Fluorescence and Photo-Chemistry. 761 



Intensity Relation between Fluorescent and 

 Exciting Light. 



It has been pretty well established that the intensity of 

 the fluorescent light is proportional to the intensity of the 

 exciting light. Knoblauch reduced the intensity of his 

 exciting beam to 1/6400 by means of dark glass, and found 

 that the relation held. As his maximum intensity fell far 

 short of the intensity at the focus of my lens (which was 

 roughly 1400 times the intensity of normal sunlight, and as 

 it was very essential for the interpretation of the results of 

 certain experiments about to be described to be sure that the 

 relation held for the very high intensities used, the following 

 very simple and instructive experiment was made. 



The theory of the experiment will be made clear by fig. 1. 

 The 6-inch lens a was illuminated by sunlight, and a small 



Fig. 1, 



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prism b of about 10° was mounted near one edge of it. The 

 area of this prism which was exposed was 4 per cent, of the 

 area of the lens not covered by the prism. This ratio of 

 areas was chosen for reasons which will appear presently. 

 The powerful beam concentrated by the lens illuminated the 

 small bottle c, which was filled with a solution of fluorescein. 

 The beam deflected by the prism was reflected by the silvered 

 mirror d and concentrated on the bottle e, which also con- 

 tained fluorescein. The ratio of the illuminating intensities 

 was thus 4: 100. If the fluorescent ratios were in the same 

 ratio, then the image of the fluorescent spot on bottle c seen 

 reflected in a single surface of glass (4 per cent, reflexion 

 coeff.) should have the same intensity as the fluorescent spot 

 on bottle e viewed directly. The reflexion was taken from 

 the surface of a small prism /, the opposite surface having 

 been painted with black paint to prevent reflexion. 



As this prism had a razor-edge, it was possible by holding 



