compton: size and shape of electrons 5 



Hull and Rice*^ have shown that for wave-lengths in the neigh- 

 borhood of 0.17 X 10-« cm. the value of o-/p for aluminium is 

 about 0.12. From curve I this corresponds to an electronic 

 radius of 2.2 X lO-^o cm., while curve II gives 2.3 X lO""' cm. 

 Ishino" finds that the value of ajp, using the hard 7-rays from 

 radium-C, is about 0.045. Taking the effective wave-length to 

 be« 0.093 X 10-^ cm., curve I gives a = 2.1 X 10"^" and curve 

 II gives a = 2.5 X 10^^^^ cm. Using either formula the agree- 

 ment between the two values of the radius is within the limits 

 of probable experimental error. The unusually low absorption 

 coefficient for 7-rays can therefore be quantitatively explained 

 on the hypothesis that the electron is a spherical shell of elec- 

 tricity of radius about 2.3 X 10-"^ cm. 



Another difficulty that is found in J. J. Thomson's simple 

 theory is that it predicts that if a beam of X-rays is passed 

 through a thin plate the intensity of the scattered rays on the 

 two sides of the plate should be the same. It is well known, 

 however, that the scattered radiation on the emergent side of 

 the plate is much more intense than that on the incident side, 

 both in the case of relatively soft X-rays and in the case of hard 

 7-rays. Barkla and Ayres^ have shown that for rather hard 

 X-rays and for those substances of low atomic weight whose 

 absorption coefficient can be calculated accurately by equation 

 (1) this prediction of Thomson's, theory is also valid. In the 

 case of the heavier atoms and the longer waves, however, the 

 rays scattered at a small angle with the incident beam by the 

 different electrons in the atom are so nearly in the same phase 

 that the intensity is considerably increased, while at large angles 

 the phase difference is much greater, and the intensity is much 

 smaller. This explanation cannot, however, be applied to the 

 excess scattering of 7-rays of short wave-length, since experi- 

 ment shows^" that for longer waves the light elements show no 



"Hull and Rice. Phys. Rev. 8:326. 1916. 

 UsHixo. Phil. Mag. 33: 129. 1917. 



« Rutherford and Andrade. Phil. Mag. 28: 263. 1914. 

 9 Barkla and Ayers. Phil. Mag. 21 : 271. 1911. 

 10 Barkla and Ayers. Phil. Mag. 21: 271. 1911. 



