722 Prof. Rutherford and Mr, Richardson : Analysis 



an illumination of g^ metre-candle the maximum visibility 

 is at 466 /jl/jL. 



Three hours was not long enough to obtain regular 

 luminosity curves from one observer. Hence, although 

 the mean was certainly accurate for the average eye, the 

 individual curves were only approximate for the individual 

 eyes. 



All the observers tested had normal colour vision. Two 

 had the shape and position of the " strong " and " weak " 

 luminosity curves exactly the same. This is supposed to 

 denote no rod vision. The women students tested, of whom 

 there were nine, showed no systematic dfferenee from the 

 men. 



LXXVT. The Analysis of the Gamma Rays from Radium B 

 and Radium C. By Professor E. Rutherford, F.R.S., 

 and H. Richardson, B.Sc, Graduate Scholar, University 

 of Manchester % '. 



T has long been recognized that the penetrating <y rays 

 emitted by a y ray salt were complex in character. 

 The examination of the radiation has been made by the 

 electric method in two ways : 



(1) by measuring the absorption of the 7 rays in different 

 materials over a wide range of thickness ; 



(2) by an examination of the absorption of the secondarv 

 and scattered <y rays which appear when 7 rays traverse 

 matter. 



Initial experiments on the absorption of the 7 rays of 

 radium by different materials were made by Rutherford f 

 and McClelland J. These were extended by later investi- 

 gations of Eve §, Tuomikoski ||, WiggerU, and S. J. Allan**. 

 The experiments showed that the absorption of the y rays 

 in lead rapidly decreased for the first two centimetres of 

 thickness, but became approximately exponential for greater 

 thicknesses. The whole question was re-examined with great 

 detail and thoroughness by Mr. and Mrs. Soddy and 

 A. 8. Russell ft? who determined the absorption of the 



* Communicated by the Authors. 



t Rutherford, Phys. Zeits. iii. p. 517 (1902). 



X McClelland, Phil. Mag. viii. p. 67 (1904). 



§ Eve, Phil. Mag. xvi. p. 224 (1908) ; xviii. p. 275 (1909). 



|| Tuomikoski, Phys. Zeit. x. p. 372 (1909). 



H Wigger, Jahrb. Radwakt. ii. p. 430 (1905). 



** Allan, Phys. Rev. xxxiv. p. 311 (J912). 



tt Soddy and Russell, Phil. Mag. xviii. p. 620 (1909); Mr. and 

 Mrs. Soddy and Russell, Phil. Mag. xix. p. 725 (1910) ; Russell, Proc. 

 Roy. Soc. A. lxxxvi. p. 240 (1911). 



