474 



Dr. C. G. Barkla. 



[Jan. 21, 



" Polarised Rontgen Kadiation." By CHARLES G. BARKLA, D.Sc., 

 B.A., King's College, Cambridge, Oliver Lodge Fellow, 

 University of Liverpool. Communicated by Professor J. J. 

 THOMSON, F.E.S. Eeceived January 21, Bead February 16, 

 1905. 



(Abstract.) 



Experiments on secondary radiation from gases and light solids 

 subject to X-rays showed that the character of this radiation differs 

 only very slightly from that of the radiation producing it, and that the 

 energy of this radiation is proportional merely to the quantity of matter 

 through which a beam of Eontgen radiation of definite intensity 

 passes, being independent of the kind of matter. 



These results, and the agreement between the energy experimentally 

 determined and that calculated, led to the conclusion that this radiation 

 is due to what may be called a scattering of primary X-rays by the 

 corpuscles or electrons constituting the molecules of the substance. 



On the hypothesis that Eontgen rays consist of a succession of 

 electro-magnetic pulses in the ether, each electron in the medium 

 through which these pulses pass has its motion accelerated by the 

 intense electric fields in these pulses, and consequently is the origin of 

 a secondary radiation, which is most intense in the direction perpen- 

 dicular to that of acceleration of the electron, and vanishes in the direc- 

 tion of that acceleration. The direction of electric intensity at a point 

 in a secondary pulse is perpendicular to the line joining this point and 

 the origin of the pulse, and is in the plane passing through the direction 

 of acceleration of the electron. 



On this theory, a secondary beam whose direction of propagation is 

 perpendicular to that of the primary, will be plane polarised, the 

 direction of electric intensity being parallel to the pulse front in the 

 primary beam. If the primary beam be plane polarised, the secondary 

 radiation from the charged corpuscles or electrons has a maximum 

 intensity in a direction perpendicular to that of electric displacement 

 in the primary beam, and zero intensity in the direction of electric 

 displacement. 



The secondary radiation from light substances was too feeble to allow 

 accurate measurement of the intensity of the tertiary radiation. 



A consideration of the method of production of primary Eontgen 

 rays in an X-ray tube, however, leads one to expect partial polarisation 

 of the primary beam proceeding from the antikathode in a direction 

 perpendicular to that of propagation of the impinging kathode rays, 

 for there is probably at the antikathode a greater acceleration along the 

 line of propagation of the kathode rays than in a direction at right 



