240 CARNEGIE INSTITUTION OF WASHINGTON. 



been completely measured at Groningen under the direction of Profes- 

 sor Kapteyn and Dr. van Rhijn. Results for the Areas 1 to 25, includ- 

 ing about 7,000 stars, are now in our hands, and their combination with 

 our own measures is practically complete. 



Photovisual Magnitudes for the Selected Areas. 



The determination of the photovisual magnitudes by Mr. Seares and 

 Mr. Shapley for 43 of the Selected Areas has progressed slowly, owing 

 to concentration of effort on the program for the photographic magni- 

 tudes. The photographs, with the exception of a dozen plates, have all 

 been obtained. The measures by Miss Joyner and Miss Richmond are 

 well advanced, but little has thus far been done with the reductions. 



Constitution of the Galactic Clouds. 



In continuation of the study of the distribution of the stars, a series 

 of photographs has been made by Mr. Seares with the 60-inch reflector 

 of fields in and near some of the conspicuous galactic clouds, for the pur- 

 pose of determining the brightness and number of stars of which they 

 are composed. 



To avoid the very great labor which would be involved in deriving 

 magnitudes for the individual stars, the results will be based on counts 

 of stars to the limits determined by a definite series of exposures. In 

 general, six or eight fields are chosen for each of the clouds studied, at 

 intervals of about a degree, so that the series extends across the cloud 

 and connects with the regions of smaller star density on either side. 

 Exposures of 20 seconds and 1, 3, 9, and 27 minutes are made in suc- 

 cession on each field, under good observing conditions. In addition, 

 at least one series of control exposures is made during each night of 

 observation on a neighboring Selected Area, for which the magnitudes 

 are known. The latter series will serve to determine the limiting 

 magnitudes corresponding to the various exposure-times. Care is 

 taken to use only plates of the same emulsion on any given night. It 

 is believed that this procedure will yield reliable results with a min- 

 imum of labor. 



Brightness op the Galactic System. 



The recent determination of the density and luminosity laws by Pro- 

 fessor Kapteyn and Dr. van Rhijn has led Mr. Seares to compute the 

 surface brightness of the galactic system as it would appear from a dis- 

 tant external point in the direction of the galactic pole. The equiv- 

 alent apparent magnitude of an area of 1 square second of arc at the 

 center of figure of the system seen in projection is approximately 23. 

 The decrease in brightness from the center outward is slow, amounting 

 to less than 2 magnitudes in a distance of 5,000 parsecs. 



The results bear upon the question of the relation of the spiral nebulse 

 to the galactic system. The surface brightness of all nebulse known is 



