XCII. The reflexion of X-rays from imperfect crystals

Mr. C. G. Dar~vin on the Reflexion ofproblem led to a formula for the intensity of reflexion.The experimental arrangements to which the calculations were adapted were those in vogue at that time--that is, with crystal fixed during each observation.This was the arrangemen~ used in the experiments of H. G. J. Moseley with the present writer*, and it was to them that the theoretical calculations were applied.In the course of those experiments a single, but a fairly good, measure was made of the absolute intensity of the reflexion of "white X-rays" and also the curve of reflexion against angle was found.By means of a quadratm'e it was therefore possible to measure the effect of a single atom, and the result was of the right order for the number of electrons anticipated.Yet it was apparent that the theory was defective, for it was calculated that the diffraction pattern of the reflexion could at most be a few seconds across, and that even if all the available radiation in this breadth were reflected the total would still be far short of the observed amount.2~ow, if the reflexion was perfect over any region, it could not be legitimate to treat of the atoms as all scattering independently.In D. it.therefore the mutual influence of the successive planes was included.It was found that over a breadth of a few seconds the reflexion was perfect, and that in this region the ordinary absorption of the rays by the crystal was swamped by a tkr more powerful special extincflea.These principles led to a modified reflexion formula~ but one which could explain the magnitude of the reflexion no better than the old.A way was found out of the difficulty by supposing the crystal to be a conglomerate of small blocks of perfect crystal all orientated approximately in the same direction, for such a conglomerate would reflect the radiation at many of its blocks~ internal as weli as external, and this would much increase the total amount reflected.No attempt was made to treat the problem at all fully, but a general line of argument suggested that the effect would be approximately to reinstate the formula of D.i. without the objections that had before attached to it.All the calculations of these papers were based on experiments in which the crystal was fixed.A. H. Compton t carried out a somewhat similar process quite independently, but based it on the experimental arrangement which has, in fact, proved more convenient in the study of monochromatic