I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 269 



In the case of fluorescent liquids it was found by one of my 

 former pupils, Dr. Waggoner," that even with a special form of 

 phosphoroscope, by means of which observation less than a thou- 

 sandth of a second after the cessation of intense illumination were 

 possible, no phosphorescence could be detected. It should be re- 

 membered, however, that i/io,ooo or even 1/1,000,000 of a second 

 is a very long time measured in terms of the frequency of light, 

 since the particles of a phosphorescent body emitting green light 

 would oscillate some 500,000,000 times in a millionth of a second. 

 We are not in position therefore to say that fluorescent liquids, in 

 none of which phosphorescence has been observed, dififer from 

 phosphorescent bodies save in the rapidity with which the light 

 decays. 



By the use of this instrument Dr. Waggoner was likewise able to 

 trace the phosphorescence of various compounds back to its very 

 source at the cessation of excitation and to show how in the cases 

 which he studied fluorescence merged into phosphorescence without 

 discontinuity and the quality, or distribution of wave-lengths in any 

 single band in the spectrum remained unchanged during the first 

 few thousandths of a second. Professor Merritt and the present 

 writer^ had previously shown that in the case of a substance of 

 slow decay (Sidot blende) the phosphorescence spectrum is iden- 

 tical, so far as the single band under observation was concerned, 

 with the fluorescence spectrum and that " if any change occurs in the 

 form of the phosphorescence spectrum during decadence, this change 

 is extremely small." 



According to this view the relation of phosphorescence to fluores- 

 cence would seem a very simple one but more detailed study de- 

 velops complications such that the complete theory of the subject 

 is as yet far from being perfected. Some of these complications 

 are brought out particularly when we subject fluorescent or phos- 

 phorescent substances to change of temperature and it is with some 

 of the phenomena accompanying such changes that I propose to 

 deal in the present paper. . 



' Waggoner, Physical Reviciv, Vol. XXVH., p. 209, 190S. 



* Nichols and Merritt, Physical Review, Vol. XXI., p. 257, 1905. 



