﻿158 Intensity of Radiation from Radioactive Sources. 



the diagrams shown in fig. 7. ABC is the cross section of 

 the triangular rod. The regions a, b, and c, bounded by the 

 continuations of the sides of the triangles, are clearly shown 

 in the photograph, since the radiation outside these triangular 

 areas is due to one side alone. 



Fiar. 7. 



b 



c 



The photograph obtained with the hexagonal rod presents 

 several points of interest. A six-armed cross isiclearly seen 

 as well as the outline of the six small equilateral triangles 

 obtained by the continuation of alternate sides of the hexagon. 

 The explanation of this six-armed cross and the smaller equi- 

 lateral triangles is clear from the diagram in fig. 7, where 

 the full lines represent the hexagon, and the dotted lines the 

 boundaries of the arms of the cross and o£ the triangles. 



A sudden change of intensity occurs at every point where the 

 tangential radiation from another side o£ the rod adds its effect. 



Experimental examples of this kind can easily be multiplied, 

 but sufficient have been given to illustrate the striking* 

 variations of the intensity of the radiation round a radiating 

 body when no cosine law of distribution applies. There is 

 no doubt that similar effects could be obtained if the {$ 

 instead of the a rays were used as a source of rays. The 

 lines of demarcation of the shadows would not, however, be 

 so marked as in the case of the a rays, on account of the 

 considerable penetrating power of the ft rays. 



Theoretically, the distribution should be quite different if 

 the radiation comes from a thick instead of a thin layer of 

 radioactive matter. It is difficult to obtain such radioactive 

 sources in definite geometric form, but we should expect that 

 the distribution for a thick layer should approximate closely 

 to that observed for light sources where the cosine law of 

 emission applies. 



I am indebted to my colleague, Professor Cox ; for his 

 assistance in explaining these effects. 



McGill University, Montreal, 

 June 5, 1906. 



