226 Dr. W. Makower on the Efficiency of 



that the curves given by Taylor appear to be slightly 

 steeper. 



It will be seen that the results of the investigation approach 

 considerably more closely to the requirements of theory than 

 those previously obtained, but, as might be expected, the 

 agreement between theory and experiment is not perfect. 

 The facts that the photographic film, though thin, is still of 

 finite thickness, and that there may be imperfections of the 

 surface emitting the radiation, lead to the anticipation of 

 straggling greater than is demanded by theory. Moreover, 

 as the end of the range is approached the silver grains 

 become feebler and feebler and consequent! } r more difficult 

 to count, and it seems not unlikely that the same difficulty 

 in the case of the scintillation method may lead to even more 

 serious errors. For, whereas a photographic film can be 

 studied again and again at leisure, scintillations have to be 

 counted at the moment of impact; of the a particles. If, 

 when giving only feeble scintillations near the end of their 

 range, some of these escape detection, erroneous results will 

 be obtained. But, however this may be, it will be seen that 

 the photographic method gives a more rapid falling off in 

 the number of a particles at the end of their range than was 

 previously indicated by the scintillation method and tends to 

 confirm the law given by Bohr's theory. 



Physical Laboratories, 

 The University, Manchester. 



XXIII. The Efficiency of Recoil of Radium D from 

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



INHERE is evidence that when radium A is deposited on 

 a platinum surface half of the radium B formed is 

 shot into the metal so that the radium C produced from it 

 is situated at varying depths below the surface. The recoil 

 stream of radium I) subsequently emerging traverses dif- 

 ferent thicknesses of platinum, and therefores escapes with 

 all possible velocities f. On this account the efficiency of 

 recoil of radium D from radium C should be low, and it is 

 of interest to determine its value. 



It has been pointed out to me by Dr. N. Bohr that the 

 problem is a very simple one, for if the recoil-streams have 



* Communicated by the Author. 



t Wood and Makower, Phil. Mag. Dec. 1915. 



