On the Photographic Action of a, ß and j Kays emitted from Radioactive Substances. 13 



calculated value of r is also within tlie range over wliich tlie size 

 of the grains actually measured by means of the microscope varies. 



9. Deflexions of a 'particles, on the passage tJirough the eynulsion 

 film. Evidences have already been given by the previously cited 

 investigators that some of the a particles suffer sudden deflexions 

 on the passage through the emulsion fihn. 



In dealing with the deflexions of a rays, the importance of 

 utilizing a single source may be emphasized. If a set of radial a 

 ray tracks are obtained with an active needle in the way already 

 described, the possibility is excluded that two tracks, running in 

 different directions, happen to fall at a common point and present 

 themselves as if they were a single track suffering a sudden 

 deflexion. 



In the microphotograph in flg. 10, which is enlarged 1,500 

 diameters, the tracks of four « particles, running from left to right, 

 are visible. The source of the particles lies outside the figure to 

 the left about 15 centimetres on this scale from the left end. We 

 can see that while the second « particle from the top passed 

 straight on, the other three suffered sudden deflexions of 10° to 

 15° downwards after traversing some distances nearly parallel to 

 one another. 



We have examined hundreds of sets of the radial a ray tracks, 

 but so far we have not been able to find any which can be said 

 with certainty to have suffered the deflexion of an angle so large as 

 90°. The smallness of the proportion of the largely deflected tracks 

 to the total will not be inconsistent with the experimental results 

 arrived at by Geiger^-" and by Geiger and Marsden''^^ and also with 

 the theory worked out by Rutherford'^ It must be borne in 

 mind that we are, in the present case, observing only such 

 deflexions, which have taken place within a very thin layer 

 bounded by planes parallel to the surface of the fllm, whereas the 

 deflexions occur, in general, equally in all planes which contain 

 the initial line of motion. 



1) H. Geiger, Proc. Roy. Soc. A, 81 (1908), p. 174-, and 83 (1910), p. 492. 



2) H. Geiger and E. Marsden, Proc. Roy. Soc. A, 82 (1909), p. 495. 



3) E. Rutheiford, Phil. Mag. 21 (1911), p. ÜÜ9. 



