decay of phosphorescence; in sidot blende. 



65 



by the violet light from the mercury arc passing through the stop whose 

 constant was to be determined, the intensity was measured by shifting 

 the position of the comparison flame on the opposite side of the photometer. 

 vSuitable glass screens were used to equalize the colors on the two sides. 

 To avoid errors resulting from the flickering of the mercury arc ten settings 

 were made for each determination. 



400 



' 



55- 



Effect of varying the intensity of the 

 exciting light. Exposure in each case 

 20 seconds. The relative intensity of 

 the exciting light is marked on each 

 curve. 



60 80 



Seconds 



In Fig. 55 decay curves are shown for different intensities of the exciting 

 light, the excitation in each case lasting for 20 seconds. In Figs. 56 and 57 

 similar sets of curves are shown for which the excitations were respectively 

 40 seconds and 2 minutes. 



A study of these curves shows that there is some approach to intensity 

 saturation; in other words, the intensity of phosphorescence is nearly 



400 



300 



200 



100 



Fig. 56. 



Effect of varying the intensity 

 of the exciting light (as indi- 

 cated on each curve). Ex- 

 posure in each case 40 seconds. 



20 



40 



60 80 



Seconds 



100 



120 



proportional to the intensity of the exciting light for small values of the 

 latter, but increases less rapidly than the excitation when the exciting 

 light is strong. This point is well brought out by curve A in Fig. 58, in 

 which the ordinates are proportional to the initial intensity of phosphores- 

 cence, 7 . The values of 7 were determined from the data of Figs. 55, 56, 

 and 57 by extrapolation, upon the assumption that the relation between 



