ABSORPTION PRODUCED BY FLUORESCENCE. 



103 



If there is no change of absorption (a) ought to be of the same intensity as (b). 

 As a matter of fact, it was always found to be much denser in the negative. The 

 intensities of (b) and ((/) were never exactly equal, but, on the contrary, (d) was 

 always darker than (6). Now, the width of the slit for (d) was double that for (b), 

 but the exposure was half, which shows that if we double the intensity of the light 

 to which a photographic plate is exposed, the exposure must be less than half in 

 order that the same effect should be produced. In other words, the photographic 

 effect is not proportional to the intensity and to the time conjointly. 



The actual width of the slits and the exposures in one particular instance were as 

 follows : 



Superposed 



(.31 



8 m. 



The relative intensity of (a) and (c) was not constant, which shows that the 

 average illumination may be different at different times, and it was consequently 

 necessary to make comparisons of photographs taken simultaneously, the symmetry 

 of the illumination having first been secured. 



Fifteen plates were obtained, which are in the possession of the Royal Society and 

 can be inspected. 



Three photographs show the degree of approximation in the uniformity of the 

 illuminations. 



Some others exhibit the remarkable difference in the relative intensities under 

 conditions such that if there were no change in the absorptive power the two 

 intensities should be equal. 



Three photographs show the superior and inferior limits of the value ot , the A 2 

 cube being screened, and the slits are s = 4, s' = 2'5. 



The relative intensities as given appear to be the nearest approach to the limits in 

 the width of s' we should wish to arrive at. Thus we obtain for the limits as 



follows : 



4 

 1 + a < ; therefore a < 0'48, 



4 

 > -^ 



> 0-43. 



The most probable value for obtained by the eye observations was 0'455. 

 Three other photographs give the limits for /3, A' t and A, being in this case 

 screened. 



s lies between 3 and 3 '5, so that 



