MOUNT WILSON OBSERVATORY. 203 



Photovisuai. Magnitudes for the Selected Areas. 



Plans for this investigation by Mr. Scares and Mr. Shapley have 

 been outhned in previous reports. The observations have been 

 continued, and 120 of the 257 photographs required have now been 

 obtained. The measures, which have been made by Miss Winn and 

 Miss Helen Burns for the greater part, are nearly complete to date. 



Investigation of the Color of Stars. 



The annual report for 1916 included an account of some preliminary 

 experiments by Mr, Seares on the measurement of the color of stars 

 by a method which seems not to have been used before. Several 

 successive exposures are made on an isochromatic plate with exposure- 

 times forming a geometrical progression. Owing to the relatively 

 low j^ellow-sensitiveness of such plates, the resulting images are 

 largely due to blue and \'iolet light, and are called "blue" images. 

 Two or more additional exposures on the same plate through a yellow 

 filter register the intensity of the yellow hght. The ratio of the 

 exposure-times necessary to produce blue and yellow images of the 

 same size, or more conveniently the logarithm of this ratio, which is 

 easily derived from the observational data, is a measure of the star's 

 color. Several sources of error depending upon variations in the color- 

 curve of the plate and other peculiarities in the photographic process 

 immediately suggest themselves, but by suitable precautions they 

 are easily controlled. 



The colors of the North Polar Standards to magnitude 15.5 have 

 been determined by this method and compared with the color-indices 

 obtained from the investigation of their photographic and photo- 

 visual magnitudes.* The agreement is excellent and demonstrates 

 the usefulness of the method, which has the advantage of being both 

 precise and expeditious. 



In the meantime, much observational material relating to various 

 problems has been accumulated and is now under discussion. Among 

 other questions the dependence of color upon absolute magnitude in 

 stars of the same spectral type has received much attention. For red 

 stars the phenomenon is important, the difference between giants 

 and dwarfs amounting to 0.5 or 0.6 mag. in the color-index. The 

 relation of color to spectral type for constant absolute magnitude is 

 also being studied, and in this direction numerous peculiarities require 

 consideration. The spectroscopic investigations by Mr. Adams have 

 shown that many features of the general spectrum which commonly 

 follow the changes in the hydrogen lines in the normal succession of 

 types do not in all cases show the usual correlation, and for many 

 stars the type determined from the hydrogen fines differs notably 



♦Proceedings National Academy of Sciences, 3, 29, 1917. 



