influence; of red and infra-red rays upon sidot blende. 8 1 



upon a straight line, and thus give a check upon the accuracy of the obser- 

 vations. 



The remarkable reversal in the effect of infra-red in passing through the 

 luminescence spectrum received ample qualitative confirmation. With 

 the spectrophotometer set for some region in the blue or violet the bright- 

 ness of the field increased noticeably for a few seconds when the screen was 

 exposed to infra-red during decay, even when the exposure first began 

 several seconds after the end of excitation. The same effect was observed 

 in the case of " Kmanations-pulver " and Balmain's paint. In the case of 

 the latter substance the flash that accompanied exposure to the longer 

 waves developed more slowly and lasted longer than in the case of Sidot 

 blende. 



Two interpretations of the results brought out in Figs. 70 to 75 suggest 

 themselves. Neither, however, is wholly satisfactory. 



t ' 3 



Fig- 74- 



Decay curves with and without infra-red. 



4 Sec. 5 ^seey 



Fig- 75- 

 X =0.464 m (Fig. 74) and X =0.445 m (Fig. 75). 



In all of these experiments the luminescence spectrum consisted of two 

 bands, namely, the green band at 0.5 1 /x and the violet band at about 0.45 /x. 

 It is possible that the infra-red rays retard the decay of phosphorescence 

 in the case of the violet band and accelerate it in the case of the green band. 

 In the curves of Fig. 75 we are dealing with the violet band only; in Figs. 

 70, 71, 72, and 74 the light is almost entirely from the green band; but at 

 wave-lengths 0.474 M (Fig. 73) and 0.464 /x (Fig. 74) the light entering the 

 collimator slit comes partly from one of these bands and partly from the 

 other. The violet band apparently decays more rapidly than the green 

 band. (Compare the slant of the line for J~* in Fig. 75 and Fig. 72.) 

 If the violet band is initially the brighter of the two the retarding effect 

 of the infra-red upon the decay of this band will predominate in the early 

 stages of decay. Later, when the violet band has nearly died out and the 

 light is chiefly due to the green band, the opposite effect will predominate. 

 The two curves / and /' will therefore intersect, as shown in Fig. 72 and 



Fig. 74- 



Two objections may be urged to this explanation. If the light in the 

 case of Figs. 73 and 74 is from two bands that decay at different rates we 

 should hardly expect the relation between / and J~* to be as simple as the 



