134 Mr. M. Ishino on the Scattering and 



The experiments of Barkla and Ayres *, Owen f, and 

 Crowther J, however, showed that this expression was 

 inadequate for X rays. Florance § and Madsen || showed 

 that the distribution of scattered gamma rays was very 

 different from that observed for scattered X rays ; but they 

 did not find any simple expression for the distribution-law. 

 At the present stage the data are not sufficiently definite to 

 evaluate the integral in (6'). 



In the case where the distance of the plate at Rj (fig. 2) 

 from the electroscope is very large compared with the linear 

 dimensions of the receiving surface of the electroscope, 

 the amount of the rays scattered by the material entering 

 the electroscope is negligible, and therefore we have 

 approximately 



I-L = V-tM-*)*, (7) 



in place of (3). If the plate be thin, we may, to a first 

 approximation, neglect the absorption of the scattered rays 

 in the material ; and we may also assume that, when the 

 plate is close to the electroscope, the whole of the emergent 

 scattered rays enter the electroscope. 

 We have then, in place of (5), 



I 2 = V-"' (8) 



The equation does not take into account the small fraction 

 of the radiation scattered in the backward direction ; this 

 point will be discussed later. , By the formulse (7) and (8) 

 we can find fx and a from the observed values of I , I 1? 

 and I 2 . The deviation of the values of fi and a so obtained 

 from their true values should decrease as the thickness of 

 the plate is reduced. Therefore such measurements should 

 be made with very thin plates. The experiments with very 

 thin plates, however^ did not give concordant results, since 

 very small changes in the sensitiveness of the electroscope 

 and errors of observation have a relatively large effect in 

 deducing the values of /jl and a. It was found, however, 

 that the values for very thin plates could be deduced by 

 extrapolation from observation on thick plates. 



* Barkla and Ayres, Phil. Mag. xxi. p. 270 (1911). 

 t Owen, Proc. Cambr. Phil. Soc. xvi. p. 161 (1911), 

 t Crowther, Proc. Koy. Soc. A. lxxxv. p. 29 (1912). 

 § Florance, Phil. Mag. xx. p. 921 (1910). 

 || Madsen, Phil. Mag. xvii. p. 423 (1909). 



