DECAY OP PHOSPHORESCENCE IN SIDOT BLENDE- 59 



chronograph. When the mercury lamp was used in excitation a piece of 

 blue glass, B, was placed between the lamp and the screen, so that only the 

 violet lines in the spectrum were used. 



The right-hand side of the screen was of white paper and could be 

 illuminated by means of the acetylene lamp A. Two pieces of green 

 glass, G, served to produce a sufficient color match. The acetylene flame 

 A was in a metal box, and the light used emerged from a small opening 

 immediately in front of the central part of the flame. Small changes in 

 pressure were therefore without effect upon the intensity of this comparison 

 source. 



The procedure in taking observations was as follows: 

 One observer, with his eyes suitably protected from stray light, watched 

 the decay of the phosphorescence after the shutter had been closed, and 

 recorded by means of a key the instant at which the phosphorescent light 

 and the comparison field were equal. The second observer then shifted 

 the position of the comparison flame A to a slightly greater distance from 

 the photometer, and the time when equality was again reached was recorded 

 as before. In this way it was often possible to get as many as 15 points 

 on the curve of decay, although a smaller number than this was more 

 usual. 



EXPERIMENTS WITH SIDOT BLENDE. 



In preliminary experiments the light from the carbon arc was used in 

 excitation. It was thought that simpler conditions would be obtained 

 if exciting light having only a small range of wave-lengths was used, and a 

 spectrum of the arc was therefore formed, the violet end of which was used 

 for excitation. The gain in the intensity of phosphorescence which re- 

 sulted from cutting off the red and infra-red rays was very noticeable. 



In our previous experiments with Sidot blende it was found that the 

 decay of phosphorescence during the first 7 seconds was in accordance 

 with the law 



(a+btY 



so that upon plotting the curves with T and I~- as coordinates a straight 

 line was obtained. Since the distance between the photometer screen 

 and the acetylene flame is proportional to I~-, it was convenient to plot 

 the results of the present experiments in the same way. 



The general character of the decay curves when plotted in this manner 

 is exhibited by the curves of Fig. 47. The violet end of the carbon arc 

 spectrum was used in excitation, and the duration of excitation was varied 

 as indicated. 



Inspection of Fig. 47 shows that while each curve starts as a line which 

 is nearly straight in the neighborhood of the origin, it soon begins to bend 

 over and ultimately changes into a second straight line having a different 

 slant. This behavior is especially noticeable in the case of long exposures. 



If phosphorescence is due to the recombination of ions that have been 

 separated by the action of the exciting light, as assumed in the theory of 

 Wiedemann and Schmidt, it appears that the coefficient of recombination 

 does not remain constant during the whole period of decay. During the 



