MOUNT WILSON SOLAR OBSERVATORY. 261 



interval 10.5-15.5 is substantially that previously reported, and the 

 divergence for the faint stars indicated by the preliminary reduction 

 appears to be confirmed. 



In examining the table, it is to be noted that the Mt. Wilson mag- 

 nitudes and the differences Mt. Wilson — Harvard require the addition 

 of a constant, whose provisional value is +0.37 mag., to reduce them 

 to the international zero-point. 



The revision of the zero-point constant and the magnitudes of some 

 of the brighter stars is now in progress and, when completed, the 

 investigation of the photographic scale can be brought to a conclusion. 



In the meantime a satisfactory determination of the photovisual 

 scale has been made for magnitudes 2 to 17.5 by means of isochromatic 

 plates and a yellow filter. The results for the lower 6 magnitudes are 

 complete, and a preliminary connection with the stars of the sixth 

 magnitude has been made for the determination of the zero-point. 

 This and the magnitudes of the bright stars require revision, but the 

 reductions are sufficiently advanced to indicate a fairly satisfactory 

 agreement with the Harvard visual scale of the Polar Sequence. 



Important evidence bearing upon the relation of the photographic 

 and photovisual scales to each other is described below in connection 

 with the color results found for N. G. C. 1647. 



The Color of the Faint Stars. 



The extension of the photographic and photovisual scales to very 

 faint stars gives at once values for the color of these objects. The 

 dependence of color index (photographic minus visual magnitude) upon 

 spectral type is well known; its behavior with increasing magnitude 

 may be studied by comparing the photographic and photovisual scales. 

 Such a comparison has been made for the Pole and for the regions of 

 S Cygni and N. G. C. 1647, the scales for the last two regions being 

 established by comparisons with the Pole. In each case it was found 

 that the color index of the whitest stars gradually increases with 

 increasing magnitude. At the seventeenth magnitude the minimum 

 index is about +0.6 mag. Since the corresponding value for the bright 

 Bo stars is —0.4, the increase is about 1 mag. 



The interpretation of this result is uncertain. It may mean a pre- 

 ponderance of later type spectra among the fainter stars; it may be 

 the consequence of absolute luminosity, for the fainter stars are less 

 luminous, on the average, than the brighter objects; or it may be the 

 effect of a scattering of light in its passage through space. 



If the color indices are to be accepted as an indication of spectral 

 type, the conclusion would be that there is a gradually increasing 

 suppression of the earlier types with increasing magnitude, and hence 

 also with increasing distance, for the majority of the faint stars are 

 faint because of distance. 



