Page:Popular Science Monthly Volume 64.djvu/520

516 throughout the whole of every clear night. The photographs thus made are usually examined in Cambridge, where a number of assistants are employed for the purpose. Only in exceptional cases is more than a preliminary examination made in Arequipa,

The largest instrument in the observatory is the twenty-four-inch Bruce telescope. This telescope is a doublet, that is, it has a combination of four lenses, giving good definition over a large field. The scale is the same as that of the instrument used in the international photographic survey of the sky, but the region covered by each plate is six times as great, so that the work of covering the whole sky is much less. With such instruments the work of making a photographic Durchmusterung of the stars to any desired magnitude would be comparatively simple, since a pair of these telescopes, one in the northern, and the other in the southern, hemisphere could furnish all the plates needed within two or three years. The Bruce telescope, after a year's trial in Cambridge, was mounted in Arequipa, in 1895, by the writer. Nearly the whole sky has been photographed with exposures of ten minutes, showing stars to about the eleventh magnitude. Good progress has also been made on plates having exposures of sixty minutes, which show stars to about the fifteenth magnitude. A set of plates has also been begun, having exposures of four hours. These can only be made on moonless nights, and a number of years will be required to cover the whole sky. The approximate number of stars has been determined on some of these plates. The number varies, in general, from one thousand to ten thousand stars per square degree. Four hundred thousand stars have been photographed on a single plate. The whole number of stars which will be recorded in this splendid set when completed will probably approach one hundred millions. In addition to such vast numbers of stars, these plates will also contain numerous star clusters and nebulæ, together with occasional asteroids, comets and meteors. This set of plates alone would furnish two or three astronomers with materials for a lifetime of study. A large part of the plates thus far obtained with this instrument have been made by Dr. Stewart and Mr. Frost.

An instrument, which has been in constant use since the beginning of Professor Pickering's photographic researches in 1886 is the Bache telescope, which has an aperture of eight inches, and a focal length of four feet. It was employed for several years in Cambridge, then for a year and a half on Mount Harvard, and since that time in Arequipa. Altogether, more than thirty thousand photographs of the stars have been made with this instrument. By its use with an objective prism photographs of the spectra of all stars to about the eighth magnitude have been made. A study and classification of these spectra have been carried out by Professor Pickering as a memorial to the late Dr. Henry