450 Prof. B. W. Wood on the 



clear from reference to fig. 1, in which AB represents the 

 upper, and CD the lower surface of a celluloid film stained 



Fig. 1. 

 F F J 



A— — — — - i 



/ \ \ 



/ \ s 



/ \ 



/ 



C ' G ~ H ~"~~ ^™° _ D 



-with rhodamine. Let X represent a fluorescent molecule 

 from which radiation of equal intensity escapes in all 



* directions. Of all the rays leaving X it is clear that only 

 «those within the solid cones EXF and GXH can escape into 

 the air. The rays within the cones FXH and EXG are 

 imprisoned by total internal reflexion, the ray XJ for 

 example being reflected back and forth between the parallel 

 surfaces of the film. Silvering the lower surface will cause 

 the rays in the cone GXH to escape through the upper 



• surface, but will not affect the paths of the other rays. If 



now we bring a white matt surface into optical contact with 



ithe lower surface of the film, it will liberate all of the rays 



which were totally reflected. Take the ray XJ for example : 



it is diffusely reflected at K, and that portion of the energy 



within a cone of the same aperture as that of the cone EXF 



-escapes through the upper surface. The rest is totally 



reflected from the upper surface, and returned to the white 



^surface, when the process is repeated, all of the energy, 



-except that lost by absorption, eventually passing out through 



the upper surface. The action of the matt surface liberates 



the energy imprisoned by internal reflexion. If there were 



/no absorption the edges would appear of dazzling brilliancy, 



.as all of the imprisoned energy would be thrown out here. 



The greater brilliancy of the edges of a fluorescent film was 



utilized by Becquerel in observing the faint luminosity of 



•certain compounds of uranium, and by Greinacher in the 



case of the fluorescence of mica films caused by radio- 



.tellurium (Pluys. Zeit. 1906). 



We may test the above theory of the action of a matt 

 surface in increasing the brilliancy of the fluorescence by 

 iinaking photometric measurements. The ratio of the energy 

 within the two cones of rays which can escape without the 

 .aid of the matt surface, to the energy imprisoned by reflexion, 



