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Mr. R. W. Varder on the 



The initial drop of the curves varied with the velocity ; 

 this is very probably due to the partial absorption of some of 

 the rays at the edges of the opening C (fig. 1). After this 

 initial drop, the curves are similar in type. In the diagrams 

 the thickness of aluminium is expressed in grains per sq. cm. 

 This thickness in the diagrams is uncorrected for the thickness 

 of the mica window D. The initial rise of the curve BB (fig. 2) 

 is due to /3 radiation excited in the screens under the condi- 

 tions of the experiment by the y rays from A. The current 



Fio-. 3. 



100 



•20 



GRAMS PER CM' 



increased with the thickness of the absorbing screens fairly 

 rapidly at first and then very slowly. This effect is very 

 important when the ratio of the /S-ray ionization in E to the 

 7-ray ionization is small. In the experiment with Rp = 2000 

 gauss cm. — the strong part of the spectrum— this ratio was 

 about 2. In this way absorption curves were taken from 



