MOUNT WILSON OBSERVATORY. 261 



Photographic and Photovisual Magnitudes in the Selected Areas. 



The combination of results obtained by Professors Kapteyn and 

 van Rhijn with those derived by Mr, Seares with the assistance of 

 Miss Joyner and Miss Richmond has continued. Some 25,000 or 

 30,000 stars measured at Mount Wilson fall outside the limits of the 

 Groningen measures. These are in process of transfer to cards for 

 arrangement in catalogue form. The results, including the checking 

 of the coordinates, are practically complete for Areas 1 to 67. 



The observing program for photovisual magnitudes in 42 of the 

 Selected Areas has been finished. With the exception of a small num- 

 ber of plates, the measures are complete; the reductions are well 

 advanced, although little has been done during the year because of 

 concentration of effort in other directions. 



Photometric Survey of the Pleiades. 



With the assistance of Miss Richmond, Mr. Shapley has finished a 

 survey of the photographic and photovisual magnitudes of about 850 

 faint stars in the Pleiades, many of which are certainly members of 

 the physical system. The magnitudes were determined by comparison 

 with the North Polar Standards, and will be of importance for statis- 

 tical discussions and for comparisons of the Mount Wilson photo- 

 metric system with that of other observers. The interval covered is 

 from magnitude 10 to 15.5. Within this interval the members of the 

 cluster are dwarfs, of absolute magnitudes 5 to 10.5. Beyond the 

 eleventh magnitude the mean color index of cluster and background 

 stars is sensibly constant and equal to about 0.7 mag. 



North Polar Standards of Magnitude. 



In connection with the work of the International Commiittee on 

 Magnitudes, of which Mr. Seares is chairman, several tests of the 

 Mount Wilson photometric scale have been undertaken. These con- 

 cern mainly the relation of the standards below the tenth magnitude 

 to those near the sixth magnitude, which define the international 

 zero-point. To obtain evidence involving the use of an instrument 

 other than the 60-inch reflector, with which the standards were es- 

 tablished, Mr. Humason has begun a series of observations with the 

 10-inch refractor. 



A second test is based on a comparison of the results described in 

 the preceding paragraph with a long series of photographic magni- 

 tudes and color-indices of stars in the Pleiades, placed at our dis- 

 posal by Professor Hertzsprung. The scale for the latter series was 

 established by means of an objective grating attached to the 80-cm. 

 refractor of the Potsdam Observatory and is referred to the zero-point 

 of the Gottingen Aktinometrie. The color indices depend on measures 

 of effective wave-length on photographs taken some years ago by 



