In an extended examination carried on chiefly by Richardson and K. T Compton, Hughes, and Millikan, it was brought out that light imparts en ergy to individual electrons in amounts proportional to its frequency and finally that the factor of proportionality between energy and frequency is just that previously deduced by Planck from the black-body spectrum. The idea of pressing the witness on the latter point had come from Einstein who outplancked Planck in not only accepting quantization, but in conceiving of light quanta as actual small packets or particles of energy transferable to single electrons in toto The case for a corpuscular aspect of light, now exceedingly strong, be came overwhelmingly so when in 1922 A. H. Compton showed that in cer- tain circumstances light quanta-Photons as they were now called -have elastic collisions with electrons in accordance with the simple laws of par ticle dynamics. What appeared, and what still appears to many of us as a contradiction in terms had been proved true beyond the least possible doubt light was at once a flight of particles and a propagation of waves; for light persisted, unreasonably, to exhibit the phenomenon of interference. Troubles, it is said, never come singly, and the trials of the physicist in the early years of this century give grounds for credence in the pessimistic say- ing.Not only had light, the perfect child of physics, been changed into a gnome with two heads- there was trouble also with electrons. In the open hey behaved with admirable decorum, observing without protest all the rules of etiquette set down in Lorentz manual, but in the privacy of the atom they indulged in st unna tces ways which no well-behaved mechanical system would deem proper. What was to be said of particles which were ignorant apparently of even the rudi- ments of dynamics? Who could apologize for such perversity - rationalize the data of spectroscopy? A genius was called for, and a genius appeared In 1913 Niels Bohr gave us his strange conception of"stationary"orbits in hich electrons rotated endlessly without radiating, of electrons disappearing from one orbit and reappearing, after brief but unexplained absences, in an- other. It was a weird picture -a picture to delight a surrealist-but one which fascinated the beholder, for in it were portrayed with remarkable fidelity the most salient of the orderly features which spectroscopic data were then known to possess; there was the Balmer series! and there the Rydberg constant correct to the last significant digit! It was a masterpiece. It is important to note that in achieving this tour deforce Bohr made judicious use of the constant which Planck had extracted from the black-body spectrum, the constant h388 1937 C.J.DAVISSON In an extended examination carried on chiefly by Richardson and K. T. Compton, Hughes, and Millikan, it was brought out that light imparts en￾ergy to individual electrons in amounts proportional to its frequency and finally that the factor of proportionality between energy and frequency is just that previously deduced by Planck from the black-body spectrum. The idea of pressing the witness on the latter point had come from Einstein who outplancked Planck in not only accepting quantization, but in conceiving of light quanta as actual small packets or particles of energy transferable to single electrons in toto. The case for a corpuscular aspect of light, now exceedingly strong, be￾came overwhelmingly so when in 1922 A. H. Compton showed that in cer￾tain circumstances light quanta - photons as they were now called - have elastic collisions with electrons in accordance with the simple laws of par￾ticle dynamics. What appeared, and what still appears to many of us as a contradiction in terms had been proved true beyond the least possible doubt - light was at once a flight of particles and a propagation of waves; for light persisted, unreasonably, to exhibit the phenomenon of interference. Troubles, it is said, never come singly, and the trials of the physicist in the early years of this century give grounds for credence in the pessimistic say￾ing. Not only had light, the perfect child of physics, been changed into a gnome with two heads - there was trouble also with electrons. In the open they behaved with admirable decorum, observing without protest all the rules of etiquette set down in Lorentz’ manual, but in the privacy of the atom they indulged in strange and unnatural practices; they oscillated in ways which no well-behaved mechanical system would deem proper. What was to be said of particles which were ignorant apparently of even the rudi￾ments of dynamics? Who could apologize for such perversity - rationalize the data of spectroscopy? A genius was called for, and a genius appeared. In 1913 Niels Bohr gave us his strange conception of "stationary" orbits in which electrons rotated endlessly without radiating, of electrons disappearing from one orbit and reappearing, after brief but unexplained absences, in an￾other. It was a weird picture - a picture to delight a surrealist - but one which fascinated the beholder, for in it were portrayed with remarkable fidelity the most salient of the orderly features which spectroscopic data were then known to possess; there was the Balmer series! and there the Rydberg constant! - correct to the last significant digit! It was a masterpiece. It is important to note that in achieving this tour deforce Bohr made judicious use of the constant which Planck had extracted from the black-body spectrum, the constant h