[ 2SS ] 
XXVI. Xote on X-JRays and Scattered X-JRays *. 
By Charles G. Barkla, M.A., D.Se.f 
IN a paper on Polarized Rontgen Radiation { I showed 
that on the a?ther-pulse theory o£ Rontgen rays, we 
should expect the primary beam proceeding from an X-ray 
tube in a direction perpendicular to that of propagation of 
the cathode stream to be partially polarized, and on the 
theory of scattering by electrons in light atoms the secondary 
rays proceeding in a direction perpendicular to that of pro- 
pagation of the primary, from substances of low atomic 
weight, ought to be almost completely plane polarized. 
Experiments were described which in both cases demonstrated 
the polarity §, and the effects were shown to be of the order 
of magnitude expected on these theories. The evidence in 
favour of the theories appeared so strong, that a more com- 
plete study of the distribution of intensity of the secondary 
radiation was considered unnecessary and unimportant. 
Prof. Bragg, however, in a recent paper || regards some of 
the evidence in favour of the a?tber-pulse theory as a little 
over-rated, and proposes in its place the hypothesis that an 
X-ray beam consists mainly of "neutral pairs'" (each consisting 
of a positive and a negative particle rotating in a plane con- 
taining the direction of propagation of the u pair"). This he 
considers affords an easier explanation of the phenomena of 
X-rays and is not improbable a priori. 
It appears altogether unnecessary to fully discuss such an 
hypothesis, for the study of the distribution of intensity of 
secondary radiation from light atoms affords a simple and 
apparently conclusive test between the sether pulse and the 
" neutral pair " theories. At the same time this makes the 
investigation on polarization more complete, and furnishes a 
delicate method of testing the presence of scattered rays in 
a complex radiation. 
According to the sether-pulse theory of Rontgen rays, when 
a primary pulse passes over an electron in a light atom, the 
electron is accelerated in a direction opposite to (being 
charged negatively) the direction of electric intensity in the 
* The expenses of this Research have been partially covered by a 
Government Grant through the Royal Society, 
t Communicated by the Author. 
% Phil. Trans. A. vol. ccxiv. 1905, pp. 467-479. 
§ Royal Soc. Proc. A. vol. lxxvii. 1906, pp. 247-255. 
jl Phil. Mag. [6] vol. xiv. Oct. 1907, pp. 429-449. 
