xviii. Proceedings. [Febnimy 20th, icjij. 



a particles on the 'halide grains as they pass through the film, 

 and their length represents the rangle oif the a particles in the 

 film of gelatine. It is evident, therefore, that we are dealing 

 Avith an extremely delicate method of detecting and studying 

 the emission of a rays, for each single a particle produces a 

 record of its passage through the photograp'hic film, and the 

 path taken by each particle can be studied in detail. 



Microphotographs showing the paths of a particles through 

 phoitographic films were first piublished by Walmsley and 

 MakoiWer, and soon afterwards 'by Kinoshita and Ikeuti. The 

 method adopted by the latter was to activ;ate the tip of a sewing 

 needle by gently rubbing it on a surface coated with the active 

 deposit of radium or isome other source of a radiation. In this 

 way .a trace of active matter was transferred toi the point of 

 the needle, which was then placed for a short time in contact 

 with a p'hotog'raphic film. The grains affected by the « particles 

 can be clearly seen radiaJting out in straight lines from centres 

 representing the points at' which the needle had been brought 

 into contact with the films. 



A defect of the method when applied to quantitative measure- 

 ments is the difficulty' of obtaining photographic plates capable 

 of development without showing under the microscope a number 

 of blackened grains, even when the plate has not been exposed 

 to light or any radio-active source. For some reason most photo- 

 graphic plates, if developed without exposure to light or other 

 stimulus, though showing noi visible fogging to the naked eye, are 

 found on examination under the microscope to be covered with 

 blackened silver grains. It is, therefore, important to use a plate 

 which, when suitably developed, is free from this defect. After 

 a' careful search it was found by Sahni that this condition is well 

 satisfied by Wratten and Wainwright's lantern plates. Later ex- 

 periments ha,ve shown that iSchumann plates are even more suit- 

 able, and by this means the photographic method has been used 

 to attack a number of problems such as the determination of the 

 ranges of a particles, and the variatiion in the number of 

 particles near the end of their range. 



Ordinary Meeting, March 6th, 191 7. 



The President, Professor SYDNEY J. HiCKSON, M.A., D.Sc, 

 F.R.S., in the Chair. 



Mr. G. P. Varley, M.Sc, and Mr. W. C. Jenkins, F.R.A.S., 

 were nominated Auditors of the Society's Accoimts for the 

 Session 191 6-1 7. 



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