Mr Growther, On the Transmission of ^-rays. 447 



known manner, by means of a shutter sliding over a plate of 

 uranium oxide. By placing a second small sample of radium 

 near the compensator, the 7-ray leaks through the two chambers 

 could be made to exactly neutralize each other, and thus the 

 effect due to the /3-rays alone could be measured directly. 



Unfortunately the double system transmitted so very fev,^ 

 y8-rays that it was not found possible to measure the effect on the 

 velocity of interposing different screens between them. Enough, 

 however, was transmitted to test the efficacy of the systems as a 

 means of producing a beam of homogeneous rays. Keeping the 

 first field constant and varying the second it was found that when 

 the mean velocity of the rays transmitted through the first system 

 was 2'77 X 10^" cms. per sec, the extreme values for the velocities 



of the rays, as measured by means of the second system, were 

 2"75 X 10^" cms. per sec. and 2*79 x 10^" cms. per sec. The extreme 

 velocities of the rays transmitted through one of the systems 

 therefore do not differ by more than one per cent., plus or minus, 

 from the mean value, and the system may be regarded therefore 

 as giving a fairly homogeneous pencil of rays. 



In order to be able to work with absorbing screens it was 

 necessary to make a further compromise in order to obtain a 

 measurable effect. The first system was left unaltered so that 

 a homogeneous pencil of rays of known velocity fell upon the 

 screen. The distance AD in the second chamber was, however, 

 reduced to 1 cm. The field across the first system in any given 

 experiment was kept constant while that across the second was 



29—2 



