222 Dr. W. Makower on the 



(V.) The effect of the positive ions produced at the same 

 time as the secondary corpuscles was examined, and the 

 potential difference to stop them determined. The effects 

 due to Rontgen rays produced from gaseous molecules by 

 the primary rays were also investigated. 



In conclusion, I desire to express my best thanks to 

 Sir J. J. Thomson for his many suggestions during the 

 course of the investigation, and also for his kind permission 

 to carry out this research at the Cavendish Laboratory. 



The Cavendish Laboratory, 

 University of Cambridge. 



XXII. The Straggling of a Particles. 

 By W. Makower, M.A., D.Sc* 



nPHE variation in the number of a particles from radium C 

 JL and polonium near the end of their ranges in air' was 

 first studied by Geiger f. Later, Taylor J repeated the 

 observations with radium C and extended the investigation 

 to the cases of hydrogen and helium. The method adopted 

 by both investigators was to count the number of scintil- 

 lations on a zinc sulphide screen kept at a fixed distance 

 from the radioactive source and to vary the pressure of the 

 gas through which the a rays passed. The results obtained 

 by Geiger and Taylor for the variation in the number of 

 a. particles from radium C in air are in substantial agree- 

 ment, but the straggling thus indicated near the end of the 

 range is considerably larger than is predicted by theory §. 

 It seemed, therefore, desirable to test the point further by 

 an independent experimental method to see whether there 

 is any defect inherent in the scintillation method giving rise 

 to the discrepancy between theory and experiment. 



The method adopted in this investigation consisted essen- 

 tially in substituting for the zinc-sulphide screen a photo- 

 graphic plate which, after development, could be examined 

 under a microscope. The density of the silver grains in 

 the film gives a record of the number of a particles which 

 fell upon the plate ||. It is therefore only necessary to count 

 the number of grains visible in a definite area in the field 



* Communicated by the Author. 



t Geiger, Proc. Kov. Soc. A, vol. Ixxxiii. p. 505 (1910). 

 % Taylor, Phil. Mag. Sept. 1913. 



§ Flamm, Sitzungsber. d. K. Akad. d, Wiss. Wien, Math.-nat. KL 

 cxxiii. II. A (1914) ; Bohr, Phil. Ma?. Oct. 1915. 



j| Kinoshita, Proc. Hoy. Soc. A, vol. Ixxxiii. p. 432 (1910). 



