MOUNT WILSON OBSERVATORY. 



229 



contains a concave grating used in conjunction with either a collimating 

 mirror or lens to render the light falling upon the grating parallel. The radius 

 of the grating is 1 meter. Mr. Merrill has employed this instrument to photo- 

 graph the spectra of a number of stars in the region to the red of Ha. 



The second spectrograph is designed for use on very faint stars and nebulae 

 and is attached to the plate-holder frame of the large reflector. Guiding is 

 by means of an auxiliary star in the field. The large ratio of the focal length 

 of collimator to camera, 8 to 1, should make this instrument efficient in 

 operation. 



The number of spectrograms obtained during the year with the Cassegrain 

 spectrographs was 1,443, of which 585 were made with the 100-inch telescope 

 and 858 with the 60-inch. The larger instrument was used mainly for stars 

 of large proper motion and for variable stars, while the general program of 

 observations on stars of the sixth and seventh magnitudes was carried on 

 with the 60-inch reflector. The spectra of a considerable number of compara- 

 tively bright stars of type A were observed with this instrument to provide 

 the material for an investigation of spectroscopic methods of deriving the 

 absolute magnitudes of such stars. 



The spectrograms were distributed as follows, according to the apparent 

 magnitudes of the stars observed : 



Radial Velocities. 



A large number of stars has been added to our lists during the year, many 

 of which are being observed for absolute magnitude and not for radial velocity. 

 For this reason, and because of the numerous variable stars upon our pro- 

 gram, the proportion of stars whose radial velocities have been determined is 

 somewhat less than usual. 



The more important results in this line of work may be summarized as 

 follows : 



(1) The radial velocities of 140 stars have been determined from three or 

 more spectrograms. With the completion of these observations a total of 

 over 1,000 stars with constant velocities is now available and the results are 

 being prepared for publication. This number does not include the results 

 for the Md stars by Mr. Merrill, nor those for the stars of the Selected Areas 

 by Mr. Humason. 



(2) Fifteen stars with variable radial velocities have been discovered, 

 among them several faint stars of the Cepheid and Algol types of light varia- 

 tion. 



(3) The orbits of 4 spectroscopic binaries have been derived by Mr. San- 

 ford, and 18 others are under observation, some of which are nearly completed. 

 About 130 spectrograms of spectroscopic binaries have been obtained during 



