52 



FLUORESCENCE OF THE URANYL SALTS. 



THE EFFECT OF VARYING THE INTENSITY OF EXCITATION. 



To determine the effect of the intensity of excitation, a series of 

 measurements were made with the spark-gap at various distances from 

 the phosphorescent surface. The substance observed in these experi- 

 ments was uranyl rubidium nitrate. It was found possible to make 

 observations of the decay of phosphorescence with the excitation 

 reduced to a two-hundredth of that usually employed. 



From the curves obtained, of which four are given in figure 41, it 

 will be noted that all three processes are present, whatever be the 

 intensity of the exciting light; also that, taken roughly, processes 1 

 and 2 are of nearly equal duration, and that with decreasing intensity 

 of excitation the duration of each of these processes diminishes. 



DURATION OF PROCESSES 

 WITH EXCITATION 



E. 

 .40 



Ifc2 



.30 



.20 



.10 



.001 



.002 



FIG. 42. 



.003 



SEC. 



These relations are better shown in figure 42, in which the duration 

 of process 1 and the sum of the duration of processes 1 and 2, counting 

 from the close of excitation, are plotted, with the intensity of the ex- 

 citing light as ordinates. Approximately in both cases the duration is 

 proportional to the natural logarithm of the excitation. (See table 17.) 



This decrease in the duration of the two processes with falling 

 excitation affords an obvious explanation of the varying character of 



