74 STUDIEvS IN LUMINESCENCE. 



able condition as being due to some new grouping of the molecules, then 

 the condition of steady fluorescence is reached when these favorable groups 

 are being broken up, either spontaneously or through the action of some 

 outside agent, just as rapidly as they are formed by the action of the excit- 

 ing light. 



It is clear that the intensity of steady fluorescence will be made less by 

 any agent which increases the rate at which the assumed favorable group- 

 ing is destroyed. Now it is precisely this effect that is exerted by the 

 red and infra-red rays; and we should therefore anticipate that the fluo- 

 rescence of vSidot blende would be diminished by the action of longer rays. 



This effect of the longer waves, which does not appear to have attracted 

 much attention heretofore, may readily be made very marked indeed. 

 Thus the rays from a projecting lantern after passing through a sheet of 

 hard rubber 0.2 mm. thick are able to reduce the fluorescence of Sidot blende 

 so greatly as to leave the intensity only a few per cent of its normal value, 

 and this too with very intense excitation. If the ultra-violet rays of a 

 spark are used for excitation numerous lecture experiments may be devised 

 for demonstrating the existence of the invisible rays at the two ends of the 

 spectrum. As compared with the experiments first proposed by Dahms, 1 

 in which the effect of the infra-red rays upon phosphorescence is utilized, 

 this procedure has the advantage of giving a persistent rather than a fleet- 

 ing effect. 



In studying the influence of the longer rays upon fluorescence we have 

 directed our attention especially to the distribution of the effect through- 

 out the fluorescence spectrum. While the result of exposure to longer 

 waves is to diminish greatly the total brightness of the fluorescence light, 

 it might be that in certain restricted regions of the spectrum the intensity 

 would be increased rather than diminished, or at least that the effect of the 

 infra-red rays would vary greatly in magnitude in different parts of the 

 fluorescence spectrum. 



For the study of this phase of the subject the Sidot blende screen was 

 mounted in front of a Lummer-Brodhun spectrophotometer, with an acety- 

 lene flame as a comparison source as in our previous work. The infra- 

 red rays from an arc fell upon the screen after passing through hard rubber. 

 The intensity of fluorescence was then measured in different parts of the 

 spectrum, first with and then without the action of the longer waves, the 

 excitation remaining constant. As exciting source a mercury-vapor lamp 

 was first used, the lamp being made of the so-called "Uviol" glass, which 

 possesses an unusual transparency to the ultra-violet rays. 



In Fig. 65, curve /, shows the ordinary fluorescence spectrum of Sidot 

 blende produced by the mercury lamp, while curve 77 shows the spectrum 

 as modified by exposure to the infra-red. In the case of curves /' and 77' a 

 sheet of ordinary glass was interposed between the lamp and the screen, so 

 that the ultra-violet rays were in large part removed from the exciting light. 

 It is clear that the ultra-violet rays of the Uviol lamp introduce a band 

 at about 0.49 /x which overlaps and distorts the usual green band at 0.51 n. 



A more annoying source of disturbance in these experiments, however, 

 was the light reflected from the screen, which was mixed with fluorescence 



Dahms, /. c. 



