GREAT COMET, NOV. 14TH, 1882. (Exposure 2hrs. 20m.)
By kind permission of Sir David Gill. From this photograph originated all stellar chart-photography.
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 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 30,000,000; and that of all the stars visible in the greatest modern telescopes is probably about 100,000,000.
Nebulæ and Star-clusters.—Our knowledge of nebulæ 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 nebulæ might be clusters of innumerable minute stars at a great distance. Then he recognised the different classes of nebulæ, and became convinced that there is a widely-diffused “shining fluid” in space, though many so-called nebulæ 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 nebulæ” might show a stage of evolution from the diffuse nebulæ, 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,[[22]] 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 nebulæ.
In 1864 all doubt was dispelled by Huggins[[23]] in his first examination of the spectrum of a nebula, and the subsequent extension of this observation to other nebulæ; 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.