Deflexion of a. Particles through Large Angles. 613 



In making the observations the disk carding the foils was 

 placed in position about 1*2 cm. from the glass plate C. 

 The apparatus was then completely exhausted through a 

 tube not shown in the diagram, charcoal cooled by liquid 

 air being used for the final exhaustion. After the source of 

 radiation had been placed in position, the microscope was 

 adjusted at that part of the zinc-sulphide screen where the 

 scintillations appeared at a rate convenient for counting. 

 With a source of 30 millicuries of Ra C this was usually the 

 case for an angle of deflexion of from 20° to 30°. The area 

 of the screen visible through the microscope was about 

 1 sq. mm., whilst the main beam of a particles covered an 

 area of about 3 sq. mm. 



As soon as the RaA in the source had decayed com- 

 pletely (i. e. after 20 minutes) countings were commenced. 

 Measurements were first taken with the layers of foils of 

 smaller thickness, and as the source decayed they were ex- 

 tended to the thicker foils. From the known decay of the 

 active deposit of radium the measurements could all be 

 corrected for the variation in activity of the source, the results 

 being* verified by making observations on the same foils at 

 different times. An experiment generally extended for about 

 80 minutes. After that time the decav corrections for the 

 source were not always reliable owing to small quantities of 

 radium emanation associated with it, as has been mentioned 

 above. Owing to the relatively short time available in each 

 experiment for the completion of the measurements, only 

 about 100 to 200 scintillations could be counted with each 

 foil. 



As in the experiments on the variation of scattering with 

 angle, some scintillations appeared on the zinc-sulphide 

 screen even when no scattering foil was interposed. It was 

 found that these scintillations were due to a particles which 

 had been scattered from the edges of the diaphragm limiting 

 the beam. Experiments were made with paper diaphragms 

 and with aluminium diaphragms of only T z - mm. thickness, 

 whilst a diaphragm D ! (fig. 4) was also introduced to prevent 

 scattering from the inside of the glass tube Gr carrying the 

 main diaphragm D. Even with these precautions the effect 

 was still so large that accurate experiments with foils of low 

 atomic weight would have been impossible. The difficulty 

 was, however, successfully overcome by intercepting the 

 stray a particles by a screen K, which could be turned by 

 means of a ground-glass joint (^ in fig. 3) about a vertical 

 axis passing through A so as to be just outside the main 

 pencil. The adjustment was made by observation of the 

 scintillations produced by the main beam on the zinc-sulphide 



