SPECTROPHOTOMETRY STUDY OF KATHODO-EUMINESCEXCE. 1 45 



3. Phosphorescence following excitation by kathode rays is less intense 

 and more fleeting than the phosphorescence excited in the same substance 

 by light. 



The explanation of this fact is probably to be found in the relatively 

 slight penetrating power of the kathode rays. The excitation is thus con- 

 lined to a thin layer at the surface of the active substance, while in the case 

 of photo-luminescence the excitation extends to a considerable depth. As 

 will be pointed out in Chapter XV of this memoir, 1 the decay of phosphor- 

 escence would be more gradual in the latter case. 



4. The effect of infra-red rays upon kathodo-luminescence of Sidot 

 blende during excitation is small compared to the same effect upon the 

 photo-luminescence of this substance and produces a barely observable 

 decrease in the intensity of luminescence. 



Nichols and Merritt. Physical Review, xxvii, p. 373, 1908. 



