﻿840 The Photographic Action of a, /3, and y Rays. 



thickness of aluminium foil weighing 7*5 mgs. per cm. 2 , the 

 active point being held next to the foil. A still greater 

 enlargement of the edge of a photograph has also been 

 studied with camera lucida sketches, noting down carefully 

 the grains which became visible at different depths of the 

 film. The distribution of the grains is so irregular that it is 

 difficult to say with certainty whether there are definite 

 tracks due to individual /3 particles. But if such tracks are 

 present, they are certainly not straight. 



It will be noticed at once that the /3-ray photographs are 

 quite different in character from those obtained with a rays. 

 The following points of distinction may be noted : — 



1. The distinct radial character of the a photographs is 

 altogether absent in the (3 photographs. Compare specially 

 a photographs fig. 2 (a, b, and c) with /3 photograph fig. 5. 

 They were produced with the same active needle, and the 

 magnification is also about the same. 



2. In a /3-ray photograph no straight tracks are visible. 



3. In the case of a photograph there is no well-defined 

 centre, the dark nucleus, when formed, being irregular and 

 nebulous. 



Photographs have been taken by interposing an increasing 

 number of aluminium foils of known thickness between the 

 active source and the sensitive plate, activated plates as well 

 as needles being used. The general character of the /3-ray 

 photographs, as indicated above, is maintained in all cases, 

 the only difference noticed being a decrease in the photo- 

 metric density or of the number per unit area of separate 

 silver grains. A platinum plate which had been exposed to 

 radium emanation for a couple of hours a month previously 

 showed clearly the emission of both a and /3 rays due to the 

 growth of the long-period active deposit. 



The method is also being used for studying the weak 

 activity of potassium and rubidium. Preliminary experi- 

 ments show that they give off/3 rays "*. It may be possible 

 to discover other substances of still weaker activity by 

 sufficiently increasing the period of exposure. Experiments 

 in this direction are proceeding. 



It will thus be clear that the Wratten lantern-plate affords 

 an easy method for determining the emission or otherwise of 

 a and /3 particles from a given substance, as well as of 

 studying the complex character of ft rays. 



The action of y rays. — A sensitive plate was exposed to 

 64 mgs. of radium contained in a thick glass tube at a 

 distance of 4 cm. In one case, the rays had to penetrate 



* Campbell & Wood, Proc. Camb. Phil. Soc. xiv. p. 15 (1907). 



