The Degradation of Gamma-Ray Energy. 749 



The deflexion of the ray is therefore 4m/E,, the value 

 ■verified by the Solar Eclipse Expeditions. 



Since the deflexion can be thus established without anv 

 appeal to the Principle of Least Time or any other idea's 

 of pre-relativity Physics, there is no a priori reason for 

 adopting assumption (b) above. 



The following- argument suggests that the proposed alter- 

 native is more in accordance with the ideas of Kelativity. 



Consider two light pulses leaving A at times t A , t^ + dti 

 .and arriving at B at times t B and t B -\-dt B . 



Since, by the Principle of Equivalence, ds = along the 

 world line of each pulse, it seems that the Einstein interval 

 ds A between the two departures from A is equal to the 

 interval ds B between the two arrivals at B. That is, the in- 

 terval ds is transmitted by the radiation. 



'University of Queensland, 

 Brisbane. 



LXVTEI. The Degradation of Gamma-Ray Energy. By 

 Arthur H. Oompton, Ph.D., Way man Crow Professor of 

 Physics, Washington University *. 



[T has long been known that when matter is traversed by 

 gamma rays, it becomes a source of secondary f gamma 

 radiation. The relation between the primary and the 

 secondary gamma rays, however, has not been definitely 

 established. Although the secondary radiation is very 

 appreciably less penetrating than the primary rays, it has 

 usually been considered to be due principally to true 

 scattering J. It is the purpose of the present paper to 

 investigate the nature and the general characteristics of 

 secondary gamma rays, and to study the mechanism whereby 



* Communicated by Prof. Sir E. Rutherford, F.RS. 



T In this paper the term '-secondary" gumma radiation is used to 

 denote any radiation of the gamma type excited either directly or 

 indirectly by the passage through matter of primary gamma rays. By 

 " scattered " radiation is meant the radiation emitted by the electrons 

 in matter (that due to the positive nuclei is theoretically negligible in 

 comparison) due to the accelerations to which they are directly subjected 

 by the primary rays. The term "fluorescent " radiation signifies as usual 

 radiation of the energy absorbed from the primary bean and stored 

 temporarily in the kinetic and potential energies of the electrons. Its 

 frequency therefore depends jointly upon the frequency of the primary 

 rays and the nature of the radiator. 



X Cf. e.g. E. Rutherford, 'Radioactive Substances, etc.,' p. 282. 

 J. A. Gray, Phil. Mag. xxvi. p. 611 (.1913). D. C. II . llprance, Phil. 

 Mag. xxvi'i. p. 225 (1914). K. W. F. Kohlrausch, Phys. Zeitschr. xxi. 

 p. 193 (1920). 



Phil. Mag. S. 6. Vol. 41. No. 245. May 1921. 3 D 



