1937 C. DAVISSON so to speak, and it was natural that the first applications should be to the atom. No thought was given at this time, it appears, to electrons in free flight. It was implicit in the theory that beams of electrons like beams of light would exhibit the properties of waves, that scattered by an appropriate grating they would exhibit diffraction, yet none of the chief theorists men- tioned this interesting corollary. The first to draw attention to it was El- sasser, who pointed out in 1925 that a demonstration of diffraction would establish the physical existence of electron waves. The setting of the stage for the discovery of electron diffraction was now complete. It would be pleasant to tell you that no sooner had Elsasser's suggestion appeared than the experiments were begun in New York which resulted in a demonstration of electron diffraction-pleasanter still to say that the work was begun the day after copies of de Broglie's thesis reached America. The true story contains less of perspicacity and more of chance. The work ac- tually began in 1919 with the accidental discovery that the energy spectrum of secondary electron emission has, as its upper limit, the energy of the pri- mary electrons, even for primaries accelerated through hundreds of volts; that there is, in fact, an elastic scattering of electrons by metals Out of this grew an investigation of the distribution-in-angle of these tically scattered electrons. And then chance again intervened; it was covered, purely by accident, that the intensity of elastic scattering varies with the orientations of the scattering crystals. Out of this grew, quite naturally, an Inves tigation of elastic scattering by a single crystal of predetermined orientation. The initiation of this phase of the work occurred in 1925, the year following the publication of de Broglie's thesis, the year preceding the first great developments in the wave mechanics. Thus the New York exp iment was not, at its inception, a test of the wave theory. Only in the summer of 1926, after I had discussed the investigation in England with Richardson, Born, franck and others did it take on this character. The search for diffraction beams was begun in the autumn of 1926, but not until early in the following year were any found -first one and then twenty others in rapid succession. Nineteen of these could be used to check the relationship between wavelength and momentum and in every case the correctness of the de Broglie formula, 2=h/p was verified to within the limit of accuracy of the measurements I will recall briefly the scheme of the experiment. A beam of electrons of predetermined speed was directed against a (Ill) face of a crystal of nickel as indicated schematically in Fig. 1. A collector designed to accept only elas-390 1937 C.J.DAVISSON so to speak, and it was natural that the first applications should be to the atom. No thought was given at this time, it appears, to electrons in free flight. It was implicit in the theory that beams of electrons like beams of light would exhibit the properties of waves, that scattered by an appropriate grating they would exhibit diffraction, yet none of the chief theorists men￾tioned this interesting corollary. The first to draw attention to it was El￾sasser, who pointed out in 1925 that a demonstration of diffraction would establish the physical existence of electron waves. The setting of the stage for the discovery of electron diffraction was now complete. It would be pleasant to tell you that no sooner had Elsasser’s suggestion appeared than the experiments were begun in New York which resulted in a demonstration of electron diffraction - pleasanter still to say that the work was begun the day after copies of de Broglie’s thesis reached America. The true story contains less of perspicacity and more of chance. The work ac￾tually began in 1919 with the accidental discovery that the energy spectrum of secondary electron emission has, as its upper limit, the energy of the pri￾mary electrons, even for primaries accelerated through hundreds of volts; that there is, in fact, an elastic scattering of electrons by metals. Out of this grew an investigation of the distribution-in-angle of these elas￾tically scattered electrons. And then chance again intervened; it was dis￾covered, purely by accident, that the intensity of elastic scattering varies with the orientations of the scattering crystals. Out of this grew, quite naturally, an investigation of elastic scattering by a single crystal of predetermined orientation. The initiation of this phase of the work occurred in 1925, the year following the publication of de Broglie’s thesis, the year preceding the first great developments in the wave mechanics. Thus the New York exper￾iment was not, at its inception, a test of the wave theory. Only in the summer of 1926, after I had discussed the investigation in England with Richardson, Born, Franck and others, did it take on this character. The search for diffraction beams was begun in the autumn of 1926, but not until early in the following year were any found - first one and then twenty others in rapid succession. Nineteen of these could be used to check the relationship between wavelength and momentum and in every case the correctness of the de Broglie formula, l = h/p was verified to within the limit of accuracy of the measurements. I will recall briefly the scheme of the experiment. A beam of electrons of predetermined speed was directed against a (III) face of a crystal of nickel as indicated schematically in Fig. 1. A collector designed to accept only elas-