DEMONSTRATION OF ELECTRON WAVES 549 



conceive an atom model capable of yielding precisely the Rydberg 

 constant and the complete spectrum of atomic hydrogen. The casual- 

 ties included two properties previously allotted to the electron as a 

 matter of course: the property of radiating energy during orbital mo- 

 tion, and the property of revolving about the nucleus in an orbit con- 

 sonant with classical dynamics and determined by initial conditions 

 which might be regarded as arbitrary. Bohr excluded from the 

 infinity of such orbits all but a special series. The motion of the elec- 

 tron remained planetary, but all else was new and bizarre. 



A great initial success had, however, been attained and hope of a 

 thorough conquest of spectra ran high, — too high as it now appears, 

 for beyond a few other quantitative successes, further achievements 

 were qualitative to a greater or less extent and consequently less im- 

 pressive. It turned out also that advancement in the elucidation of 

 spectra could be made only at the cost of an ever increasing array of 

 special rules and prohibitions — additional equipment of the same ar- 

 bitrary nature as that of Bohr's original postulates. Out of this neces- 

 sity appeared the one new idea regarding the electron which had 

 emerged in twenty years — the idea advanced by Goudsmit and Uhlen- 

 beck that the electron spins and possesses in consequence a magnetic 

 moment. 



It was recognized a decade ago by Bohr and others that the attack 

 upon the atom, despite its propitious beginning, had in a considerable 

 measure failed; and this because it had lacked, so to speak, a proper 

 base of operation. It was felt that the many arbitrary rules and 

 restrictions required to correlate the data of spectroscopy must flow 

 in a natural and unforced way from fundamental mechanical principles 

 as yet undiscovered. A system of mechanics was envisaged which 

 would degenerate to classical mechanics for large scale phenomena, but 

 which would present an entirely different aspect on the atomic scale, 

 and be capable, of course, of explaining atom dynamics as revealed by 

 spectra. 



Attempts to discover these underlying principles led to the formula- 

 tion by Heisenberg in 1925 of what is known as matrix-mechanics, 

 and led L. de Broglie in the same year to put forward his ideas concern- 

 ing a so-called wave-mechanics. These proposals are said to be state- 

 ments in different forms of one and the same principle — so far, at any 

 rate, as applications to atom dynamics are concerned. It is the wave- 

 mechanics, however, which has appealed most strongly to the physicist, 

 and it is with this only that I will here concern myself. 



The basic idea of the wave-mechanics was supplied by a paradoxical 

 situation which had existed for some years in the theory of optics. It 



