MOUNT WILSON OBSERVATORY. 229 



spectively to oxygen and water- vapor. Their increasing importance as 

 reference lines requires more definite knowledge of their behavior under 

 varying conditions (pp. 259, 260). 



INFRA-RED SOLAR SPECTRUM. 



The infra-red solar spectrum has been photographed by Mr. Brackett 

 to X9900 on Seed 23 plates bathed with dicyanin. Prehminary analy- 

 sis, by means of a Hilger monochrometer, was effective in removing 

 the scattered light which had obhterated the finer structm-e on pre- 

 vious photographs. 



The region between X9000 and X9350 proves to be particularly 

 rich in fine lines. On the other hand, the region between X9300 and 

 X9500 is characterized by broad diffuse radiations covering several 

 angstroms, um^esolved with a dispersion of 3 angstroms to the milU- 

 meter. Bej^ond this point the finer structure agains appears until 

 it is lost because of the rapidly falling sensitiveness of the plate. 



Solar-rotation plates have been obtained as far as X97o0, which serve 

 to distinguish the solar from the telluric lines. 



Preliminary determinations of wave-lengths have been made 

 throughout the whole region. More than 550 lines in all have been 

 measured, of which roughly 10 per cent prove to be solar. The larger 

 portion is probably due to water-vapor, as shown by the remarkable 

 increase in intensity during times of high humidity. 



RESEARCHES ON STARS AND NEBUL/E. 

 OBSERVING CONDITIONS. 



The conditions for night observing for the year ending August 31, 

 1920, are shown in detail by the subjoined tables, compiled from records 

 of the 60-inch reflector. The conditions were slightly better than the 

 mean for the past eight years. The 60-inch telescope was in use 65 per 

 cent of the hours of darkness. The total exposure-time was 71 per cent 

 of the available observing weather and 46 per cent of the total hours 

 of darkness. Weather conditions recorded by the Government in- 

 struments at the Observatory show for the year a mean temperature of 

 54° F.; a maximmn of 95° F. on August 13, and a minimum of 15° F. 

 on November 28. The total precipitation was 31.19 inches, being a 

 little below normal; the total snowfall, 43 inches. The average wind 

 velocity was 10.7 nfiles per hour. 



The most destructive forest fire ever kno\^Ti in the vicinity broke out 

 on September 12, 1919. Two serious fires were in progress at the same 

 time, one about 20 miles to the northeast and one about the same dis- 

 tance northwest of the Observatory. The smoke became so dense at 

 Mount Wilson that observations were entirely prevented on several 

 nights. These fires were beyond control for two weeks, and at times 

 threatened to reach the observatory grounds. Fortunately, a heavy 

 rainstorm on September 26 extinguished the fires completely. 



