MOUNT WILSON SOLAR OBSERVATORY. 189 



Thirteen plates containing four exposures each were taken by Mr. Fath 

 for the comparison of the magnitudes of the Kapteyn selected areas with 

 those of the Pritchard-Kapteyn areas. These plates have not been measured. 



STELLAR SPECTROSCOPY. 



A large part of the stellar spectroscopic work done by Mr. Adams and 

 Mr. Kohlschiitter during the year has been a continuation of the determina- 

 tion of the radial velocities of certain selected stars, mainly of types A and 

 B, whose motion is of particular importance in the study of star streaming. 

 Between September 1, 191 1, and September 1, 1912, about 850 spectrograms 

 were obtained with the Cassegrain spectrograph, and all of the photographs 

 have been measured and reduced by at least one computer. In most cases 

 two or more measures are available. 



Two modifications of the spectrograph have been made. The first was the 

 substitution of a camera of 102 cm. focal length and one prism for the 

 shorter camera of 46 cm. focal length and two prisms. This has been found 

 of decided advantage in the study of the spectra of A and B type stars, 

 which have few lines and do not require high revolving power. The ex- 

 posure times with the two combinations are not far from equal, a slight ad- 

 vantage lying with the shorter camera. A second modification has been the 

 use of the 46 cm. camera with a single prism for observations of very faint 

 stars. This optical system was first employed in the case of Nova Gemi- 

 norum No. 2 after the star had become too faint to be photographed with 

 the more powerful apparatus, and it has proved of great value in the study 

 of faint stars, especially of the solar type of spectrum. Under good condi- 

 tions we have secured fully timed spectrograms of stars with a visual mag- 

 nitude 7.2 in 40 minutes. The faintest star so far observed with this instru- 

 ment is of magnitude 8.2 on the visual scale. 



The principal results of the observations with the Cassegrain spectrograph 

 during the year may be summarized as follows : 



( 1 ) The discovery of about 45 new spectroscopic binaries, making a total 

 of about 95 so far found with this instrument. Many other stars are upon 

 the list of probable binaries and are at present under observation. Most of 

 these stars are of types A and B. 



(2) The radial velocities of about 70 stars with spectra of types A and B 

 which have constant radial velocities have been determined from three or 

 more observations. 



(3) One or more observations of about 140 other stars of types A and B 

 have been secured during the year. 



(4) The radial velocities of about 40 stars of types F to M, mainly fainter 

 than magnitude 5.5, have been determined from three or more observations. 

 Most of these stars are of known proper-motion and parallax, and accord- 

 ingly their velocities in space can now be obtained. An interesting feature 



