﻿Intensity of Radiation from Radioactive Sources. 155 



be shown that the intensity at the extreme edge of the image 

 may be very much greater than that in the centre. This is 

 the experimental case already considered (fig. 1 A, PI II.). 



The mathematical theory of the distribution of intensity of 

 the radiation for the case of a radiating sphere or cylinder, 

 when no cosine law applies, can be readily worked out, but 

 it has not been thought necessary to include it here. It 

 suffices to say that the observed distribution of the intensity 

 of the radiation from the radioactive sphere or cylinder 

 receives a simple explanation when we take into account 

 that 



(1) the layer of radioactive matter is so thin that the a rays 

 suffer no appreciable absorption in passing through it, i. e. 

 no cosine law of distribution applies ; 



(2) the a, particles are, on an average,, projected equally 

 in all directions. 



The experimental results obtained afford an indirect proof 

 that the a. particles are, on an aA'erage, projected equally in 

 all directions from a radioactive source. 



Our experience of the distribution of the radiation from 

 luminous sources is so bound up with the cosine law of 

 emission, that we are liable to overlook how different this 

 distribution would be if no cosine law applied. We have 

 seen, for example, that if the radiation from the sun came 

 from a thin non-absorbing superficial layer, the sun would 

 appear to us as a circular disk, the edge of which was marked. 

 by a brilliant ring of light, the luminosity falling off rapidly 

 from the edge to the centre. 



Consider also another example. If a small plane disk 

 emits light according to the cosine law, the intensity of the 

 radiation at any point some distance from it is greatest when 

 the plane of the disk is normal to the line joining the point 

 and the disk. The intensity of the radiation falls off as the 

 plate is orientated, and is very small when the light is emitted 

 nearly tangentially from the surface. Consider now a similar 

 disk coated w r ith a thin film of radioactive matter, and, in 

 order to avoid the disturbance due to the absorption of the 

 a rays in air, suppose the experiments are made in a vacuum. 

 Since the cosine law does not apply, the intensity of the 

 radiation at some distance from the small plane disk is prac- 

 tically independent of the orientation of the plate, and is 

 nearly the same for normal and. tangential emission. 



We shall now consider some farther experiments which 

 illustrate these points in a striking manner. 



A brass rod of square cross section, about I cms. long, was 

 exposed for some hours to the radium emanation, in order to 



