8o 



STUDIES IN LUMINESCENXE. 



In all these figures the curve marked I was taken without infra-red 1 and 

 that marked /' with infra-red. In general the exposure to infra-red began 

 at the instant the excitation ceased, the shutter being arranged so that the 

 same movement that cut off the exciting rays allowed the infra-red rays to 

 fall upon the screen. In the case of Fig. 71 exposure to the infra-red did 

 not begin until about 1.4 seconds after the end of excitation. Numerous 

 curves of this kind were taken in which the exposure to infra-red began at 

 different times after the beginning of decay. All of these curves show the 

 same sudden drop in intensity at the instant that the long waves begin to 

 act. 



The curves of Figs. 70, 71, and 72, except for the method of plotting, are 

 quite similar to the curves for the total light obtained by the photometer 

 method first described. Apparently the action of the longer waves during 

 the first few seconds of decay is quite similar to its action later. It will 



be observed that the curves of Figs. 

 70 to 72 refer to regions of the phos- 

 phorescence spectrum either near 

 the maximum or toward the red 

 edge of the band. 



Figs. 73, 74, and 75 show the in- 

 fluence of the longer waves upon 

 those regions of the fluorescence 

 spectrum lying near and beyond the 

 violet edge of the green band. Here 

 the effect seems to be entirely dif- 

 ferent. At 0.445 M (Fig. 75) the 

 effect of exposure to infra-red is to 

 retard the decay of phosphorescence 

 instead of to accelerate it. In the 

 region lying between the green band 

 (0.51 /x) and the violet band (0.45 /j.) 

 the effect of the infra-red is at first 

 to retard the decay and later to ac- 

 celerate it (Figs. 73 and 74). 



Owing to the faintness of the spec- 

 trophotometer field and the rapidity 

 with which the phosphorescence decays it is difficult to determine the form 

 of the curves in the early stages of decay with accuracy. Especially is this 

 true in the blue and violet, owing to the small luminosity of this region of 

 the spectrum. Each point plotted represents, however, the average of a 

 number of separate readings. For each pair of points the observations 

 for the time of decay with and without infra-red were taken alternately. 

 No special precautions were taken to keep the exciting source constant. 

 The slight initial curvature of the line I~- in Figs. 70 and 73, where the 

 exciting light was from the carbon arc, is perhaps to be explained as the 

 result of variations in this source. In the case of the other observations, in 

 which a spark was used in excitation, the points for I~* lie reasonably well 



'The carbon arc was used as a source of infra-red, a piece of dense ruby glass serving as a filter. Red 

 light, and perhaps a little yellow light, was therefore present in addition to the infra-red. There has been 

 nothing in our experiments to indicate that the presence of these visible rays modifies the results in any way. 



Fit 



4- Sec. 5 



73- 



Decay curves with and without infra-red. 

 X =0.474 M- 



