ABSORPTION PRODUCED BY FLUORESCENCE. 



101 



in the width of the slits. It was really a blank experiment to test whether the 

 illumination was uniform or not. 



All the four cubes were exposed to the exciting illuminations. 



(A.) 



The table shows the width of the slits for each of the five photographs. The 

 nearest approach to equality was obtained in 2 and 3, and the relative intensity was 

 reversed. The exposures were twelve minutes each. The plate was, however, con- 

 siderably fogged. The next plate (B) was taken in the same way as before, but the 



width of the slits was diminished. 



(B.) 



The equality was here between 2 and 3. The photographs were still too dense, 

 and the fogging too great, to make it possible to say with any degree of accuracy 

 which way the inequality, if any, went ; and it was really difficult to say if there 

 was any difference between 3 and 4. 



(C) was taken with an experiment of six minutes. 



In this case the equality was between 3 and 4. 



It was not possible to obtain a greater accuracy than 5 per cent, in any single 

 plate, but it did not seem necessary in the determination of the absorption to attain 

 even to this. 



In order to test whether the absorption was the same whether the substance was 

 fluorescing or not, the experiment was arranged in the following way : 



1. A photograph is taken with the screen, as shown in fig. 3. Let p be the ratio 

 of intensity of the light emitted by / to that emitted by a t ; then 



E (1 



