DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE. 



63 



It seems possible that the action of these rays in destroying the residual 

 effect in the phosphorescent substance is similar to the effect of jarring or 

 tapping in destroying the residual magnetism of a bar of iron. Ignorance 

 of the existence of hysteresis would evidently lead to confusing results in 

 the case of either of these two classes of phenomena. Our delay in recog- 

 nizing" the effect of previous history has in fact made it necessary for us to 

 discard all of our earlier observations. 



Fig- 5i. 



Hysteresis loop. Ordinates 

 give the intensity of phos- 

 phorescence 30 seconds 

 after the end of excitation. 

 The curves from which these 

 points were determined 

 were observed in the order 

 indicated by the arrows. 



20 40 60 80 



Duration of excitation 



100 sec 



In our later work the screen was exposed to the red and infra-red rays 

 as described for one minute before each exposure. With this precaution 

 to avoid the effects of hysteresis the curves shown in Fig. 52 were taken 

 to determine the effect of varying times of exposure. 



Effect of duration of excitation. Violet of mercury arc used for excitation. Screen exposed to 

 infra-red for i minute before each curve. The times of excitation were as follows: 

 Curve A, 1.2 sec; B, 5.4 sec; C, 12.0 sec; D, 37 sec; E, 60 sec; F, 15 min. 



Since our previous experiments have shown that the curves are accurately 

 straight in the neighborhood of / = o, it is possible to determine the initial 

 intensity of phosphorescence by prolonging the curves in each case until 



