MOUNT WILSON OBSERVATORY. 



239 



brought out by these photographs, and the structure rendered visible 

 by the aid of the stereoscope, should make them of considerable value to 

 selenologists. 



A few planetary photographs have also been taken, but not under 

 good atmospheric conditions. 



STELLAR PHOTOMETRY. 



The observational part of the investigations in stellar photometry by 

 Mr. Seares and Mr. Shapley includes 632 photogi'aphs made with the 

 60-inch and the 100-inch Hooker reflectors, distributed as follows: 



Selected areas 153 



Clusters 100 



Color photographs 126 



NebuljE 43 



Galactic clouds 182 



Miscellaneous 28 



632 



Photographic Magnitudes for the Selected Areas. 



The determination of the magnitudes for Areas Nos. 1 to 139, to the 

 limit shown by an exposure of 15 minutes with the 60-inch reflector, 

 has been finished by Mr. Seares with the assistance of Miss Joyner and 

 Miss Richmond. The results are on the normal scale, and have been 

 reduced to the international zero-point by methods which provide a 

 series of closing conditions similar to those of a triangulation net. 



The closing errors, in hundredths of a magnitude, are shown by the 

 accompanjdng table. A ''circuit" includes, in general, 6 fields, one of 

 which is the standard region at the pole. For the zones, the number of 

 fields is 6 at 75°, 12 at 60°, and 24 for each of the remaining zones. 

 These outstanding deviations were made the basis for the final adjust- 

 ment of the zero-points in each of the 139 areas. Rectangular coordi- 

 nates of the stars in all the areas but three have been measured with 

 sufficient precision for identification. 



The final catalogue will include these results, combined with meas- 

 ures on photographs with multiple exposures of 60 and 5 minutes 

 which should extend the scale about a magnitude below that of the 

 plates of 15 minutes' exposure. The long-exposure photographs have 



