MOUNT WILSON OBSERVATORY. 227 



ment of 2T5 in the 11>^ years' interval between the plates. Three new 

 novae in the Andronieda nebula and two new variable stars in the Trifid 

 nebula have been found on these photographs (p. 258). 



The investigation of the spectra and colors of nebulous stars by Mr. 

 Seares and Mr. Hubble has been completed for most of the known 

 objects brighter than the 13th magnitude. Their exceptional color 

 is strikingly illustrated by 17 faint stars involved in nebulosity near 

 o Persei, which show an average color excess of over half a magnitude 

 (p. 259). In contradistinction to this effect, probably a scattering by 

 the surrounding lumirious nebulosity, Mr. Seares has found no positive 

 evidence that the obscured areas, or "dark nebulse," in Perseus and 

 Taurus, produce any increased redness of the neighboring stars. Mr. 

 Hubble has photographed an abnormally high percentage of late- 

 type stars in these regions, but most of these are presumably dwarfs 

 lying in front of the obscuring clouds, which are comparatively near us 

 and cut off completely the light of the more distant stars (p. 262). 



Mr. Seares and Mr. Hubble have also made an extensive study of 

 the correlation of stellar luminosity with spectral tj^pe and color. By 

 plotting absolute magnitudes against specti'al types or equivalent 

 color-indices some interesting relationships appear. The curve of modal 

 values for the helium stars runs smoothly into that of the dv/arfs. The 

 Cepheids and pseudo-Cepheids of our own system present, in the 

 mean, a correlation of color with luminosity similar to that of the 

 Cepheids and the giant stars in globular clusters. From a corresponding 

 correlation it is also inferred that the stars in the galactic clouds are 

 similar in luminosity to these classes of stars. The frequency curve 

 for stars of all tj^ies is in good general agreement with the luminosity 

 and density functions of Kapteyn and van Rhijn (p. 277). 



With the aid of data bearing on absolute magnitude, Mr. Seares has 

 determined the mean total masses of visual binaries of the various 

 spectral types. These show the following remarkable correlation with 

 spectral type: 



BO, mass = 18; B5, 14; AO, 10; A5, 7; FO, 4.4; F5, 2.7; 

 GO, 1.7; G5, 1.3; KO, 1.2; K5, 1.1; M, 1.0. 



From FO on the masses in this summary are those of the dwarfs 

 (p. 276). Further reference to this important investigation will be 

 mads after the completion of studies still in progress. 



Photometric observations by Messrs. Seares, Shapley, Hubble, 

 Humason, and Lindblad have covered a wide range. Mr. Hubble has 

 nearly completed the determination of the photovisual and photo- 

 graphic magnitudes of 250 stars associated with nebulae and nebulous 

 clusters (p. 259). Mr. Seares has given special attention to the deter- 

 mination of the colors of nebulous stars and the stars in galactic clouds 

 and special fields (p. 263) . His color-comparisons of the Selected Areas 

 in the +30° zone with the North Pole is half completed, and the com- 



