MOUNT WILSON SOLAR OBSERVATORY. 217 



provides the means for the determination of their absolute motion in 

 space. Their spectra in general are of types F to M. 



In addition to these two main lists a few photographs have been 

 obtained of the spectra of stars of type N (Secchi's Type IV) pre- 

 hminary to a study by Mr. van Maanen of the spectra of these stars 

 and a determination of their radial velocities. Mr. van Rhyn has 

 also employed the Cassegrain spectrograph in the determination of 

 the spectral classification of certain stars with a view to selecting 

 such as are best adapted for the study of some problems relating 

 to the absorption of light in space. On a few of his negatives the 

 spectra of two stars having as nearly as possible identical spectra 

 have been photographed side by side, one star having a large and the 

 other a small proper motion. 



The observational work has been carried on by Mr. Adams, Mr. 

 Kohlschiitter, Mr. ^Vhitney, and (during Mr. Kohlschiitter's absence) 

 Mr. van Maanen. Miss Lasby has also assisted in a part of the 

 observations. A summary of the observations for the year Sep- 

 tember 1, 1912, to September 1, 1913, follows: 



Spectrograma of stare types A, B, and Oe 788 



Spectrograma ot stars types F to M 258 



Spectrograms of stars type N 10 



Spectrograms of Nova Geminorum No. 2 5 



Total number of spectrograms 1 ,061 



The use of reduction tables has enabled the department to make 

 rapid progress in the measurement and reduction of this large amount 

 of observational material, and at present two, and in many cases 

 three, independent sets of determinations of the radial velocity are 

 available for each photograph. This part of the work has been 

 carried on by Miss Lasby, Miss Burwell, and Miss Ensign, of the 

 Computing Division, and by Mr. Adams and Mr. Kohlschiitter. 

 Miss Shumway has assisted in the reduction of the measures. 



Some of the principal results of the radial- velocity work may be 

 summarized as follows : 



(1) The total number of stars of types B and A for which three 

 or more determinations of the radial velocity are available and which 

 have constant velocities is 245. These stars are for the most part 

 between the magnitudes 5.0 and 6.5 on the Harvard scale. 



(2) The total number of spectroscopic binaries of types B and A 

 (so far found) is 99. A considerable number of additional stars show 

 variations which are probably real. In view of the difficult character 

 of their spectra, however, additional observations are required to 

 establish their variation. 



(3) The radial velocities of 18 stars between magnitudes 5.5 and 

 7.2 having spectra of types F8 to Mb and small proper motions have 

 been determined from three or more observations. 



