104 ON THE CHANGE OF ABSORPTION PRODUCED BY FLUORESCENCE. 



> -V, > > 075, 



4 



< < 0-89, 



the value with the eye observations p being 0'78G. 



Assuming the value of a to be 0'48, and that there is no difference between it and 



f3, we obtain 



+ ) = 0-32. 



Consequently if we screen A.\ and adjust 



s = 1-3, 

 s' = 4-0, 



we should obtain equality of intensity on a photographic plate. This, however, does 

 not appear to be the case, as shown in a photograph in which two successive images 

 .of the horizontal slit are given, one being the result of reversing the screws and 

 screen, so that A } was for the second image sheltered from the illuminations, and A'j 



exposed, and 



s=4-0, 



s' = 1-3. 



It shows, moreover, that a reversal in the arrangement also reverses the effect. 



Another plate exhibits the effect of superposing two such photographs, showing the 

 absence of want of symmetry in the illumination. 



Some other photographs show the equality of illumination and change of 

 absorption from the calculated values, and two others the result of superposing two 

 photographs in the following way. 



Firstly, A\ is screened, and s = s' = 4 ; and an exposure of 30 minutes gives an 

 effect nearly proportional to E /3 on the left and E (l + a) on the right. Then s is 

 closed completely, and the screen altered from A'j to A'^ and an additional exposure 

 is given without changing the position of the photographic plate, so that an increase 

 proportional to E is produced on the left. Now, since y3 does not differ so very much 

 from unity, we may take the resultant effect as equal to the sum, though in reality, 

 as shown on p. 103, it is less. The effect on the left side of the plate is then 

 proportional to E (1 + /3), and that on the right due to a single exposure to 

 E (1 + a), and if a =/J, these two should have been equal. The plates show that 

 they are not, but give /J > a. This is clearly the simplest method of exhibiting the 

 phenomenon. If the photographic effect were proportional to the intensity the 

 difference would clearly be more marked. 



I desire to thank Professors ARTHUR SCHUSTER and G. F. FITZ&ERALD for many 

 useful suggestions and much valuable advice throughout this research, and also 

 Professor J. H. POYNTING, to whom the investigation owes its origin. 



