6 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



on the radiat'ion of a standard incandescent lamp supplied by the 

 Bureau of Standards. After applying the proper correction for stray 

 thermal losses and spherical reduction factor and reducing the read- 

 ings as taken to the standard distance of 50 cm. employed throughout 

 this investigation, the constant of the thermopile galvanometer 

 system was found to be 1 mm. = 1 scale division = 35.3 ergs per 

 second per square cm. By this constant the observed deviations 

 were reduced to absolute dynamical measure. 



As a matter of convenience and to establish an approximate ratio 

 between the ultra violet radiation from the various sources studied 

 and the radiation in the visible spectrum, an absorption cell which 



'MO IIUO 



liiOO 



500 700 



Figure 3. Absorption curve of water. 



1500 



eliminated nearly all the infra red was kept in front of the thermopile 

 window. This cell, Figure 2, was of glass, ground flat and exactly 1 cm. 

 thick, 44 mm. external diameter and 35 mm. internal diameter. The 

 glass ring was provided with a hole for filling and was closed by two 

 quartz plates cut across the axis, each 2.25 mm. thick and 44 mm. 

 diameter. These were fastened with hard shellac to the glass cell, 

 and the cell in use was filled with distilled water. The absorption of 

 a layer of distilled water of this thickness is shown in Figiu'e 3 taken 

 from Nichols's experiments.^*^ Quartz has no material absorption in 

 the part of the infra red spectrum transmitted and neither quartz nor 



1° Nichols, Physical Review, Vol. 1, p. 1. 



