Februaby 14, 1919] 



SCIENCE 



153 



tudes has been generally adopted as an in- 

 ternational standard. When to these ob- 

 servations, most of which were made by 

 Professor Pickering himself, are added his 

 numerous measures upon variable stars, 

 satellites and other objects, the whole num- 

 ber of photometric settings which he per- 

 sonallj' made rises to the amazing total of 

 more than a million and a half. 



He was also a pioneer in stellar photog- 

 raphy, and especially in the use of the doub- 

 let lenses which combine great light grasp 

 with a wide angle of field, and can with an 

 exposure of an hour or two, record on a 

 single plate the positions and magnitudes of 

 a number of stars which may run into the 

 hundreds of thousands. The Harvard 

 equipment includes instruments of this 

 type ranging from the 2-1-inch Bruce tele- 

 scope at Arequipa and the 16-inch Metcalf 

 instrument at Cambridge to the little lenses 

 of one inch aperture which are used to 

 photograph as large a portion of the visible 

 heavens as possible on every clear night. 

 The plates are developed, indexed, and filed 

 in the great "Harvard Photographic Li- 

 brary," which its creator described as "a 

 library of 250,000 volumes, every one 

 unique, and with but a handful of readers 

 to work in it." The very magnitude of the 

 mass of information stored in this vast col- 

 lection makes it impossible to extract it all ; 

 but whenever an object of unusual interest 

 is discovered, it is only necessary to refer 

 to the Harvard plates to find out just where 

 and how bright it was on some three or 

 four hundred dates during the last thirty 

 yeare. Among the most notable examples 

 of this may be mentioned the recognition of 

 images of the asteroid Eros upon plates 

 taken two and four years before its discov- 

 er}', and the recent tracing of the historj- of 

 the brilliant new star in Aquila through an 

 interval of thirty years, up to the very day 

 before the great outburst. 



The third principal field of work is in 

 stellar spectroscopy. Pickering led again 

 in the photography of stellar spectra with 

 the objective prism, and in the more precise 

 classification of stellar spectra which this 

 made possible. Assisted financially by the 

 liberal aid of the Henry Draper Memorial, 

 he and his very distinguished assistants, 

 Mrs. Fleming and Miss Cannon, studied 

 these spectra, devised the empirical classi- 

 fication of the original Draper Catalogue, 

 and improved upon this by omitting some 

 of the original classes and rearranging 

 others, until the resulting classification 

 proved so convenient, and so remarkably 

 representative of the actual facts, that it 

 was adopted without a dissenting voice by 

 the International Union for Solar Research 

 as a universal standard. The fact, which 

 was first brought out by this investigation, 

 and served as the basis of the final classifi- 

 cation, that the spectra of almost all the 

 stars fall into a single sequence, along 

 which each tj-pe grades almost impereep- 

 tiblj' into the next, is now recognized as the 

 ver}- foundation of modern astrophysics, 

 and the progress of discovery serves stead- 

 ily to emphasize the importance of classifi- 

 cation according to spectral type in the 

 most diverse problems of sidereal astron- 

 omy. In this field, too, the Harvard work 

 is of imposing extent, culminating in the 

 "New Draper Catalogue" containing the 

 spectra of about 2] 5,000 stars, classified by 

 Miss Cannon. Professor Pickering took 

 the liveliest interest in this monumental 

 work, and in the admirably arranged plans 

 for its production ; and it is cause for 

 gratification that the first volume saw the 

 light while he was alive to enjoy it. 



One other scries of investigations that 

 should not be passed over deals with photo- 

 graphic photometry. This was one of the 

 chief interests of his later years, and an in- 

 creasing part of the work of the observa- 



