THE DIFFRACTION OF ELECTRONS BY A CRYSTAL 105 



beams are due to the scattering of electrons by the adsorbed gas and 

 therefore we shall not consider them at this time. 



In closing I should like to say a few words about the conceptual 

 difficulty in which these experiments involve us. When Laue and 

 his collaborators investigated the scattering of x-rays by crystals 

 the results of their observations were accepted at once as establishing 

 the wave theory of x-rays. It was a very simple matter for W. H. 

 Bragg and others to give up the corpuscular theory because of the 

 hypothetical nature of the x-ray corpuscle. It was only necessary 

 to recognize that Laue's results were contrary to hypothesis and the 

 corpuscle disappeared. 



If the electron were not the well-authenticated particle we know it 

 to be, it is possible that the experiment I have described would cause 

 it to vanish in like manner. We do not, however, anticipate any 

 such event. The electron as a particle is too well established to be 

 discredited by a few experiments with a nickel crystal. The most 

 we are apt to allow is that there are circumstances in which it is 

 more convenient to regard electrons as waves than as particles. We 

 will allow perhaps that electrons have a dual nature — when they 

 produce tracks in a C. T. R. Wilson cloud experiment they are particles, 

 but when they are scattered by a crystal they are waves. 



A quite similar situation exists, of course, in the case of x-rays. 

 It has been evident for some years that the adherents of the corpuscular 

 theory of x-rays were too enthusiastic in their recantations. X-rays 

 also exhibit a dual nature — when they give rise to diffraction patterns 

 they are waves, but when they exhibit the Compton effect or cause 

 the emission of electrons from atoms they are particles — quanta or 

 photons. 



This state of affairs is one that should appeal to us as intolerable. 

 There must, it would seem, be comprehensive modes of description 

 applicable to all electron and x-ray phenomena, but what these are 

 we do not yet know. We do not know whether we shall eventually 

 believe with de Broglie and Schroedinger that electrons and x-rays 

 are waves that sometimes masquerade as particles, with Duane that 

 electrons and x-rays are particles that sometimes masquerade as 

 waves, or whether eventually we shall believe with Born that we are 

 dealing in both cases with actual particles and phantom waves. 



I believe, however, that for the present and for a long time to 

 come we shall, in describing experiments, worry but little about 

 ultimate realities and logical consistency. We will describe each 

 phenomenon in whatever terms we find most convenient. 



