MOUNT WILSON SOLAR OBSERVATORY. 



257 



which should reduce the effect of accidental errors in determining the 

 location of the points observed. The first order, scale 1 mm. = 0.7 a, 

 is used for the major part of the observations, but for some special pur- 

 poses the second order, scale 1 mm. = 0.35 a, is employed. The regions 

 include that between X 4220 and X 4280, covered by all observers in com- 

 mon, and the special region X 5100 to X 5300, assigned to this Observa- 

 tory by the Committee of the International Union for Cooperation in 

 Solar Research. Observations are made at latitudes 0°, 30°, 60°, 75°, 

 80°, 85°, and 90°, with particular emphasis upon the higher latitudes. 



INVESTIGATIONS OF STARS AND NEBULAE. 

 OBSERVING CONDITIONS. 



During the year ending August 31, 1914, the 60-inch reflector was 

 in operation 188 entire nights and during parts of 83 nights, while on 

 94 nights no observations could be made. The instrument was in use 

 2,215 hours out of a total of 3,600 hours of darkness, or 61.5 per cent 

 of the total night-time. There were 1,315 hours lost because of weather 

 conditions, and 70 hours on account of silvering the 60-inch mirror and 

 repairs to the instrument. The statistics for each month are given in 

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



The total exposure time for the year is 1,607 hours, which is 70 per 

 cent of the working time and 45 per cent of the hours of darkness. The 

 exposures on individual plates range from 1 minute to 30 hours, the 

 average being 49 minutes. 



As appears from the above table, the weather conditions during the 

 winter months were unusually poor, while those of sunamer have been 



