332 Theory of X- Ray Reflexion. 



15. The Absence of Resonance. 



A very interesting question arises with regard to the 

 natural periods o£ the electrons. If there are such periods 

 in the X-ray region it should be possible to observe a 

 marked change in the refractive index in their neigh- 

 bourhood. It should also be found that a substance scatters 

 a particular wave-length much more efficiently than any other. 

 The early work on refraction was all with heterogeneous 

 rays and so cannot be taken as evidence, and no one has 

 worked at the scattering of characteristic radiation. In 

 Moseley's experiments the reflexion of the iron rays by 

 potassium ferrocyanide was nut specially strong, but we 

 know it in all cases to be nearly as good as it can be. But 

 in spite of the absence of direct evidence, it seems almost 

 incredible that an increase of about a million times (which 

 is what would be expected from (8)) should not affect the 

 apparent absorption. Barkla found no special absorption 

 by a substance of its own radiation, but that it needed for 

 example] rays as hard as those from nickel, to be strongly 

 absorbed in iron. Superficially this bears some resemblance 

 to a resonance effect. Indeed, before anything was known 

 about the wave character of the characteristic rays, it might 

 be thought of as an example of the fact that a strongly 

 damped vibration responds best to a different frequency from 

 that of emission. Even without further information this 

 explanation would hardly work, for whatever the damping 

 it will be found that the response is always nearly as good 

 to the emission frequency as to the optimum. But Moseley's 

 work has shown that the characteristic rays are extremely 

 homogeneous, so that this view becomes quite untenable. 

 On the whole it is simplest to suppose that Barkla's results 

 depend in some way on an intermediate secondary electron. 

 With regard to there being some real resonance effect, we 

 can only hold that it seems rather improbable that it exists. 

 If it does not, we must conclude that a substance radiating* 

 its characteristic X-ray spectrum is in some abnormal state. 

 It would be exactly analogous to the fact that the hydrogen 

 lines from a spectrum-tube are not absorbed in hydrogen. 



Summary. 

 (i.) The structure is discussed of the lines photographed by 



Moseley in his method of finding the X-ray spectra of 



the elements, 

 (ii.) Proceeding on a theory exactly analogous to that of 



light, the intensity of reflexion is found both for 



monochromatic and for heterogeneous radiation. 



