764 ENERGY AND VISION 



phere interposed and the brightness of the sky. Although his ex- 

 periments were made with great care, the differences from one day 

 to another are important. However, when he expresses the energy 

 in absolute units, he always refers to the same mean amount of energy 

 radiated by the sun on 1 sq. cm. This amount is certainly not 

 constant, if one judges from the differences observed in two measure- 

 ments of sensitivity of the eye of the same individual at different 

 dates. On the contrary, for a given wave length, our measurements 

 always agreed closely, as our source of radiation was very nearly 

 constant, owing to the absence of a varying amount of water vapor 

 interposed. This may in some way account for the discrepancies 

 observed. 



I wish to express my thanks to Dr. Harry Clark of The Rockefeller 

 Institute for the valuable help he was kind enough to give me in 

 solving certain difficulties which were encountered during this work. 



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