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hour's exposure was good, so he tried three hours. The result was such a display of sharp star images that he resolved on the Cape Photographic Durchmusterung, which after fourteen years,
By kind permission of Sir David Gill. From this photograph originated all stellar chart-photography.
with Kapteyn's aid in reducing, was completed. Meanwhile the brothers Henry, of Paris, were engaged in going over Chacornac's zodiacal stars, and were about to catalogue the Milky Way portion, a serious labour, when they saw Gill's Comet photograph and conceived the idea of doing the rest of their work by photography. Gill had previously written to Admiral Mouchez, of the Paris Observatory, and explained to him his project for charting the heavens photographically, by combining the work of many observatories. This led Admiral Mouchez to support the brothers Henry in their scheme." Gill, having got his own photographic work under way, suggested an international astrographic chart, the materials for different zones to be supplied by observatories of all nations, each equipped with similar photographic telescopes. At a conference in Paris, 1887, this was decided on, the stars on the charts going down to the fourteenth magnitude, and the catalogues to the eleventh. This monumental work is nearing completion. The labour involved was immense, and the highest skill was required for devising instruments and methods to read off the star positions from the plates. Then we have the Harvard College collection of photographic plates, always being automatically added to; and their annex at Arequipa in Peru. Such catalogues vary in their degree of
* For the early history of the proposals for photographic cataloguing of stars, see the Cape Photographic Durch musterung, 3 vols. (Ann. of the Cape Observatory, vols. iii., iv., and v., Introduction.)
accuracy; and fundamental catalogues of standard stars have been compiled. These require extension, because the differential methods of the heliometer and the camera cannot otherwise be made absolute. The number of stars down to the fourteenth magnitude may be taken at about 30, ooo, ooo; and that of all the stars visible in the greatest modern telescopes is probably about 1oo, ooo, ooo. Nebulae and Star-clusters. — Our knowledge of nebulae really dates from the time of W. Herschel. In his great sweeps of the heavens with his giant telescopes he opened in this direction a new branch of astronomy. At one time he held that all nebulae might be clusters of innumerable minute stars at a great distance. Then he recognised the different classes of nebulae, and became convinced that there is a widely-diffused “shining fluid’’ in space, though many so-called nebulae could be resolved by large telescopes into stars. He considered that the Milky Way is a great star-cluster, whose form may be conjectured from numerous star-gaugings. He supposed that the compact “planetary nebulae'' might show a stage of evolution from the diffuse nebulae, and that his classifications actually indicate various stages of development. Such speculations, like those of the ancients about the solar system, are apt to be harmful to true progress of knowledge unless in the hands of the ablest mathematical physicists; and Herschel violated their principles in other directions. But here his speculations have attracted a great deal of attention, and, with modifications, are accepted, at least as a working hypothesis, by a fair number of people. When Sir John Herschel had extended his father's researches into the Southern Hemisphere he was also led to the belief that some nebulae were a phosphorescent material spread through space like fog or mist. Then his views were changed by the revelations due to the great discoveries of Lord Rosse with his gigantic refractor," when one nebula after another was resolved into a cluster of minute stars. At that time the opinion gained ground that with increase of telescopic power this would prove to be the case with all nebulae. In 1864 all doubt was dispelled by Huggins” in his first examination of the spectrum of a nebula, and the subsequent extension of this observation to other nebulae; thus providing a certain test which increase in the size of telescopes could never have given. In 1864 Huggins found that all true nebulae give a spectrum of bright lines. Three are due to hydrogen; two (discovered by Copeland) are helium lines; , others are unknown. Fifty-five lines have been
photographed in the spectrum of the Orion nebula. It seems to be pretty certain that all true nebulae are gaseous, and show almost exactly the same spectrum. Other nebulae, and especially the white ones like that in Andromeda, which have not yet been resolved into stars, show a continuous spectrum; others are greenish and give no lines. A great deal has to be done by the chemist before the astronomer can be on sure ground in drawing conclusions from certain portions of his spectroscopic evidence. The light of the nebulae is remarkably actinic, so that photography has a specially fine field in revealing details imperceptible in the telescope. In 1885 the brothers Henry photographed, round the star Maia in the Pleiades, a spiral nebula 3’ long, as bright on the plate as that star itself, but quite invisible in the telescope; and an exposure of four hours revealed other new nebulae in the same district. That painstaking and most careful observer, Barnard, with Ioł hours' exposure, extended this nebulosity for several degrees, and discovered to the north of the Pleiades a huge diffuse nebulosity, in a region almost destitute of stars. By establishing a 10-inch instrument at an altitude of 6,ooo feet, Barnard has revealed the wide distribution of nebular matter in the constellation Scorpio over a space of 4° or 5° square. Barnard asserts