﻿154 Prof. E. Rutherford: Distribution of the 



Let the circle P (fig. 2) represent the cross section of a 

 cylindrical uniformly radiating body. The rays pass through 

 a narrow rectangular slit CD, and fall on a screen SS. The 

 radiation which falls on the central point of the image 



Fig-. 2. 



comes from a length AB of the circle, which is intersected by 

 lines drawn from touching the edges of the slit. The 

 radiation which reaches 0', a point on the extreme edge of 

 the luminous area, comes from a length A'B' of the circle, 

 which is seen to be considerably greater than the length AB. 



In the case of a self-luminous body, this increase of 

 radiating surface which supplies light at a point near the 

 edge of the image is exactly compensated for by the cosine 

 law of distribution, and the intensity of the radiation is 

 consequently uniform over the cross section of the image. 



In the radioactive case, however, the radiations come from 

 an extremely thin film of radioactive matter, in which the 

 a rays are not appreciably absorbed. There is, in consequence, 

 no cosine law of distribution, but, on an average, the a. radia- 

 tion from any point is equally intense in all directions. We 

 see that, under such conditions, the intensity of the radiations 

 reaching (disregarding small variations in distance) is pro- 

 portional to the length AB, and the intensity at 0' to the 

 length A'B'. The intensity of the radiation is consequently 

 greatest at the extreme edge of the image, and falls off 

 towards the centre. 



If the distances OC, CP are very large compared with the 

 radius of the cylinder and the slit very narrow, it can readily 



