MOUNT WILSON OBSERVATORY. 237 



provided by using the same grating at the 30-foot focus of the spec- 

 trograph in conjunction with the same interference apparatus as 

 before. The flatter field of the long-focus lens, combined with the 

 increased auxiliary dispersion and greater slit-'W'idth required, should 

 prove of material assistance in such observations. 



POLARIZATION OF THE NIGHT SKY. 



At the request of Lord Rayleigh (then Professor Strutt) observations 

 were begun by Mr. Babcock during the past spring with a view to 

 finding out whether the background of the night sky, which is by no 

 means absolutely black, consists of scattered sunlight. Theoretical 

 considerations as well as laboratory data have shown that if such is the 

 case, the feeble light which constitutes this background will be polar- 

 ized in a plane passing through the sun, even though the scattering be 

 produced by hj^drogen extremely attenuated. A large Savart polari- 

 scope was loaned to us by Lord Rayleigh for making the observations, 

 but a smaller instrument, more economical of light and at the same 

 time more sensitive than the Savart polariscope, was developed in our 

 laboratory- and used instead. The photographs were taken with every 

 precaution to eliminate false effects. They extended over a period of 

 4 or 5 months, and 8 plates were secured under conditions thought to 

 be satisfactory for the purpose. These photographs indicate that prac- 

 tically none of the light in the background of the night sky is polarized. 



INVESTIGATION OF STARS AND NEBUL/E. 

 OBSERVING CONDITIONS. 



The obser\nng conditions at night on Mount Wilson for the year 

 ending August 31, 1919, are indicated by the accompanying tables 

 prepared by Mr. Hoge, night assistant at the 60-inch reflector. The 

 data show conditions slightly above the average for the past 7 years. 

 Observations were carried on during all of 204 nights and a part of 

 97 nights. On 64 nights no observations were made on account of 

 the weather. The 60-inch telescope was in use a total of 2,378 hours, 

 which is 66 per cent of the total hours of darkness. The total exposure 

 time for the year was 70 per cent of the available observing weather 

 and 46 per cent of the total hours of darkness. The month of June 

 gave 100 per cent of observing weather at night. This is the first 

 time our records show the nights of any calendar month to have been 

 entirely free from clouds. The telescope was in use during the whole 

 of each night from May 31 to July 15 inclusive, 46 consecutive nights, 

 the longest uninterrupted observing period we have recorded. 



The past year was the driest since the beginning of our record 15 

 years ago. The total precipitation was 20.62 inches, which is 13.69 

 inches below the normal. The total snowfall was 55 inches. A very 

 severe and remarkable wind-storm occurred on November 24, 1918, 



