504 PROCEEDINGS OF THE AMERICAN ACADEMY. 



(2) A scale of photographic magnitudes. It was shown later that 

 these are convertible into visual magnitudes through reference to the 

 spectral types of the stars. 



(3) A system of classification of variable stars. Light curves have 

 been determined for a large number of these and many thousand 

 measures of their brightness have been made on a uniform scale for 

 all of the sky. 



(4) A system of classification of stellar spectra which has been 

 universally adopted. The new Henry Draper Catalogue contains 

 estimates based on this system of all stars to approximately the ninth 

 magnitude, about 200,000 in number. 



Through the establishment of an observatory at Arequipa in 1891 

 after two years of preliminary study it became possible to include 

 measurements made on the stars throughout the southern heavens 

 within the scope of the work of the Harvard College Observatory. 



At a later date, 1911, an observing station was established at Mande- 

 ville, Jamaica, which has been devoted particularly to the study of the 

 moon and the j^lanets. 



In this short article it is not possible to go into detail or even to 

 mention the great variety of investigations carried on to a successful 

 completion. The volumes of the Harvard Annals, more than eighty 

 of which were published during Professor Pickering's directorship, can 

 alone give any idea of this. 



A word, however, should be said of the "photographic library" 

 which now contains over a cjuarter of a million photographic plates 

 that together weigh one hundred and twenty tons. Through the use 

 of short focus lenses and automatic following apparatus there has 

 been kept a "sky patrol" the results of which furnish the history of 

 all the stars down to the tenth magnitude and measuring back for 

 many years. By its use whenever any noteworthy stellar change is 

 discovered the plates will reveal its past history and character, while 

 otherwise one might have to wait years to understand the nature of 

 the phenomena. 



Through the use of this library, it is possible to find the history of 

 the stars, as one turns back the pages of a book already printed. 

 What may still be hoped for from this crystallized past of the heavens 

 is shown by what it has already done in recording the extraordinarily 

 favorable position for observation of the minor planet Eros at its 



