88 MR. JOHN BURKE ON THE CHANGE OF 



other four were quite distinct. The spectrum seemed to me to be of the nature of 

 maxima and minima. 



In the summer of 1895 I made some experiments at Trinity College, Dublin, on 

 the electrical conductivity of fluorescent solutions, and endeavoured to determine 

 whether the action of violet or ultra-violet light upon them gave rise to any 

 alteration in their conductivity. 



The many difficulties which the experiment entailed prevented me from arriving 

 at any conclusion upon the matter, and I commenced to look for some such effect as 

 Professor POYNTING had suggested. 



The experiments have since been carried out in Professor SCHUSTER'S laboratory 

 at Owens College. 



1. The leading conception in the following paper may briefly be stated thus : 

 Suppose that a body A is transmitting light from a similar 



body B, which is fluorescing. It is quite conceivable, as 

 Professor POYNTING remarked, that the amount of light J 

 from B transmitted by A should be different according as A 

 h fluorescing or not. 



O '-t P- JW P- 



2. There were some reasons which pointed to the con- 

 clusion that something of the sort might happen. It seemed quite possible that the 

 action, if any, of the fluorescent light from B might be to strengthen the fluorescence 

 of the body A when already fluorescing ; not that the emitted light would be 

 increased merely by scattering, which would be precisely the same in amount 

 whether the body were fluorescing or not, but that the emission of fluorescent 

 radiation would be increased if light of the same frequency were incident on the 

 radiating body. 



The experiments which are described in the sequel do not confirm this supposition, 

 but they prove that the amount of fluorescent light apparently transmitted is very 

 different according as the body through which it passes is fluorescing or not, the 

 amount being very much less, only about one-half (0'57), when fluorescence is taking 

 place than when the body is unacted upon by exciting light. 



3. It seemed possible that this effect might be due, indirectly at least, to an 

 increased absorption of the incident fluorescent light, if it possessed the property of 

 destroying the fluorescence of the body through which it passed. BECQUEREL has 

 shown that certain infra-red rays have the power of destroying phosphorescence in a 

 substance which is exposed to the phosphorescence-exciting rays. It was therefore 

 possible that the result obtained by me was an effect of this kind, but I think that 

 the experiment described on p. 92 sufficiently proves that, though such an expla- 

 nation would involve an increased absorption of the annulling rays in order that 

 they might be capable of destroying the fluorescence, the effect I have observed is 

 really not essentially of this kind, but is rather due to an increased direct absorption. 



It may be useful if, before entering into an account of the experiments, I give, 



