78 



STUDIES IN LUMINESCENCE. 



flash may be observed immediately after exposure to infra-red rays even 

 in the case of Sidot blende. These observers find that "there are two stages 

 in the decay of phosphorescence, merging gradually one into the other. In 

 the first stage the effect of long waves is to cause a sudden drop in intensity ; 

 in the second stage the long waves cause a temporary increase of brightness 

 before decay. In the intermediate stage the accelerated decay appears to 

 the unaided eye to be delayed in starting." With intense infra-red rays 

 the preliminary flash could be made to occur as early as 30 seconds after 

 the end of excitation. 



Perhaps the most striking feature of the curves in Fig. 68 is the nearly 

 exact parallelism that exists between the later part of curve 7/7 (after the 

 infra-red rays were cut off) and the straight part of the ordinary decay 

 curve. The action of the longer waves appears to be to bring the material 

 quickly into the same condition as regards ability to emit light that it 

 would have acquired at the end of a much longer period of ordinary 

 decav. The results of Ives and Luckiesh are not in accord with this view. 





Fig. 69. 



Influence of exposure to infra-red during decay 

 upon the form of the decay curve. 



Curve /. Ordinary decay curve. 



Curve //. Screen exposed to infra-red after the 

 decay had proceeded for 18 seconds. 



Curve ///. Screen exposed to infra-red during exci- 

 tation and for first 16 seconds of decay. 



Curve IV. Screen exposed to infra-red during exci- 

 tation and for first 9 seconds of decay. 



ZQ 



40 



CO 



60 Cc. 100 



In the brief abstract of their work referred to on page 77 they state that 

 "the decay curve after brief action of red or infra-red does not correspond 

 with the original curve with origin shifted." 



In the case of curves III and I V of Fig. 69 the screen was exposed to 

 infra-red during excitation and during the early stages of decay. The 

 infra-red rays were cut off at the points indicated by the break in the curves. 

 It will be noticed that the latter portion of curve III is nearly parallel to the 

 straight part of the ordinary decay curve; but in curve IV, where the infra- 

 red was cut off earlier, the straight part of the curve is not even approxi- 

 mately parallel to curve I. The results shown in Fig. 69 are thus in agree- 

 ment with the statement of Ives and Luckiesh. 



For the early stages of decay a number of curves were taken by means 

 of the spectrophotometer, the method being that described in Chapter III. 

 With this method it is impracticable to follow the decay for more than a 

 few seconds, since the illumination of the spectrophotometer field soon 

 becomes too faint for accurate measurements. The method possesses a 

 great advantage, however, in the fact that the effect of the long waves can 

 be determined for different parts of the phosphorescence spectrum. 



