ELECTRONS AND QUANTA 219 



the evidence that Hght is in some sense corpuscular. This idea had 

 its inception in Einstein's speculations in regard to Planck's theory 

 of the distribution of energy in the spectrum of a black body radiator. 

 It was conceived that the energy radiated by one atom remained in 

 some way localized in space, and could be delivered in toto to another 

 atom or resonator suitably constituted to receive it. From this 

 hypothesis Einstein predicted the relation which was later found to 

 obtain between frequency of radiation and maximum electron energy 

 in photoelectric emission, and the idea has become more and more 

 essential to the understanding of the photoelectric phenomenon as the 

 facts concerning it have been more and more fully revealed to us by 

 experiment. Energy in amounts hv is absorbed from radiation of 

 frequency v, not by slow accretion from waves, but instantaneously as 

 from particle impact. The picture seemed clear enough; the energy 

 of a beam of light is carried by corpuscles, each corpuscle transporting 

 the amount hv. If we were willing, for the sake of pursuing this 

 idea further, to disregard the whole gallery of interference phenomena 

 with which it apparently conflicted, we could show that if the cor- 

 puscles possess energy hv they must possess also momentum hvjc — 

 this relation being required to explain the observations on light 

 pressure in terms of impinging particles. 



Thus the corpuscular theory seemed to be required to explain the 

 photoelectric phenomena, and it might be made to explain also the 

 phenomenon of radiation pressure. On the other hand the light 

 corpuscle seemed a strange and ephemeral sort of particle, lacking 

 that continuity in time which we attribute to electrons and atoms. 

 Apparently it was manufactured within an atom for the express 

 purpose of carrying away a part of its energy and was later destroyed 

 in another part of the material universe to which this quantum of 

 energy was delivered. It was difficult to regard an apparently 

 transient entity of this sort as a particle in good standing to be classed 

 with electrons and alpha rays. 



If a certain suspicion still attaches to the light quantum in respect 

 to its continuity, this suspicion has at any rate been considerably 

 allayed, and the reputation of the quantum as an authentic particle 

 correspondingly enhanced, by the discovery of the Compton effect, 

 and again quite recently by the discovery of the Raman effect. In 

 the first of these phenomena we see the quantum surviving an en- 

 counter with an essentially free electron, with which it exchanges 

 energy and momentum in accordance with the ordinary laws of 

 elastic collision; in the latter we see the quantum preserving its 

 identity through an encounter with a molecule to which it imparts 

 a part only of its energy. 



