PHYSICAL METABOLISM 239 



contradictory facts? Of their truth there cannot 

 be any question. Their contradiction therefore 

 must only be apparent. The answer is to be found 

 in the fact that the absorption of the fluorescent light 

 takes place only during the period of fluorescence, 

 and the transparency can be observed only when 

 the body is inactive. In other words, the absorp- 

 tion spectrum of the body undergoes a change 

 whilst it is fluorescing, and this I have attributed 

 to a temporary change in the body during the 

 action of the exciting light. 



The phenomenon of the change of absorption 

 produced by fluorescence was observed by the 

 author some years ago, notably in the case of 

 uranium glass, and has led up to the theory of 

 fluorescence herein put forward. 



The experiment is as follows : — ^ 



Two cubes of uranium glass, A and B (as in the 

 figure), are illuminated as in the drawing by the light 

 from a spark of a Ley den jar between cadmium 

 electrodes (the jar is not shown). 



A is first made to fluoresce, whilst B is screened 

 from the radiation; the light from A in the 

 direction B A being photographed on the plate P. 

 Then A is screened whilst B is illuminated, and 

 the light from B transmitted by A is superposed 

 on the previous eflfect, the exposures being of equal 

 duration. The plate P is then slightly displaced 

 in a vertical direction and the effect of the two 

 cubes taken when illuminated simultaneously, the 

 1 See Philosophical Transactions, Vol. 191 (A). 



