102 ME. JOHN BURKE ON THE CHANGE OF 



2. If p 2 is the ratio of the intensity of the light emitted by / to that emitted by 

 ! in the arrangement fig. 5, then 



__ 



EO (1 + ) ~ 



Hence 



and if a = /3, 



Pi+2h= I- 



The experiment is then performed by first determining p lt and putting p. z = 1 p, 

 and observing whether the two lights so obtained are of equal intensity. 



Again p 2 is first obtained, and p } put = 1 p 2 , and equality again looked for. 



In the first case it is found necessary to diminish s', or widen s, to obtain equality, 

 and since the radiation from a } and a\ are inversely as the width of the slits, it 

 follows that the real value of p. 2 is greater than the calculated one, and in the second 

 it is also found necessary to diminish ./ or widen s, which shows that p l + p 2 > 1, 



1 _L /Q 



or that - - > 1, and therefore /3 > a. Hence the absorption is greater when the 



substance is fluorescing than when it is not, a and /S being the coefficients of trans- 

 mission. 



To test this photographically, first of all an exposure is given when the screening 

 of the cubes is as in (1), and then, without altering the position of the camera, the 

 screening is arranged as in (2). The slits are such that s' = 2s, since we want the 



1 -4- R 1 -4- R 



ratio - , not ; r --- - . Thus, if the photographic effect be proportional to the 



time and the intensity, the effect in the first instance on the photographic plates may 

 be represented thus : 



1 



and in the second 



Hence the resultant effects, when superposed, are 



E (1 + 0) ; E (1 + ). 



The camera is then slightly raised, s' is made equal to s, all the screens are 

 removed, and a photograph is taken on the same plate with an exposure merely 

 equal to that of each of the other two, the object being to obtain on the one plate 

 a comparison, not merely with reference to the absorption, but also with that of the 

 illumination. The effects on the photographic plate ought to be as follows : 



E a + of.) 



