( J28 Dr. J. Chadwick and Mr. E. S. Bieler on the 



§ 4. Apparatus. 



The apparatus was almost the same as that described by 

 one of us* in a recent paper on the scattering of a particles. 

 The general arrangement is shown in fig. 2. 



Fig. 2. 



To Pump 



The sheet of paraffin was held on a diaphragm placed 

 at A, equidistant from the source R and the ZnS screen. 

 The opening was covered with aluminium foil of 3*7 cm. 

 air-equivalent, and absorbing screens of aluminium could be 

 inserted between the opening and the ZnS screen. These 

 screens were necessary to prevent scattered a particles 

 from hitting the ZnS screen, and were also used in mea- 

 suring the range of the H particles. 



The ZnS screen was observed through a microscope, M, 

 which had a numerical aperture of 0*45 and a field of view 

 of 7*7 sq. mm. area. 



The box containing the source and diaphragm was placed 

 between the poles of an electromagnet. This served to 

 deflect the /3-rays emitted from the source, and was only 

 necessary with the diaphragms containing central holes. 



The paraffin sheets were cut by means of a microtome, 

 and their thickness was found by weighing. 



The source of a. particles was a -brass disk of 3 mm. 

 diameter, coated on the face with Ra(B-f-C). Its initial 

 * J. Chadwick, Phil. Mag. vol. xl. p. 734 (1920). 



