the Electrical Conduction produced by it. Ill 



fogging of the plate, during the long exposures required, by 

 the vapours of substances * is liable to obscure the results. 

 On the other hand the method of testing the electrical dis- 

 charge caused by the radiation is much more rapid than the 

 photographic method, and also admits of fairly accurate 

 quantitative determinations. 



The question of polarization and refraction of the radiation 

 can, however, only be tested by the photographic method. 

 The electrical experiment (explained in § 2) to test refraction 

 is not very satisfactory. 



§ 2. Polarization and Refraction. 



The almost identical effects produced in gases by uranium 

 and Rontgen radiation (which will be described later) led me 

 to consider the question whether the two types of radiation 

 did not behave the same in other respects. 



In order to test this, experiments were tried to see if 

 uranium radiation could be polarized or refracted. Becquerel f 

 had found evidence of polarization and refraction, but in 

 repeating experiments similar to those tried by him, I have 

 been unable to find any evidence of either. A large number 

 of photographs by the radiation have been taken under 

 various conditions, but in no case have I been able to observe 

 any effect on the photographic plate which showed the presence 

 of polarization or refraction. 



In order to avoid fogging of the plate during the long- 

 exposures required, by the vapours of substances, lead was 

 employed as far as possible in the neighbourhood of the 

 plate, as its effect on the film is very slight. 



A brief account will now be given of the experiments on 

 refraction and polarization. 



Refraction. — A thick lead plate was taken and a long- 

 narrow slit cut through it ; this was placed over a uniform 

 layer of uranium oxide ; the arrangement was then equivalent 

 to a line source of radiation and a slit. Thin prisms of glass, 

 aluminium, and paraffin-wax were fixed at intervals on the 

 lead plate with their edges just covering the slit. A photo- 

 graphic plate was supported 5 mms. from the slit. The plate 

 was left for a week in a dark box. On developing a dark 

 line was observed on the plate. This line was not appreciably 

 broadened or displaced above the prisms. Different sizes of 

 slits gave equally negative results. If there was any appreci- 

 able refraction we should expect the image of the slit to be 

 displaced from the line of the slit. 



* Russell, Proc. Roy. Soc. 1897. f C, R. 1896, p. 559. 



