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XLY. The Tracks of the u Particles in Sensitive Photo- 

 graphic Films. By S. Kinoshita, Assistant Professor of 

 Physics, and H. Ikeuti, Research Student, Imperial 

 University, Tokyo *. 



[Plate VII.] 



IN 1910 one of the writers | showed that each a. particle 

 produces a detectable effect on a photographic film, i. e. 

 whenever an « particle strikes a grain of silver halide in the 

 sensitive film, that grain is subsequently capable of develop- 

 ment. It was also shown that this is the case throughout 

 the whole range o£ the a particles. These conclusions were 

 afterwards confirmed by the experiment of Reinganum %. 

 On a microscopic examination of a photographic plate, to 

 which the a rays had been tangentially projected, he observed 

 that the path of each a particle appeared as a trail of silver 

 grains. Reinoanum drew attention to the fact that some of 

 the trails showed the effect of scattering. Experiments on 

 this subject were later made by Michl § and Mayer || in 

 more detail. Recently, Walmsley and MakowerU succeeded 

 in taking some microphotographs on which the deflected 

 paths of the a particles were beautifully demonstrated. We 

 have also been engaged in the study of the same problem 

 and now allow ourselves to give a brief account of the 

 results. 



In investigating the photographic traces of the a rays it 

 was considered effective to work with a possibly small source 

 of the rays. For, if a point-like source be established and 

 placed on a photographic plate, the expelled a particles will 

 leave on it a set of radial traces, which can be followed with 

 greater ease and certainty. 



What we have utilized for the source of the a rays was a 

 sewing-needle, carrying at its pointed end a minute quantity 

 of the active deposit of radium. This could be easily 

 prepared by lightly rubbing the point on a metal piece which 

 had previously been exposed to a few millicuries of radium 

 emanation. After the active needle had been in contact with 

 a photographic plate for a short time, the plate was developed 

 in the usual way, when a fine spot became visible to the 



* Communicated bv Prof. II. Nagaoka. 



t Si Kinoshita, Proc. -Roy. Soc. A. lxxxiii. p. 432 (1910). 



% Reinganum, Phys. Zeits. xii. p. 1076 (1911). 



§ Michl, Wien. Per. cxxi. 2 a, p. 1431 (1912). 



|| Mayer, Ann. d. Phys. xli. p. 931 (1913). 



% Walmsley and Makower, Proc. Phys. Soc. xxvi. p. 261 (1914). 



