DEMONSTRATION OF ELECTRON WAVES 551 



In developing his idea of a possible wave aspect of the electron, de 

 Broglie was led to the conclusion (partly, it appears by intuition and 

 partly by considerations based on relativistic mechanics) that if elec- 

 trons possess wave properties, the correlation between their wave and 

 corpuscular aspects will be exj^ressed by these same two formula-. 



It would lead us too far afield to follow even cursorily the further 

 development of de Broglie's idea toward its original objective of ex- 

 plaining the behavior of atoms as disclosed by their spectra. These 

 interesting matters must be left with the mere statement that the 

 mathematical researches of Schrodinger and others have led to a con- 

 ception of the atom in which standing wave patterns replace the per- 

 mitted electron orbits of the Bohr model, and from which it is possible 

 to derive certain laws of spectra which previously could be given only 

 as empirical rules. 



The immediate object which de Broglie had in view in postulating a 

 wave aspect of the electron has thus been attained, and its attainment 

 argues strongly, of course, for the soundness of the underlying concep- 

 tion. It is not this spectroscopic evidence, however, which reveals 

 most clearly the wave as a real and actual property of electrons, but the 

 more direct and unequivocal evidence supplied by experiments de- 

 scribed in following paragraphs in which streams of electrons are 

 difTracted by crystals. 



It was implicit in de Broglie's earliest writings regarding electron 

 waves that a stream of electrons moving with uniform speed along 

 parallel lines will exhibit in appropriate circumstances the properties 

 of a beam of monochromatic waves. De Broglie's first step was, in- 

 deed, to associate a train of plane parallel waves with an electron 

 moving with uniform speed along a straight line. It remained, how- 

 ever, for the young German physicist Elsasser to point out the logical 

 conclusion to which these speculations lead, and to indicate the crucial 

 experiment by which they might be tested: to wit, that a beam of 

 electrons scattered by an appropriate grating should exhibit the 

 phenomenon of diffraction, and that the appropriate grating for this 

 purpose is a crystal, since the wave-lengths calculated from de Broglie's 

 formula for electrons of moderate speeds are like those of X-rays, of 

 the order of one Angstrom unit. 



It is the demonstration of this phenomenon — the diffraction of 

 electrons by crystals — which constitutes now, as has been intimated, 

 the chief experimental evidence in support of de Broglie's conception, 

 and it is with demonstrations of this kind that I am here primarily 

 concerned. The first of these was made by Davisson and Germer, who 

 showed in 1927 that beams of electrons are diffracted by a crystal of 



