7 2 



STUDIES IX LUMINESCENCE. 



The phenomenon in question is illustrated by the curves of Fig. 50, p. 62. 

 If we compare curves A, C, and D, all corresponding to the same excitation, 

 it is clear that exposure to the longer waves before excitation exerts a very 

 marked influence upon the rate at which the phosphorescence, excited after 

 this exposure, will decay. While the semi-permanent change produced 

 by excitation is partly lost as the result of prolonged rest in the dark, rest 

 alone is not a very satisfactory means of restoring the substance to a stand- 

 ard condition. Thus a rest of 24 hours brings about a change in the decay 

 curve following a 10-second excitation from C to A. Rest for several days 

 would shift the curve somewhat farther to the left. But even a rest of 

 several weeks failed to bring the decay curve as far to the left as curve D. 



Curve D, Fig. 50, was taken after an exposure of 4 minutes to the longer 

 waves. A very much shorter exposure would have been nearly, if not quite, 

 as effective. This point is brought out by the curves of Fig. 63, which were 

 taken to determine the way in which the effect depended upon the duration 

 of exposure to the longer waves. In taking these curves the procedure was 

 as follows : In each case the screen was first exposed for 2 minutes to the 



Fig. 63. 



Effect on the decay curve of exposure to the infra- 

 red for different times. 



Curve /, 10 seconds exposure after 48 hours rest in the 

 dark; II, 2 minutes exposure; ///, 10 seconds ex- 

 posure after 1 second infra-red; IV, to seconds 

 exposure after 3 seconds infra-red; V, 10 seconds 

 exposure after 60 seconds infra-red. 



00 S r C 



mercury arc ; the phosphorescence was then allowed to decay for 2 minutes, 

 at the end of which time, while still visible, it was too faint for measurement. 

 The screen was then exposed to the rays of a 50 c.p. lamp at a distance of 

 5 inches, a piece of ruby glass being interposed between the lamp and the 

 screen ; the duration of this exposure was 1 second for curve 7/7, 3 seconds 

 for curve 1 1 r , and 60 seconds for curve 1 '. After this exposure to the longer 

 waves the screen was excited by the mercury arc for 10 seconds, and the 

 decay curves shown in Fig. 63 were observed by the procedure described 

 on page 58. 



It will be observed that even an exposure of only 1 second is more effec- 

 tive than 48 hours rest. It will be noticed also that 3 seconds exposure to 

 the longer waves is nearly as effective as an exposure of a minute. 



With red and infra-red rays of less intensity a longer time is required 

 to destroy the effect of previous excitation. The curves of Fig. 64 were 

 taken with a procedure similar to that for Fig. 63, except that the distance 

 of the 50 c.p. lamp from the Sidot blende screen was 30 inches instead of 5 

 inches. Exposure for 60 seconds to these less intense rays produces as 



