CHAPTER V. 



THE INFLUENCE OF THE RED AND INFRA-RED RAYS UPON 

 THE PHOTO-LUMINESCENCE OF SIDOT BLENDE.i 



The effect of the red and infra-red rays in suppressing the phosphorescence 

 of various substances has long been known, 2 and has frequently been 

 utilized in the study of the infra-red spectrum. The effect is exhibited 

 by vSidot blende more strongly perhaps than by any of the other phos- 

 phorescent sulphides. In Chapter IV we have called attention to another 

 effect produced by the longer waves, namely, the restoration of a screen of 

 Sidot blende, after the excitation and complete decay of phosphorescence, 

 to a standard condition, so that the result of a subsequent excitation shall 

 be unaffected by the previous history of the substance. While this new 

 effect is doubtless connected in some way with that first mentioned, the 

 nature of the relationship between the two is by no means clear. For this 

 reason, and because of the bearing of the phenomena upon the general 

 theory of luminescence, we have investigated the influence of the longer 

 waves upon the luminescence of Sidot blende under a variety of different 

 conditions. 



The work naturally falls under several heads, as follows: 

 i . The effect upon the luminescence of Sidot blende of exposure to the 

 longer waves before excitation. 



2. The effect of the longer waves during excitation. 



3. The effect of the longer waves after excitation, i. e., during the decay of 

 phosphorescence . 



4. The influence upon the effect studied of the wave-length of the red 

 and infra-red rays used. 



THE EFFECT OF THE LONGER WAVES BEFORE EXCITATION. 



Experiments described in the preceding chapter indicate that when Sidot 

 blende is excited to luminescence " some change is produced in the material 

 by the action of the exciting light, and that this change persists for a con- 

 siderable period after a visible phosphorescence has ceased. In other words, 

 the effect of a given excitation in producing phosphorescence depends upon 

 the previous history of the phosphorescent substance. If the screen is 

 allowed to rest in the dark for a number of hours this semi-permanent effect 

 of exposure in part dies out. But rest alone does not restore the screen 

 completely even if continued for several days." An exposure of a few 

 seconds to the rays from a 50 c. p. lamp seen through ruby glass is, however, 

 sufficient to restore the screen to what seems to be a definite standard 

 condition. 



'The substance of this chapter essentially as here given appeared in the Physical Review, xxv, p. 362. 

 -References to the literature of the subject are given by Dahms, Annalen der Physik, 13, p. 425. 



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