SI 6 POPULAR SCIENCE MONTHLY. 



throughout the whole of ever}' clear night. The photographs thus 

 made are usually examined in Cambridge, where a number of assist- 

 ants 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 com- 

 bination 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 com- 

 paratively 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 prog- 

 ress 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 deter- 

 mined 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 com- 

 pleted will probably approach one hundred millions. In addition to 

 such vast numbers of stars, these plates will also contain numerous 

 star clusters and nebulae, 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 



