments on lunar heat, 269

Rost, nature of sun-spots, 54

Roszel, mass of asteroids, 287

Rowland, photographic maps of solar spectrum, 210, 440; elements in run, 213; concave gratings, 439, 440

Rümker, observation of Encke's comet, 90

Russell, H. C., red spot on Jupiter, 295; change in Argo nebula, 404; photographs of Nubeculæ, 425

Russell, H. N., atmosphere of Venus, 254

Rutherfurd, lunar photography, 268; star spectra, 372; photographs of the Pleiades, 410; diffraction gratings, 439

Sabine, magnetic and sun-spot periods, 127, 128, 130

Safarik, secondary light of Venus, 256; compression of Uranus, 304

Satellites, discoveries, 110, 282, 293; transits, 291, 292; variability, 292, 302; origin, 309, 318

Saturn, low specific gravity, 298; rotation, 302; spectrum, 303

Saturn's rings, first disclosure, 85; dusky ring, 86; stability, 298, 300; meteoric constitution, 300; eventual dispersal, 301

Savary, orbits of double stars, 46

Savélieff, solar radiation, 164, 225

Sawerthal, discovery of a comet, 366

Schaeberle, discovery of Procyon's satellite, 42; coronal photographs, 187, 188; theory of corona, 191; meteoric photography, 339; discovery of a comet, 355

Schaeberle and Campbell, observations of Jupiter's satellites, 292

Scheiner, Father, nature of sun-spots, 52, 54; equatoreal instrument, 120 _note_; solar rotation, 146; darkening of sun's limb, 221

Schiener, Dr. J., photospheric structure, 165; spectrographic researches, 384, 405; spectrum of Andromeda nebula, 403; stars and nebulæ in Orion, 407

Schiaparelli, rotation of Mercury, 247; of Venus, 251, 252; spots on Mars, 275; snow-cap, 277; canals, 278-280; compression of Uranus, 304; comets and meteors, 327, 331, 332, 338; anomalous tail of great comet, 364; Pons's comet, 365; origin of comets, 370; measures of double stars, 419

Schmidt, A., circular refraction in sun, 159

Schmidt, J., sun-spot period, 126; lunar rills, 263; lunar maps, 265; disappearance of Linné, 267; cometary appendages, 363; new stars, 393

Schönfeld, extension of Bonn Durchmusterung, 412, 414

Schrader, construction of reflectors, 243

Schröter, a follower of Herschel, 5; motions of sun-spots, 146; biographical sketch, 243, 244; observations on Mercury, 244, 246, 247; on Venus, 250-253, 255; on the moon, 263; a lunar city, 265; Linné, 267; spots on Mars, 275; Jovian markings, 290

Schülen, perspective effects in sun-spots, 54

Schuster, photographs of corona, 178, 185; spectra of oxygen, 214

Schwabe, sun-spot periodicity, 125, 126

Secchi, chromosphere, 70; Biela's comet, 97; cyclonic movements in sun-spots, 144; distribution, 148; profundity, 154; nature, 156, 158; constitution of photosphere, 151; eclipse observations, 166, 167; reversing layer, 171; observations of prominences, 194, 196, 199; absence of helium absorption, 213; temperature of the sun, 218; solar atmospheric absorption, 221; Martian canals, 279; spectrum of Uranus, 304; of Coggia's comet, 343; stellar spectral researches, 372, 373; carbon stars, 372, 381; gaseous stars, 377

See, stellar orbits, 42, 46; measures of Neptune, 84; measures of Uranus, 304; belts of Neptune, 306; colour of Sirius, 375 _note_; southern double stars, 419; evolution of stellar systems, 420

Seeliger, photometry of Saturn's rings, 299; rationale of new stars, 396

Seidel, stellar photometry, 420

Sherman, spectrum of Nova Andromedæ, 395

Short, reflectors, 4, 109, 115, 121; chromosphere, 68; satellite of Venus, 256; striation of Saturn's rings, 299

Sidereal science, foundation, 9, 442; condition in 1785, 10; progress, 50

Sidgreaves, spots and faculæ, 159

Siemens, regenerative theory of the sun, 312

Simony, photographs of ultra-violet spectrum, 215

Sirius, a binary star, 41; mass, 42; parallax, 42, 416; spectrum, 133, 373, 383; former redness, 375 _note_; radial movement, 386, 387

Smyth, Admiral, Donati's comet, 324

Smyth, Piazzi, oxygen spectrum, 215; lunar radiations, 269; expedition to Teneriffe, 434

Solar constant, 216, 225

Solar spectrum, fixed lines in, 133-135; maps, 133, 136, 206, 210, 211, 224, 440; distribution of energy, 222, 223

Solar system, translation through space, 15, 39, 40, 406; development, 308, 309, 313-316, 322; complexity, 441

Soret, solar temperature, 218

South, observations of double stars, 45; 12-inch lens, 113; Rosse reflector, 117; occultation by Mars, 276

Spectroscopic binaries, 387-391

Spectrum analysis, defined, 130; first experiments, 131, 132; applied to the sun, 133-135, 156; to the stars, 133, 372, 373; Kirchhoff's theorem, 135; elementary principles, 139, 140; effects on science, 141, 142; radial motion determined by, 201, 386; investigations of comets by, 342, 343; of new stars, 393, 399; of nebulæ, 401-403

Spencer, position of nebulæ, 422

Spitaler, attendants on Brooks's comet, 366

Spitta, transits of Jupiter's satellites, 292

Spörer, solar rotation, 148, 149; chromosphere, 199, 200

Stannyan, early notice of chromosphere, 68

Star catalogues, 28, 31, 32, 60, 414, 415; spectroscopic, 381, 385, 386; photographic, 412-414; photometric, 420, 421

Star-drift, 426

Star-gauging, 13, 19, 47

Star-maps, 77, 78, 81, 284, 413, 415; photographic, 413, 414

Stars, movements, 9, 10, 35, 39, 415, 426; radial, 386, 387, 406; comparative brightness, 13, 49, 50, 420, 421; distances, 35-37, 416-418; chemistry, 372, 381, 382; spectroscopic orders, 373; colours, 374; development, 375-377; actual magnitudes, 422; gregarious, 426

Stars, double, physical connection surmised, 17; proved, 18, 442; masses, 38, 42; catalogues, 43, 45, 47, 50, 418, 419; orbits, 46, 418; discoveries, 43, 46, 47, 418, 419, 435; photographs, 409; evolution, 420

Stars, gaseous, 377-380

Stars, temporary, 24, 392-401

Stars, variable, early discoveries, 9; Eta Carinæ, 48, 49, 379; sun-spot analogy, 128, 392; spectra, 379; Algol class, 390, 391; catalogues, 391, 392

Stefan, law of cooling, 219

Steinheil, stellar photometry, 420; silvered glass reflectors, 429

Stewart, Balfour, Kirchhoff's principle, 135 _note_; solar investigations, 154, 155

Stewart, Matthew, solar distance by lunar theory, 230

Stokes, prevision of spectrum analysis, 138

Stone, E. J., reversal of Fraunhofer spectrum, 172; distance of the sun, 231, 232, 236; transit of Venus, 240; Cape catalogue, 415; proper motions, 426

Stone, O., star catalogues, 415; measures of double stars, 419

Stoney, carbon in photosphere, 152; dynamical theory of planetary atmospheres, 288; perturbations of Leonids, 338; status of red stars, 375

Stratonoff, star counts in Pleiades, 411

Stroobant, satellite of Venus, 256

Struve, F. G. W., stellar parallax, 35; career and investigations, 43-45; occultation by Halley's comet, 106; Russo-Scandinavian arc, 261, 262

Struve, Ludwig, solar translation, 40

Struve, Otto, parallax of Eta Cassiopeiæ, 38; solar velocity, 40; his father's successor at Pulkowa, 45; eclipse of 1842, 62, 64; Neptune's satellite, 84; research on Saturn's rings, 300, 301; variable nebula, 403

Stumpe, solar translation, 40

Sun, Herschel's theory, 54-57, 70, 149; atmospheric circulation, 58, 59; chemical composition, 135, 211-213; mode of rotation, 146, 147; Kirchhoff's theory, 149; Faye's, 150-152; convection currents in, 150, 152, 165; dissociation, 152, 206-210; luminous outbursts, 159-161; explosions, 205; heat emission, 216, 217, 221, 222, 225, 226; temperature, 217-220, 226; problem of distance, 227; results from transits, 230, 232, 236, 240; from oppositions of Mars, 231, 237; from light-velocity, 232, 241; from measurements of minor planets, 238; concluded value, 242; maintenance of heat supply, 310-313; past and future duration, 312

Sun-spots, speculations regarding, 52, 53; Wilson's demonstration, 53, 154; distribution, 53, 58, 148; cyclonic aspect, 58, 144, 157, 158; periodicity, 126, 128, 162, 163; magnetic relations, 127, 160, 161; meteorological, 129, 164; auroral, 129, 130, 160, 162; photographs, 145, 154; level, 155; spectra, 156, 207, 208; volcanic hypothesis, 158; Lockyer's rationale, 159; planetary influence, 163; relation to Jovian markings, 297

Swan, chromosphere, 70; sodium line, 132

Swift, E., discovery of a comet, 368

Swift, L., fallacious glimpse of Vulcan, 181, 250; discovery of a comet, 368

Tacchini, eclipse of 1883, 181; white prominences, 184; prominences and chromosphere, 199, 200; spectrum of Venus, 254

Talbot, Fox, spectrum analysis, 131; spectroscopic method of determining stellar orbits, 387

Tarde, nature of sun-spots, 52

Taylor, eclipse expedition, 187; spectrum of Uranus, 305; achromatic lenses, 431

Tebbutt, comets discovered by, 326, 352; comet of 1882, 359

Telescopes, achromatic, 112, 431, 432

Telescopes, equatoreal, 84, 120, 121

Telescopes, reflecting, Short's, 4, 109, 115, 121; Herschel's, 12, 109-111; Lassell's, 83, 114, 121; varieties of construction, 109, 110; Rosse's, 115-119, 434; Common's, 407, 412, 429

Telescopes, refracting, Fraunhofer's, 34, 35, 121; Clark's, 114, 429, 430, 433, 436; Grubb's, 430, 433; with bent and horizontal mountings, 436-438

Tempel, red spot on Jupiter, 294; comet discoveries, 327; cometary observations, 352, 362; Andromeda nebula, 394; discovery of Merope nebula, 410

Temperature, of the sun, 217-220, 226; of the moon, 269, 270; of space, 270; on Mars, 277

Tennant, eclipse observations, 168, 169, 174

Terby, surface of Mars, 278, 279, 281; secondary tail of comet, 355

Thalén, basic lines, 207; map of solar spectrum, 210; solar elements, 212

Thollon, line-displacements by motion, 202, 364; atlas of solar spectrum, 211, 440; lunar atmospheric absorption, 264

Thome, comet discovered by, 361

Thomson, Sir William (Lord Kelvin), solar chemistry, 138; magnetic influence of the sun, 161; tidal strains, 257; rotation of the earth, 273; dynamical theory of solar heat, 311, 312

Thraen, period of Wells's comet, 357

Tidal friction, effects on moon's rotation, 271, 272, 318; month lengthened by, 316, 318; influence on planets, 319-322; on development of binary systems, 420

Tietjen, asteroidal orbits, 284

Tisserand, capture of comets, 98; lunar acceleration, 273; revolutions of Neptune's satellite, 305; stationary radiants, 341; perturbations of Algol, 391; director of Paris Observatory, 414

Titius, law of planetary intervals, 71, 72, 85

Todd, eclipse of 1887, 185; solar distance, 236, 241; trans-Neptunian planet, 306

Tornaghi, halo round Venus, 254

Transit instrument, 120

Trépied, reversal of Fraunhofer spectrum, 172

Troughton, method of graduation, 122

Trouvelot, veiled spots, 148; chromosphere in 1878, 175; intra-Mercurian planets, 181, 250; observations of prominences, 184, 196, 204; of Mercury, 245, 247; rotation of Venus, 252; red spot on Jupiter, 296

Trowbridge and Hutchins, carbon in sun, 212

Tschermak, origin of meteorites, 339

Tupman, transit expedition, 235; results, 236

Turner, polariscopic coronal photography, 189; employment of coelostat, 190, 438; stationary radiants, 341

Ulloa, eclipse of 1778, 69

United States, observatories founded in, 6, 7

Uranus, discovery, 5, 74, 111; unexplained disturbances, 78, 79, 307; satellites, 87, 303; equatoreal markings, 303, 304; spectrum, 304, 305; retrograde rotation, 313, 315, 322

Valerius, darkening of sun's limb, 221

Vassenius, description of prominences, 68

Venus, transits, 4, 229, 232; of 1874, 233-236; of 1882, 239, 240; atmosphere, 236, 253, 254; mountains, 252, 253; spectrum, 254; albedo, 255; ashen light, 255, 256; pseudo-satellite, 256; effects upon, of solar tidal friction, 320

Very, temperature of sun, 220; lunar heat, 270

Vesta, discovery, 75, 76; diameter, 287; spectrum, 288

Vicaire, solar temperature, 218

Vico, comet discovered by, 97; rotation of Venus, 251; Cytherean mountain, 253

Violle, solar temperature, 218, 219; solar constant, 225

Vogel, H. C., solar rotation, 202; solar atmospheric absorption, 222, 224; spectrum of Mercury, 245; of Venus, 255; of Vesta, 288; of Jupiter, 290; of Jupiter's satellites, 293; of Uranus, 304; rotation of Venus, 252; ashen light, 256; intrinsic light of Jupiter, 291; cometary spectra, 342, 343, 355, 357; carbon in stars, 374; stellar development, 375, 376; spectrum of Gamma Cassiopeiæ, 378; of Nova Cygni, 393; of Nova Andromedæ, 395; spectroscopic star catalogue, 381; radial motion of Sirius, 386; period of Mizar, 388; eclipses of Algol, 390; components of Nova Aurigæ, 397; spectrographic determinations of radial motion, 405, 406

Vogel, H. W., spectrum of hydrogen, 206 _note_, 383

Vulcan, existence predicted, 248; pseudo-discoveries, 249, 250

Wadsworth, coronal photography, 189

Ward, Nova Andromedæ, 394

Waterston, solar temperature, 218; meteoric infalls, 311

Watson, fallacious observations of Vulcan, 181, 250; asteroidal discoveries, 284

Webb, comet of 1861, 326

Weber, Baily's Beads, 62; illusory transit of Vulcan, 249

Weinek, study of lunar photographs, 268

Weiss, comets and meteors, 332, 334

Wells, comet discovered by, 356

Wesley, drawings of corona, 175

Wheatstone, spectrum of electric arc, 132; method of ascertaining light-velocity, 232

Whewell, stars and nebulæ, 422

Williams, A. Stanley, canals of Mars, 279; markings on Jupiter, 295, 297; rotation, 296; Nova Persei, 400

Wilsing, solar rotation from faculæ, 155; density of the earth, 261; system of, 61 Cygni, 419

Wilson, Alexander, perspective effects in sun-spots, 53, 154

Wilson, H. C., red spot on Jupiter, 295; compression of Uranus, 304; exterior nebulosities of Pleiades, 411

Wilson, W. E., solar temperature, 220, 222; ultra-Neptunian planets, 306

Winnecke, comet discovered by, 94; distance of the sun, 231; Donati's comet, 324, 347

Wisniewski, last glimpse of 1811 comet, 99

Witt, discovery of Eros, 284

Wolf, C., objections to Faye's cosmogony, 315; origin of Phobos, 321

Wolf, Max, photographic discoveries of minor planets, 283, 284; Nova Andromedæ, 394; Nova Aurigæ, 396; nebula near Nova Persei, 401; photographic nebular survey, 412; galactic nebulosity, 425

Wolf, R., sun-spot and magnetic periodicity, 128, 162, 163; analogy of variable stars, 128, 392; auroræ, 129; suspicious transits, 249

Wollaston, ratio of moonlight to sunlight, 49; flame spectra, 131; lines in solar spectrum, 133

Woods, coronal photography, 179, 180; Cape Durchmusterung, 412

Wrangel, auroræ and meteors, 335

Wright, G. F., Ice Age in North America, 260

Wright, Thomas, theory of Milky Way, 14; structure of Saturn's rings, 299

Wright, W. H., polarisation of cometary light, 355; spectrum of nebulæ, 400

Yerkes, donation of a telescope, 433

Young, Miss Anne, nebular hypothesis, 314

Young, C. A., spectrum of sun-spots, 156; origin, 158; spectrum of corona, 170, 177; detection of reversing layer, 171, 172; prominences and chromosphere, 194-196, 200; photograph of a prominence, 197; spectroscopic measurement of sun's rotation, 202; solar cyclones and explosions, 204, 205; basic lines, 207; spectrum of Venus, 254; red spot on Jupiter, 294; observations of Uranus, 303, 304; Andromedes of 1892, 337; spectrum of Tebbutt's comet, 355; of Nova Andromedæ, 395

Young, Thomas, absorption spectra, 136

Zach, Baron von, promotion of astronomy, 5, 6, 28; Baily's Beads, 62; search for missing planet, 72; rediscovery of Ceres, 74; use of a heliostat, 120

Zantedeschi, lines in solar spectrum, 134; lunar radiation, 269

Zenger, observations on Venus, 253, 255

Zenker, cometary tails, 348

Zezioli, observation of Andromedes, 334

Zodiacal light, relation to medium of space, 94; to solar corona, 176; meteoric constitution, 310

Zöllner, electrical theory of comets, 99, 344, 346, 347; solar constitution, 158; observations of prominences, 194, 196; reversion spectroscope, 202; solar temperature, 220; Mercurian phases, 245; albedo of Venus, 255; of Jupiter, 290; of Saturn, 303; of Uranus, 304; condition of Venus, 256; of great planets, 289; Jovian markings, 297; ages of stars, 375; polarising photometer, 420, 421

THE END

BILLING AND SONS, LTD., PRINTERS, GUILDFORD

THE SYSTEM OF THE STARS

AGNES M. CLERKE

Hon. Member of the Royal Astronomical Society; Author of "History of Astronomy During the Nineteenth Century" and "Problems in Astrophysics"

_SECOND EDITION_

_Demy 8vo. Cloth. With many Illustrations. *Price 20s. net.*_

(Post Free, Price *20s. 5d.*)

FROM THE PREFACE

Fifteen years have elapsed since the original publication of the present work; and fifteen years count as a long spell of time where sidereal research is in question. In preparing the Second Edition, accordingly, I have introduced extensive modifications. Considerable sections of the book have been recast, and all have been thoroughly revised. New chapters have been inserted, old ones have been in large part suppressed. Drastic measures of reform have, in short, been adopted, with results that certainly import progress and (it is hoped) constitute improvements. Most of the illustrations are entirely new; and I am under great obligations for the use of valuable photographs and drawings, among others, to Sir. David Gill, F.R.S., to Professor Hale and the University Press of Chicago, to the Rev. W. Sidgreaves, S.J., to Professors E. C. Pickering, Campbell, Barnard, and Frost, and to Dr. Max Wolf of Heidelberg.

"The work was admirable from the first, imparting the best knowledge of a decade and a half ago; now it retains its high quality by incorporating the newer knowledge."--_The Guardian_.

"It has the remarkable feature of combining extraordinary profusion of precise information with an elegance of literary style quite unusual in scientific authors."--_The Academy_.

PUBLISHED BY

ADAM & CHARLES BLACK . SOHO SQUARE . LONDON

BY THE SAME AUTHOR

PROBLEMS IN ASTROPHYSICS

Demy 8vo. 560 pages. Cloth.

Containing 50 Illustrations in the Text and 31 Plates

Price 20s. net

(Post free, price 20s. 6d.)

NOTE

"This is emphatically a "new century" book. It aims at stimulating progress along lines carefully marked out as immediately practicable. The same author's "History of Astronomy" is a survey of the past; "Problems in Astrophysics" looks to the future. What we already know is regarded in it as means to the end of augmenting knowledge. Astrophysics is a science still at the outset of a magnificent career. Its ways are beset with claimants for its attention. There is often much difficulty in choosing between them, yet rapidity of progress depends upon prudence in selection. Many hints for its guidance are accordingly offered in the present work, which deals, so far as possible, with answerable questions. It should, then, find its way into the hands of every astronomer who desires to keep up with the drift of thought, and to be informed of the prospects of work and discovery in the various departments of research connected with the physics of the heavenly bodies."

*SOME PRESS OPINIONS*

"The book shows every sign of profound and careful study, and the sense of scientific imagination, which is one of the greatest means of independent discovery."--_St. James's Gazette._

"The book is written with all the charm that has characterised the authoress's previous volumes, and contains a wealth of information and suggestion for work yet to be accomplished which will appeal to all who are interested in the problems of the universe."--_Daily Telegraph._

"We feel that Miss Clerke has earned, and will surely receive, the admiration and gratitude of astronomers for this new proof of her devotion to their science."--_The Times._

MODERN COSMOGONIES

Crown 8vo. Bound in Cloth.

Price 3s. 6d. net

(Post free, price 3s. 10d.)

NOTE

This volume contains a series of brief and attractive studies on current theories regarding the origin and history of the visible universe. The difficulties besetting cosmical doctrines of evolution are pointed out in them, and the expedients are described by which those difficulties have been met, though not wholly overcome. The widened possibilities connected with the new science of radiology, the unification of the physical forces that may ensue upon further discoveries concerning electrical action, the function in the world of the impalpable ether, the nature of gravity, are in turn discussed or adverted to; while the final chapter takes into consideration the crowning problem of life.

"These sixteen chapters are short studies on modern theories about the origin and mystery of the universe, by an astronomer whose writings have done much to help and popularise that science."--_The Times._

PUBLISHED BY

ADAM & CHARLES BLACK . SOHO SQUARE . LONDON

* * * * *

Transcriber's notes:

Original page line Original text left as is (sic) --- ---------- -------------------------------------------------- 072 13 The search for it, through confessedly scarcely 196 24 The first description are tranquil

page line Original text Replaced with --- ---------- ----------------------- --------------------------- 003 footnote 1 xviiie xviii^e (the e is superscript) 009 11 byeways byways 024 46 concentation concentration 043 37 Is appears from It appears from 062 37 appearances seem by him appearances seen by him 072 42 Ecole Militaire École Militaire 082 3 forgotton forgotten 092 footnote 1 11/9647000 1/9647000 (confirmed by looking up reference quoted) 093 7 phenenoma phenomena 100 17 Bredikhin Brédikhine 131 13 identifiying identifying 140 40 terrestial terrestrial 143 25 appearence appearance 149 27 bloodvessel blood vessel 152 12 Angström Ångström 169 3 undimished undiminished 171 42 sympton symptom 172 18 familar familiar 173 42 photograpic photographic 182 37 by which i structure by which its structure 199 37 Bredikhine Brédikhine 220 26 stata strata 246 30-31 of its orbit 24 hours of its orbit in 24 hours 53 seconds. 53 seconds 260 13 garden at its seasons garden as its seasons 284 21 throngh through 284 13 oparator operator 376 42 recognised. in a recognised in a 377 footnote 3 applie applied 395 42 the gaseous fields o the gaseous fields of 423 35 relatiouship relationship 434 footnote 2 Optice Optics 436 42 ofter some years after some years 436 footnote 1 (two references given, (split into two footnotes, within a single footnote. and corrected references In the text footnote 1 in the text) used twice) 450 27 1862 Conclusion of a 1872 Conclusion of a 454 40 spectographically spectrographically 454 18 spectographic spectrographic 456 4 Lyrae Lyræ 488 index Wolf, R., sun-spot and Wolf, R., sun-spot and magnetic periodicity, magnetic periodicity, 128, 164, 162; 128, 162, 163;