756 Prof. A. H. Compton on the 



out the beta rays. The results are shown in the following- 

 table :— 



Table II. 



ju- J/Io- Jo/Io- (Io-Jo)/Io- 



3-1 cm. 0-57 0-17 O'OS 0-02 



The values in the fourth and fifth columns are calculated 

 from the experimental data in the first three columns. 

 After the primary gamma rays have passed through 3*1 cm. 

 of lead, we have seen that probably not more than 10 per 

 cent, of the secondary radiation is of the scattered type, but 

 the intensity is so weak that the probable error of these 

 measurements is necessarily rather large. It may be con- 

 cluded, however, that at 45° probably not as much as 15 per- 

 cent, of the whole secondary radiation consists of scattered, 

 primary rays. 



At the larger angles the results given in Table I. show 

 that if there is any appreciable scattered radiation it must 

 be of a very soft type, and since it is unable to penetrate a 

 centimetre of lead, it cannot be identified with the hard 

 gamma rays from radium C. It will be seen from this 

 table also that the absorption coefficient of the fluorescent 

 radiation is rapidly approaching that of the primary rays at 

 the smaller angles. This fact, together with geometric 

 difficulties which prevent securing- intense secondary radiation 

 at the smaller angles, makes very difficult any effort to 

 separate the scattered and fluorescent radiation at angles 

 much smaller than 45°. The question of the presence 

 of scattered radiation at the smaller angles will be discussed 

 in another paper on the basis of some experiments of a 

 different type *. In this puper some positive evidence will 

 be presented for the existence of true scattering at angles 

 less than 15°. 



In order to find out how far the actual scattering falls 

 short of that to be expected from theoretical reasoning, a 

 measurement was made of the relative intensity of the* 

 primary and secondary radiation at several angles. If the 

 dimensions of the electron are negligible compared with 

 the wave-length of the gamma rays, and if the electrons all 

 scatter independently of each other, the usual theory f gives 

 for the intensity of the scattered radiation 



i,_!sy g+cos 2 fl) 



I ~ ~ 2m 2 / 2 C 4 ' [ ' 



* Infra, p. 770. 



t J.J. Thomson, he. cit, 



