CHAPTER XIII

_METHODS OF RESEARCH_

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

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

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

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telescopes, proposed by Steinheil in 1856, and independently by Foucault in 1857,[1630] has in a great measure superseded the use of a metallic alloy.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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

FOOTNOTES:

[Footnote 1630: _Comptes Rendus_, t. xliv., p. 339.]

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

[Footnote 1632: Newcomb, _Pop. Astr._, p. 137.]

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

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

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

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

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

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

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

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

[Footnote 1641: Holden on Celestial Photography, _Overland Monthly_, Nov., 1886.]

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

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

[Footnote 1644: _Harvard Circulars_, Nos. 2, 18, 24, 33;]

[Footnote 1645: Loewy, _Bull. Astr._, t. i., p. 286; _Nature_, vol. xxix., p. 36.]

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

[Footnote 1647: _Ibid._, vol. xxix., p. 470.]

[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 Nebulae. 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 Nebulae, 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 nebulae. 1792 Atmospheric refraction in Venus announced by Schroeter. 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 Systeme 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 _Mecanique Celeste_. 1799, May 7 Transit of Mercury observed by Schroeter. 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 Celeste_. 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 Nebulae. 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 nebulae. 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 Astronomiae_. 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 Koenigsberg heliometer mounted. 1830 Publication of Bessel's _Tabulae Regiomontanae_. 1832 Discovery by Brewster of "atmospheric lines" in the solar spectrum. 1833 Magnetic observatory established at Gottingen. 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 Maedler's _Der Mond_. 1837 Publication of Struve's _Mensurae Micrometricae_. 1837, Dec. 16 Outburst of Eta Carinae 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 nebulae. 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 Astraea 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 Angstrom. 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 nebulae. 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 Zoellner 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 Linne. 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 Angstrom'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 Nyren 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 Coude. 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 Duner'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 nebulae 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 deg. 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 Kuestner'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 nebulae. 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 Ursae Majoris and Beta Aurigae. 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 Duner'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 Lyrae. 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 Aurigae photographed at Harvard College. 1891, Dec. 20 Photographic maximum of Nova Aurigae. 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 Aurigae. 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 Duner's Investigation of the System of upsilon Cygni. 1892 Photographic investigation by Deslandres of the spectra of prominences. 1892 Photographs of the sun with faculae 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 Lactee_. 1893 Campbell detects bright H Alpha in Gamma Argus and Alcyone. 1893 Nova Normae 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_,