REPORT OF THE SECRETARY 49 



MOUNT WILSON EXPEDITION 



Messrs. Abbot and Aldrich continued the occupation of the station 

 at Mount Wilson up to September 1932 for the purpose of attempting 

 measurements of the solar spectrum for very long wave lengths, from 

 8 to 30 microns, far down in the infrared. The ultimate object of 

 these studies is to determine the transparency of the atmosphere for 

 these long wave lengths and its dependence on the atmospheric 

 humidity, but more especially on the quantity of atmospheric ozone. 

 It is believed that the temperature of the earth may probably depend 

 intimately on the absorption of outgoing earth rays produced by the 

 ozone of the atmosphere. Measurements made in the ultraviolet 

 spectrum have indicated that the ozone content of the atmosphere 

 is variable and that the variations are associated with the sun-spot 

 numbers. Inasmuch as an important band of absorption by ozone 

 occurs at exactly the only region of the terrestrial spectrum where the 

 atmosphere is otherwise highly transparent, it is very probable that 

 the variation in the atmospheric ozone is an important weather 

 phenomenon. The difficulties of observing in this region are very 

 great. The solar spectrum there is extremely feeble compared with 

 regions of lesser wave length, and a great deal of the shorter wave 

 length rays are scattered into the long-wave region which must be 

 studied. Accordingly a double spectroscopic arrangement must be 

 provided to produce a better spectrum. This not only complicates the 

 apparatus but requii'es exceedingly sensitive radiation-measuring 

 devices to observe the feeble indications of energy. Computation 

 showed that neither the bolometer, the thermopile, nor the radiom- 

 eter appeared adequate for the purpose. Dr. Abbot was fortunate 

 in recalling the special radiation-measuring device which he invented 

 about 25 years ago, and in very greatly improving its construction so 

 that it becomes of the very highest sensitiveness. The description of 

 this instrument which he prepared and used at Mount Wilson was 

 published in 1932 under the title "The Kampometer, a New Instru- 

 ment of Extreme Sensitiveness For Measuring Radiation." 



Considerable time at Mount Wilson was occupied in devising and 

 constructing and learning to use this instrument, but Messrs. Abbot 

 and Aldrich before their departure were able to make certain pre- 

 liminary measurements in the solar spectrum at very long wave 

 lengths which indicated that sufHcient sensitiveness is available. 



Attempts are under way to improve the kampometer by the sub- 

 stitution of bimetallic strips composed of cadmium and molybdenum 

 and by more exact construction. Also the double spectroscope is being 

 rebuilt, and it is believed that an expedition in 1934, if that be possible, 

 may add valuable knowledge of the extreme infrared solar spectrum. 



