MOUNT WILSON SOLAR OBSERVATORY. 265 



STELLAR PHOTOMETRY. 



The observational part of the investigations in stellar photometry 

 by Mr. Seares and Mr. Shapley includes 764 photographs, all made 

 with the 60-inch reflector and distributed as follows : 



Variables 270 



Clusters 171 



Selected Areas 189 



Orion Stars 80 



Miscellaneous 54 



Total 764 



Standard Polar Magnitudes. 



The investigation by Mr. Seares of the magnitudes of stars near the 

 pole, including those of the North Polar Sequence, has been com- 

 pleted. A summary of the results, including a list of photographic 

 and photovisual magnitudes for about 300 stars, has been published as 

 Mount Wilson Contribution No. 97, and all of the tabular data for the 

 final publication of the results have been compiled. These include 

 photographic magnitudes from 2.5 to 20.1 for about 600 stars, and pho- 

 tovisual results from 2.1 to 17.5 for about one-half this number of 

 objects. 



The appearance of Miss Leavitt's investigation in volume 71 of the 

 Harvard Annals has made it possible to compare in detail the Harvard 

 and Mount Wilson results. The large divergence between the two 

 scales in the region of the faint magnitudes is due to two circumstances 

 affecting the Harvard reduction of photographs made at Mount 

 Wilson with the 60-inch reflector: first, the neglect of the distance 

 correction; second, the application of a correction depending on the 

 order of exposure. 



The importance of the distance correction and its influence upon 

 the scale have been discussed in previous reports. The first and last 

 exposures on a plate, although of equal duration, often differ systemat- 

 ically. Some of the Mount Wilson plates showed such differences, 

 and in the Harvard reduction it was assumed that the intermediate 

 exposures, which were used for the derivation of the scale, were affected 

 proportionally to the time which had elapsed since the beginning of 

 the first exposure. It seems, however, that this assumption is unjusti- 

 fiable. The systematic effect is probably confined to the first exposure, 

 which does not enter into the scale, while apparently the intermediate 

 exposures require no correction. With allowance for these two circum- 

 stances, the Harvard and Mount Wilson measures of the plates for the 

 faint stars give identical results for the scale. 



The determination of color indices for the Polar Sequence Stars has 

 made it possible to complete the reduction of the Harvard magnitudes 

 to a homogeneous color system. The corrections thus introduced 

 improve the agreement of the Harvard and Mount Wilson scales, and,. 



