46 



Fig. 7. 



Fisf. 8. 



suits for the hard rays are collect-ed and shown in the second 

 column of the following table : — 



I. II. III. IV. 



Substance. Thickness of Screen A Calculated X for j3 rays. 



to give half-value from II., in (McClelland and 



in mm. cm— i Hackett). 



Lead 083 ... 84 ... 93 



Tin -141 ... 50 ... 52 



Copper ... -137 ... 51 ... 55 



Aluminium ... '50 ... 14 ... 14 



Celluloid ... -98 ... — ... _ 



