194 STUDIES IN LUMINESCENCE. 



In each of these two figures also we have plotted the carves for the coeffi- 

 cient of absorption (.1) and for the energy distribution in the fluorescence 

 spectrum (F). It would seem reasonable to expect that the curve of specific 

 exciting power might show some peculiarities either in the region of maxi- 

 mum absorption or in that of most intense fluorescence. It appears, how- 

 ever, that nothing of this kind occurs. 



Before beginning these experiments we were of the opinion that either 

 the specific exciting power would prove to be constant, so that the same 

 quantity of absorbed energy would produce the same fluorescence regardless 

 of its wave-length, or else that the effectiveness of the exciting light would 

 prove to be greater for the shorter wave-lengths. The readiness with which 

 fluorescence is excited in the ultra-violet spectrum made the latter view 

 seem plausible. As it turns out, neither of these views is in accord with the 

 facts. We were unfortunately unable to extend the observations to the 

 ultra-violet spectrum. It seems clear, however, that if we confine our 

 attention to wave-lengths falling within the range of one absorption band 

 the light lying near the red side of the band is more effective in producing 

 fluorescence than that lying on the violet side, and the change in the specific 

 exciting power as we pass through the band is steady, without any indica- 

 tion of anything selective in the neighborhood of the region of maximum 

 absorption. 



