358 ANNUAL 'REPORT SMITHSONIAN INSTITUTION, 1910. 



The principal line of researches followed at the Harvard Observa- 

 tory is the spectroscopic and photometric study of the stars. The 

 observers therefore constantly watch the entire system of stars down 

 to the sixth magnitude, and as much fainter as possible, so that none 

 of them may escape surveillance. In order to study the stars about 

 the southern celestial pole, which for good reasons have been so neg- 

 lected by most astronomers, the Pickering brothers established, in 

 1891, an auxiliary station at Arequipa, Peru, at an altitude of about 

 2,700 meters, and consequently under the best atmospheric conditions 

 for observations. Pickering, with praiseworthy self-denial, sent 

 there one of his most beautiful instruments, the Bruce 24-inch tele- 

 scope, which he thought would render more service there than at 

 Harvard. We will not dwell on the photometric and spectroscopic 

 catalogues published at Cambridge, nor yet upon the astounding dis- 

 coveries of spectroscopic double stars, nor the various kinds of stellar 

 hydrogen, and so on, which have been made here in the past. We 

 will limit ourselves to the methods actually in use at this establish- 

 ment and which, as we have stated, are in themselves of great interest. 



Prof. E. C. Pickering personally carries on the stellar photometry 

 and has allowed no detail to escape detection which would lead to 

 precision. The photometer is stationary and placed in a well-shaded 

 place; a siderostat, worked from the interior by an assistant, sends 

 the rays of the star under measurement into the horizontally placed 

 photometer. The astronomer is thus comfortably situated, as at the 

 equatorial-coude at Paris, with his head enveloped in a mantle of 

 black material ; he remains here continuously during the whole even- 

 ing ; the settings, the records, even the readings of the divided scale, 

 are made by his assistant. Mr. Pickering uses his sight strictly for 

 the purely photometric work, assuring himself of his maximum effi- 

 ciency in the photometric comparisons and avoiding thus a number 

 of more or less systematic errors. 



The spectroscopic classification of all the stars of the sky is done 

 principally with the aid of the objective prism of the 11-inch tele- 

 scope. The equatorial upon which this prism is mounted is in no 

 way unusual except for the electric control, which assures the accurate 

 regulation of the driving mechanism which is kept in perfect syn- 

 chronism with a controlling pendulum. As this 11-inch apparatus 

 would not have been sufficient, two new ones, both of 24 inches, were 

 constructed at the same time, thus reducing the net cost of each. 

 Nothing more need be said in praise of the objective prism ; it is well 

 known how with it. at one exposure, may be photographed the spectra 

 of all the stars visible in a given field. There is much less loss of light 

 than in the use of a slit spectroscope, so that a shorter exposure is 

 necessary. Unfortunately, accurate determinations of wave-lengths 

 can not be made with that system and, despite the most ingenious con- 



