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PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
three heads, the names we apply to the radiations produced :—-(l) scattered X-rays ; 
(2) fluorescent (characteristic)'X-rays ; (3) primary X-rays. This order has been 
chosen as probably leading to the greatest simplicity of treatment. 
SCATTERED X-RADIATION. 
When X-rays traverse matter of any kind, that matter becomes the source of a 
radiation of X-rays indistinguishable in kind from the primary radiation exciting it. # 
What in many ways appears to be simply a re-directed or scattered primary beam is, 
however, more truly a new radiation. The similarity between the primary radiation 
and the secondary radiation of this type, while perhaps tending rather to mask the 
full importance of the phenomenon in reality enhances the value of the results 
obtained. The simplicity of necessity permits of less ambiguity in the interpretation 
of the experimental results. 
The established facts which have an obvious bearing on the theory of radiation may 
be briefly stated. 
1. Slight polarization of the primary radiation—as shown by the variation in 
intensity of radiation scattered around such a primary beam,t 
2. Polarization of the scattered X-radiation, proceeding in a direction perpendicular 
to that of propagation of the primary radiation^ (such as is shown in light 
scattered from the sky), and 
3. The close agreement between wide limits, of the theoretical and experimental 
distribution of radiation scattered in a plane containing the direction of pro¬ 
pagation of the primary beam,§ 
are inexplicable except on the transverse wave theory. 
These experimental results were, in fact, looked for as necessarily following from 
such a theory. Similar results may, however, be obtained by transverse waves along 
strings or flexible tubes. An arrangement of flexible tubes may be used in lecture 
demonstrations illustrating mechanical phenomena exactly analogous to the three 
above-mentioned X-radiation phenomena. 
* Barkla, ‘Phil. Mag.,’June, 1903; May, 1904; June, 1906; February, 1908. 
t Barkla, ‘Nature,’ March 17, 1904; ‘Phil. Trans. Roy. Socvol. 204, 1905 ; ‘Phil. Mag.,’June, 
1906. 
J Barkla, ‘Roy. Soc. Proc.,’ A, vol. 77, 1906. 
§ Barkla, ‘Phil. Mag.,’ February, 1908; Barkla and Ayres, ‘Phil. Mag.,’February, 1911. AVhen 
the secondary radiation experimented upon is 'purely scattered radiation , the distribution experimentally 
determined agrees with the theoretical distribution remarkably closely between the limits of 180° and 
about 30° for 0. On the other hand the discrepancy between the two becomes very marked for small 
values of 6, indicating either the necessity for some modification of the simple theory in this direction, or 
possibly the superposition of other effects. Various possibilities might be suggested, but they could not 
at present be regarded as more than speculations. 
