Page:Popular Science Monthly Volume 42.djvu/423

 Rh the relative positions of thirty-one stars in the Pleiades. On the same day Mr. Rutherfurd communicated orally to the Academy a detailed account of his experiments, difficulties, and successes, and of the methods which he had finally adopted. His photographs of the moon are remarkable for the fineness of their details.

In 1863 Mr. Rutherfurd published in the American Journal of Science a paper dealing with the spectra of the stars, the moon, and the planets, the first published work of the kind after that of Bunsen and Kirchhoff, and the first attempt at classifying the stars according to their spectra. In this paper he said: "The star spectra present such varieties that it is difficult to point out any mode of classification. For the present, I divide them into three groups: first, those having many lines and bands, and mostly resembling the sun, viz.—Capella, β Geminorum, α Orionis, etc. These are all reddish or golden stars. The second group, of which Sirius is the type, present spectra wholly unlike that of the sun, and are white stars. The third group, comprising α Virginis, Rigel, etc., are also white stars, but show no lines; perhaps they contain no mineral substance, or are incandescent without flame." In 1864 he presented to the National Academy of Sciences a photograph of the solar spectrum obtained by means of bisulphide-of-carbon prisms, containing more than three times the number of lines that had been laid down within similar limits on the chart by Bunsen and Kirchhoff. In the course of his spectrum work, to which he now gave increasing attention, he found, as he had done in photographing, that the apparatus in use was insufficient for his purposes. He noticed that diffraction gratings of finely ruled lines upon glass and metal were preferable to series of prisms for the decomposition of light in spectral study. The best gratings in existence—still imperfect—were those of Nobert, who kept his process a secret. Mr. Rutherfurd—as usual helping himself in invention—devised a ruling engine capable of turning out much finer gratings than those of Nobert, some of which had about seventeen thousand lines to the square inch, and which have been surpassed only by those since made by Prof. Rowland. With these gratings his great photographs of the solar spectrum—more than eleven feet long—were made.

After he ceased to take an active part in astronomical work, Mr. Rutherfurd gave his instruments and photographs to Columbia College: the telescope in December, 1883—and it is now mounted in the observatory one hundred and ten feet above the ground; the machine for making measures in the same year; and his best negatives in November, 1890. This valuable collection of photographs of the sun, the moon, and the star clusters has been placed in a fire-proof vault. It contains, according to a list