THE SCIENTIFIC WORK OF C. J. DAVISSON 793 



discussion of them. On the whole of the westward transatlantic voyage 

 Davisson spent his time trying to understand Schroedinger's papers, as he 

 then had an inkling (probably derived from the Oxford discussions) that 

 the explanation might reside in them. In the autumn of 1926, Davisson 

 calculated where some of the beams ought to be, looked for them and did 

 not find them. He then laid out a program of thorough search, and on 6 

 January 1927 got strong beams due to the line-gratings of the surface atoms, 

 as he showed by calculation in the same month." 



Now I will supplement this succinct history by explanations. The first 

 name to be mentioned in the explanations must be one which does not 

 appear in the quotation: that of Louis de Broglie. 



Louis de Broglie of Paris had suggested that electrons of definite momen- 

 tum — let me denote it by p — are associated with waves of wavelength X 

 equal to h/p, h standing for Planck's constant. This suggestion he made in 

 an attempt to interpret the atom-model of Bohr, a topic which is irrelevant 

 to this article. Irrelevant also is the fact that Louis de Broglie's suggestion 

 led Schroedinger to the discovery of "wave-mechanics," but I mention it 

 here because Schroedinger's name appears in the quotation. Highly relevant 

 is the inference that the ''de Broglie waves," as they soon came to be called, 

 might be difi'racted by the lattices of crystals, and that the electrons of an 

 electron-beam directed against a crystal might follow the waves into char- 

 acteristic diffraction-beams such as X-rays exhibit. 



This inference was drawn by a young German physicist Walther Elsasser 

 by name, then a student at Goettingen. It was one of the great ideas of 

 modern physics; and, in recording that its expression in Elsasser 's letter 

 was not what guided Davisson to its verification, I have no wush to weaken 

 or decry the credit that justly belongs to Elsasser for having been the first 

 to conceive it. Dr. Elsasser has authorized me to publish that he submitted 

 his idea to Einstein, and that Einstein said ''Young man, you are sitting on 

 a gold-mine." The letter which I have mentioned appeared in 1925 in the 

 German periodical Die N aturwissenschaften. As evidence for his idea Elsasser 

 there adduced the polycrystalline scattering-patterns, in particular those 

 for platinum, that had been published by Davisson and Kunsman. But 

 Davisson as we have seen did not accept this explanation of the patterns; 

 and never since, so far as Elsasser or I are aware, has anyone derived or 

 even tried to derive the polycrystalline scattering-patterns from the wave- 

 theory of electrons. This must be listed as a forgotten, I hone onlv a tem- 

 porarily forgotten, problem of theoretical physics. 



Essential to the application of Elsasser's idea is the fact that the wave- 

 lengths of the waves associated with electrons of convenient speeds are of 

 the right order of magnitude to experience observable diffraction from a 



