66 



STUDIES IN LUMINESCENCE. 



/ and I~- is linear for small values of /. Since the early portion of each 

 decay curve is chiefly determined by the first two or three points, which 

 are the most difficult to observe, the values plotted for I are subject to 

 considerable error. Curve A is nevertheless reasonably smooth and 

 indicates nearly exact proportionality between intensity of excitation and 



20 4-0 60 80 100 



Seconds 



Fig. 57- 



Effect of varying the intensity of the exciting light (as indicated on each curve). 

 Exposure 2 minutes in each case. 



initial phosphorescence. It is only with the most intense excitation used 

 that saturation begins. 



It is to be observed that in most cases the values of I that are computed 

 from the data of Fig. 55 lie on the same curve as those obtained from 

 the data of Fig. 57. A well-marked difference is noticeable only in 



300 



200 



100 



Fig: 58. 



Effect of the intensity of the excitation upon 

 the initial intensity of phosphorescence. The 

 points marked by dots are for an exposure of 

 2 minutes; those indicated by crosses are for 

 an exposure of 20 seconds. In curves B and 

 C the ordinates show the intensity of phos- 

 phorescence 1 minute after excitation hail 

 ceased. 



10 20 30 



Intensity of excitation 



40 



the case of the points corresponding to intense excitation. In other words, 

 for weak excitations the intensity of the initial phosphorescence is the same 

 after an exposure of 20 seconds as for 1 or 2 minutes. There at first appears 

 to be a contradiction here to the results shown in Figs. 53 and 54. But 

 while this may be due to the uncertainty in the values of I , it is readily 

 explained upon the assumption that a weak excitation produces its full 



