the Absorption of Rontgen Rays. 741 



distribution of this radiation has, in all the cases investi- 

 gated, been found to be approximately that expected from a 

 theory of scattering on the sether-pulse theory. 



In addition to this, many elements — possibly all when 

 subject to a suitable primary radiation — emit a homogeneous 

 X-radiation which is characteristic of the element emitting 

 it, its penetrating power being independent of that of the 

 primary radiation exciting it. 



A primary radiation excites this homogeneous radiation 

 only when it — the primary— is of more penetrating type 

 than the homogeneous radiation. 



Regarding the distribution of this type of radiation, it has 

 been shown by one of us * that when a polarized beam 

 of Rontgen radiation is incident on Fe, Cu, Sn, Pb, &c, the 

 homogeneous secondary radiation which is emitted is equally 

 intense in directions in and perpendicular to the plane of that 

 polarization — that is, the intensity of secondary radiation in 

 any direction is independent of the position of the plane of 

 polarization of the exciting primary beam. 



Again, the homogeneous radiation from these metals has 

 been found equally intense in a direction perpendicular to 

 the direction of propagation of the primary, and one almost 

 opposite to that of primary propagation t- 



It is thus evident, and it has since been further verified, 

 that the homogeneous radiation is equally intense in all 

 directions. 



Now the energy of this secondary radiation is so great that 

 in many experiments it would produce enormous errors if not 

 taken into consideration. In measuring the intensity of a 

 beam of X-rays transmitted through an absorbing substance 

 and proceeding in the original direction of propagation, it is 

 therefore necessary either to so arrange the apparatus that 

 the effect of the secondary rays may be neglected or to make 

 a correction for it. 



It was also important to ascertain if the beam emerging 

 from the absorbing plate and proceeding in the original 

 direction of propagation was identical in properties with the 

 incident beam, or if it was transformed in any manner. 



A few simple experiments showed that all the phenomena 

 observed when a homogeneous beam of X-rays was trans- 

 mitted through a metal plate placed before the detecting- 

 electroscope could be explained qualitatively simply by 

 considering the superposition of the secondarv radiation 



* Barkla, " Polarized Rontgen Radiation/ 1 Phil. Trans. A. vol. cciv. 

 1905, po. 467-478, and later papers, 

 t Phil. Mag. Feb. 1908, pp. 288-396. 



