﻿Intensity of Radiation from Radioactive Sources. 153 



bands with a well-marked white band between them. A 

 photograph taken in a similar way, using an active circular 

 rod 5 mms. in diameter, is reproduced in fig. 8, PL II. 



This peculiarity in the trace of the rays could not be due 

 to the magnetic field, for it was observed equally clearly 

 when no magnetic field was acting. The same appearance 

 was also observed when the a rays were passed through an 

 absorbing screen. It was at first thought possible that this 

 effect was due to photographic reversal, since it was first 

 observed when the photographic effects were very intense. 

 Later work, however, showed that the effect was equally 

 marked when the photographic effect was too feeble to admit 

 the possibility of photographic reversal. 



In another experiment, a small sphere about 1 cm. in 

 diameter, which was made uniformly active by exposure to 

 the radium emanation, was used as the source of rays. A 

 metal plate was placed about 2 cms. above the sphere, and 

 the rays from the sphere passed through a small hole in this 

 plate and fell nearly normally on a photographic plate placed 

 2 cms. above the hole. The impression on the plate corre- 

 sponded to a pinhole photograph of the active sphere, taken 

 by the a. rays emitted from its surface. A drawing of the 

 photograph is shown in fig. IB, PI. II. The photographic 

 effect is greatest at the extreme edge of the circle, and falls 

 off towards the centre. 



This distribution of the intensity of the radiation for the 

 radioactive sphere and cylinder is very different from that 

 which would be observed if the photographs were takeu by 

 the luminous rays from a white-hot sphere or cylinder. In 

 a pinhole image of the sun, for example, we know that the 

 intensity of the radiation is practically uniform over its 

 disk. 



These striking differences between the distribution of the 

 intensity of radiation for ordinary light sources and for these 

 radioactive sources receive a simple explanation along the 

 following lines. 



In the case of a luminous body like the sun, the intensity 

 of the radiation emitted from any point is not uniform in all 

 directions, but is governed by the well-known cosine law, 

 sometimes known as Lambert's law. This asserts that the 

 intensity of the radiation from any point varies as the cosine 

 of the angle between the normal and the direction of the 

 emitted light. This law of distribution for a self-luminous 

 source is, I believe, usually explained by supposing that the 

 light does not come entirely from the surface, but from a 

 sensible depth of an absorbing layer. 



