MOUNT WILSON SOLAR OBSERVATORY. 263 



zero-points of the magnitude scales for the polar stars are provisional, 

 and their transfer to the cluster introduces into the color indices a 

 further uncertainty of 0.04 mag. 



Color Variation of the Cluster-Type Variable RS Bootis. 



Eighty-five ordinary and isochromatic plates, alternately exposed on 

 July 16, 17, and 18, establish with a good degree of precision the photo- 

 graphic and photovisual light-curves of the cluster-type variable RS 

 Bootis (period 9'' 03™). The photographic amplitude is 1.5 mags., the 

 photovisual about 1 mag. A clearly marked change in color is thus 

 revealed. The zero-points of the magnitude scales are yet to be deter- 

 mined, so that the absolute values of the color index can not now be 

 given. The change in the color index of 0.4 mag. agrees satisfactorily 

 with Mr. Pease's observations of the change in the spectrum. His 

 limiting values are Bg at maximum and Fq at minimum. The corre- 

 sponding color indices on Parkhurst's scale would be —0.10 and +0.40. 

 The epochs of maxima for the two curves agree within the uncertainty 

 of the determination. 



Magnitudes for the Selected Areas. 



Considerable progress has been made in the determination of standard 

 magnitudes for the Selected Areas, as described in the previous report, 

 though less than could be expected normally. Unsatisfactory atmos- 

 pheric conditions have interfered with the observational program, and 

 the reductions have been delayed in order that the information con- 

 cerning systematic errors afforded by the final discussion of the results 

 for the polar stars might be available. The completion of the latter 

 investigation now makes it possible to take up actively the reductions 

 for the Selected Areas. 



STELLAR SPECTROSCOPY. 



The stellar spectroscopic work of the past year has included observa- 

 tions of the spectra of stars selected from the following lists : 



1. Stars of types F to Mo of very small proper motion. Their spectra 

 have been used for radial-velocity determinations, for investigations of 

 the absorption of light in space, and for a study of spectral differences 

 between stars of small and stars of great absolute brightness. 



2. Stars with measured parallaxes and in general of very large proper 

 motion. These stars have been under investigation for the past four 

 years, and the amount of observational material regarding them has 

 now reached considerable proportions. Their characteristics are the 

 precise opposite of those of the stars in list 1, these stars being com- 

 paratively near to the earth, of small absolute brightness, and probably 

 of small mass and size. 



3. Stars suspected of belonging to the Perseus group. 



