MOUNT WILSON OBSERVATORY. 227 



four areas have been compared with the pole for the determination of the 

 zero-point of the color-scale, and for half of the zone series of longer exposures 

 have been made to extend the observations to the fainter stars. 



A number of additional fields of special interest have also been observed, 

 among them Selected Area No. 110, which lies between the two branches of the 

 Milky Way south of Cygnus. The star density in this region is exceptionally 

 low, and the faint stars are all reddish. Preliminary measures of 120 stars 

 between the fourteenth and nineteenth magnitudes reveal no color indices 

 less than 0.5 mag. The maximum frequency occurs for a color-index of 0.9 

 mag. 



Magnitudes of Central Stars in Planetary Nebulae. 



Mr. Hubble is engaged in the construction of a photometric catalogue of 

 the nuclei or central stars of planetary nebulae. Magnitudes are determined 

 by the method of polar comparisons, the exposure-times being sufficiently 

 short to give images of the central stars essentially free from nebulosity. 

 The photographic magnitudes of 20 such stars and the photovisual magnitudes 

 of 10 have already been determined. The color indices range from —0.2 

 to —0.7 mag. 



A photometric study of the Algol variable C. C— 37° 13034 is being made 

 by Mr. Hubble. This star is involved in the dense diffuse nebula N.G.C. 

 6727. The case is unique, and an accurate determination of the light-curve 

 may provide evidence as to the effect of a nebulous medium on orbital motion 

 and surface brightness. 



Miss Mayberry has finished the measurement and reduction of the photo- 

 graphs of M 5 taken at the request of Professor Turner for the determination 

 of the variation of comparison star k of Bailey's variable No. 33. The star 

 seems to be a cluster-type variable of small amplitude. The discussion of the 

 data for the determination of the period is in the hands of Professor Turner. 



The compilation of photographic magnitudes in the Selected Areas by Miss 

 Joyner and Miss Richmond, under direction of Mr. Seares, has continued. 

 The combination of the Mount Wilson magnitudes with those derived at 

 Groningen by Professors Kapteyn and van Rhijn is complete for Areas 1 

 to 67, and half finished for Areas 68 to 139. The Groningen results for all 

 the areas are now in our hands. 



North Polar Standards of Magnitude. 



An independent determination of the photographic and photovisual scales 

 of magnitudes for Polar Sequence stars to the fifteenth and thirteenth magni- 

 tudes, respectively, confirms closely the results published by Mr. Seares in 

 1915. The photographs were taken with the 10-inch refractor by Mr. Huma- 

 son, who has also made the measures. The discussion of these data may be 

 found in Contribution No. 234. 



A similar confirmation of the earlier results is afforded by the recent investi- 

 gation at Greenwich by Jones, and by the revision of the results obtained at 

 the Harvard Observatory. There is little doubt, therefore, of the reliability 

 of the scales now available. Mr. Seares has discussed all of the Mount 

 Wilson data bearing on this question, including a direct determination of 

 the colors of the stars by the method of exposure ratios, as a preliminary 

 to the preparation of the report of the International Commission on Stellar 



