﻿156 Prof. E. Rutherford : Distribution of the 



obtain on its surface a thin uniform layer of radioactive 

 matter. After removal, the intensity of the radiation from 

 the rod was examined, in various positions, both by optical 

 and photographic methods. In the former, the rod was 

 brought close to a screen of willemite or zinc sulphide. 



The results observed are best described by means of a 

 diagram (see fig. 3). ABCD represents the cross section of 



Fig. 3. 



/k 



the square rod. The dotted lines represent continuations of 

 the sides of the square. Suppose the phosphorescent screen 

 is placed in the position PQ, parallel to the upper surface of 

 the rod. The experiment at once showed that the part RS 

 within the dotted lines was far less illuminated than the parts 

 RP, SQ immediately beyond. This was confirmed by placing 

 a photographic plate in the same position. The intensity of 

 the impression is much less within the region RS than on 

 either side of it. The photographic negative, consequently, 

 has the appearance of a whitish band of width equal to the 

 side of the square on a dark background. The edges of the 

 band are very sharply defined, and there is no gradual trans- 

 ition of intensity from one region to another. 



It is, at first sight, somewhat startling that the intensity of 

 the photographic effect is least immediately above and closest 

 to the active surface, but a little consideration shows that 

 such an effect is to be expected. The region RS is acted on 

 only by the rays from the upper surface AD of the rod, while 

 the regions RP, SQ are, in addition, acted on by the rays 

 from the sides of the square. If the distance AR is large 

 compared with the length AB, the intensity of the radiation 



