212 



CARNEGIE INSTITUTION OF WASHINGTON. 



and averaging the results over a sufficient period to eliminate the 

 effect of changes in the transparency of the sky and in the coefficient 

 of reflection of the mirrors, it may be possible to determine whether 

 considerable variations take place. 



INVESTIGATIONS OF STARS AND NEBULA. 

 Observing Conditions. 



During the year ending August 31, 1913, the 60-inch reflector was 

 in operation 190 entire nights and during parts of 100 nights, while 

 on 75 nights no observations could be made. The instrument was 

 in use 2,383 hours out of a total of 3,605 hours of darkness, or 66.1 

 per cer.t of the total night time. There were 1,193 hours lost because 

 of weather conditions and 29 hours on account of silvering the 60- 

 inch mirror, which was done on October 19, 1912, April 16 and 

 August 10, 1913. The statistics for each month are given in the 

 following table, prepared by Mr. Hoge, night assistant. 



Since June 1, 1913, an exact record has been kept of the actual 

 times that exposures were in progress, showing a total for 3 months 

 of 407 hours and 15 minutes, or 70 per cent of the time the instru- 

 ment was in use. In other words, 30 per cent of the working-time 

 was required for making settings, changing plates, picking up guiding 

 stars, focusing, and making such adjustments as are incidental to 

 the work. The amount of time necessary between exposures varies 

 much with the nature of the observations, the Cassegrain spectro- 

 graph requiring from 5 to 11 minutes, while direct photography 

 involves much longer intervals, on account of the careful knife-edge 

 focusing, the necessity of finding suitable guiding stars, the change 

 of diaphragms in photometric work, etc. 



