INFLUENCE OF RED AND INFRA-RED RAYS UPON SIDOT BLENDE. 77 



total phosphorescent light was measured by a photometer at different 

 times after excitation had ceased, as described in Chapter IV. The violet 

 end of the carbon arc spectrum was used for excitation, and a 50 c.p. 

 incandescent lamp as a source of infra-red rays. In these experiments a 

 cell containing a solution of iodine in carbon disulphide was used instead 

 of hard rubber to remove the visible rays. The distance of the lamp 

 from the Sidot blende screen was about 60 cm. 



The curves of Figs. 68 and 69 show some of the results obtained by this 

 procedure. In Fig. 68, curve I is the ordinary decay curve without expo- 

 sure to infra-red. In the case of curve 777 the infra-red rays were allowed 

 to fall on the screen when the decay had proceeded for about 32 seconds. 

 In curve II the infra-red rays were turned on about 4 seconds after the 



JL 



Fig. 68. 



I nfluence 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 decay had pro- 

 ceeded for 4 seconds. Infra- 

 red cut off at / = 19 seconds. 



Curve ///. Screen exposed to 

 infra-red after decay had pro- 

 ceeded for 32 seconds. 



?o 



40 



60 



60 



100 



ISO Sec. 140 



end of excitation and were cut off again at the end of about 19 seconds. 

 The great increase in the rapidity of decay brought about by infra-red rays, 

 even when of such small intensity as those used in these experiments, is 

 clearly shown. 



There was no indication in these experiments of any temporary increase 

 in the brightness of phosphorescence when the rays were first turned on, 

 as has been noted by many observers in the case of Balmain's paint and 

 other phosphorescent sulphides. Dahms has already called attention to 

 this peculiarity of Sidot blende. The effect of the longer waves in suppress- 

 ing phosphorescence has sometimes been explained by assuming that these 

 rays act in the same way as does a rise of temperature; i. c, that they 

 accelerate the process which causes phosphorescence, so as to produce a 

 brief flash, due to the sudden liberation of the energy stored during excita- 

 tion, followed by a complete loss of luminescence when the stored energy 

 has been used up. While this explanation of the phenomenon may be 

 correct for the other phosphorescent substances it can not be applied with- 

 out essential modification to the case of Sidot blende. It has recently been 

 shown, however, by Ives and Luckiesh 1 that under some circumstances a 



Physical Review, xxxir, p. 240, 1911. The specimen of Sidot blende that was tested by Ives and 

 Luckiesh differed from that used by us and other observers in the fact that the law of decay after one minute 

 was I~~ 0, * 7 = a+bt instead of / '* = a-\-bt. It thus forms the only exception known to us to the law of de- 

 cay discussed in Chapter XV. 



