Observed Eatio. 



Relative 



Intensity for Pb. 



Intensity 



intensity for Cu. 



per electron 



•605 ±-012 



•86 ±02 



•601+009 



•85 ±-01 



•638 ±-008 



•89+01 



•677+023 



•95 + -03 



776 Prof. A. H. Compton on the 



the secondary rays from lead to that from copper, shown in 

 the second column, are the averages of large numbers of 

 readings : — 



Table I. 



Angle. 



30° 



20° 



15° 



10° 



In the third column the observed intensities are corrected 

 for the difference in absorption of the rays in traversing the 

 lead and copper rings, and the ratio of the relative intensity 

 per electron is calculated, taking the number of electrons per 

 atom as equal to the atomic number. It will be seen that 

 while at all angles at which measurements were made the 

 value of this ratio is slightly less than the theoretical value 

 unity, the ratio shows a tendency to increase at the smaller 

 angles. It is unfortunate that at still smaller angles the 

 energy of the secondary radiation was so low that no satis- 

 factory measurements could be made. 



It may be mentioned that Kohlrausch * has recently made 

 measurements similar to these at angles as low as 10°, and 

 that his measurements do not show this tendency for the 

 intensity of the secondary rays ffrom lead to increase at 

 the small angles more rapidly than that from the lighter 

 elements. While Kohlrausch used several times as strong a 

 source of gamma rays as that employed in these experiments, 

 his apparatus was not specially adapted to taking measure- 

 ments at small angles, and i( would appear that his probable 

 error at these angles was greater than that of the present 

 measurements. In view of the consistency of the experiments 

 here recorded, it appears probable that the observed increase 

 in the ratio at 10° is not the result of chance. 



The Wave-Length of the Gamma Rays. — A comparison of 

 the values given in Table I. with the data in fig. 1 shows 

 that the present experiments have not been carried to angles 

 sufficiently small to give values in Ipb/Icu overlapping Barkla 



* K. W. F. Kohlrausch. loc. cit. 



