92 



MR. JOHN BURKE ON THE CHANGE OF 



It is easy to see, as before, that if A' x be screened, the 

 light coming from j 



Fig. 5. 



whereas that from a\ 



Hence 



and reversing, 

 so that 



= E /3. 



/?/( I + a) = S/6-', 

 = o-'/a; 



4. 



This description of the general method will suffice for the present, and the account 

 of the general methods of carrying out an experiment will be found on p. 96. 



To Determine the Alteration in the Fluorescence on the Supposition that the 

 Fluorescent Light is Capable of Destroying Itself. 



Let us suppose that the intensity of the fluorescent light given out by each of two 

 cubes of uranium glass when not exposed to the radiation from each other that is, 

 when there is an opaque non-reflecting screen (II, fig. 6) separating them is equal to 

 E , and that when this screen is removed the intensity is equal to E' . The blocks 

 are supposed to be illuminated by rays perpendicular to the plane of the paper, and the 

 radiation, the intensity of which is under investigation, to be in the direction perpen- 

 dicular to the surfaces, a i} a',, of the cubes. The method employed for comparing 

 E and E' (J depends upon obtaining two photographs on one plate, for comparison side 

 by side. 



We proceed thus : 



Let the slit s be completely closed. Three experiments 

 are made : 



1. The light coming from a i is photographed when the 

 cube A\ is not fluorescing, or the screen II' interposed. We 

 thus obtain the effect due to E . 



2. Both cubes are then made to fluoresce, and the screen is removed, in which 

 case we get the effect due to E' . 



It can thus be found by adjusting the slit s when the photographic effects of E and 

 E' are the same, from which the ratio of the two is determined. As the result of these 

 experiments we are not justified in saying that any material difference between E 

 and E' exists other than that possibly due to scattering. 



3. The amount of scattering from A'j into A lf or rather the amount of the A\ 

 light scattered by A t in the direction outwards from a lt may be found by making A', 

 fluoresce alone, and noting the effect of the light then falling on the slit s. The 



I 



