THE DIFFRACTION OF ELECTRONS BY A CRYSTAL 91 



deflecting atom. It was with these ideas in mind that the investigation 

 was begun. What we were attempting, it will be seen, were atomic 

 explorations similar to those of Sir Ernest Rutherford and his col- 

 laborators but explorations in which the probe should be an electron 

 instead of an alpha particle. I shall not stop to recount the earlier 

 experiments of this investigation, but shall pass at once to the most 

 recent ones — those in which Dr. Germer and I have studied the 

 scattering of electrons by a single crystal of nickel. 



The unusual interest that attaches to these experiments is due to 

 their revealing the phenomenon of electron scattering in a new and, 

 I may say, fashionable role. Electron scattering is not, it would 

 seem, the mildly interesting matter of flying particles and central 

 fields that we supposed, but is instead a much more interesting 

 phenomenon in which electrons exhibit the properties of waves. The 

 experiments reveal that the way in which electrons are scattered by 

 a crystal is very similar to the way in which x-rays are scattered by 

 a crystal. The analogy is not so much with the alpha ray experiments 

 of Sir Ernest Rutherford, as with the x-ray diffraction experiments of 

 Professor von Laue. 



My task of describing these experiments is much simplified by the 

 fact that the experiments of Professor von Laue are so well known 

 and so thoroughly comprehended. I remind you very briefly that in 

 the original Laue experiment a beam of x-rays was directed against a 

 crystal of zincblende, that about the transmitted beam was found an 

 array of regularly disposed subsidiary beams proceeding outward 

 from the irradiated portion of the crystal, and that these subsidiary 

 beams could be interpreted completely and precisely in terms of the 

 then already popular wave theory of x-radiation. They could indeed 

 be explained as diffraction beams that resulted from the superposition 

 of secondary wave trains expanding from the regularly arranged 

 atoms of the crystal lattice. 



There are two features of the Laue experiment which we shall need 

 particularly to remember. The first is that diffraction beams issue 

 not only from the far side of the crystal along with the transmitted 

 beam, but also from the near or incidence side of the crystal — these 

 latter being disposed in a regular array about the incident beam. The 

 second is that each diffraction beam is characterized by a particular 

 wave-length, and that a given beam appears in the diffraction pattern 

 if the incident beam contains radiation of its characteristic wave- 

 length, or of some submultiple value of this wave-length, but not 

 otherwise. If the incident beam is monochromatic, no diffraction 

 beams appear at all unless the wave-length of the incident beam 



