CONTE MP OR A RY ADVANCES IN PHYSICS 91 



may be expressed in any of several ways, laying greater emphasis now 

 on one aspect and now on another. 



From the viewpoint of the atom, and using the notions of the un- 

 dulatory theory, one may say that tJie atom {or the molecule) modulates 

 the incident light with frequencies of its own. 



Again from the viewpoint of the atom, but using now the notions 

 of the corpuscle-theory of light, one may say that the- atom or molecule 

 may take part hut not necessarily all of the energy of an incident quantum, 

 converting this energy in any of numerous ways. 



From the viewpoint of the quantum, however, the essential feature 

 of the principle is this : a quantum may lose part of its energy or receive 

 energy in an encounter with a molecule or atom, retaining its identity 

 even though its frequency is changed. 



The first who stated the principle with anything like its proper 

 generality was probably Smekal; in the following year (1924) it was 

 developed by Kramers and Heisenberg. They knew of no examples 

 but the Compton effect, and curiously enough no one was tempted 

 to search for other instances, though Foote and Ruark considered 

 whether any of the phenomena already known in optical spectra 

 could be related to it.^^ Partial anticipations crop out here and there, 

 especially in the work of Compton, Jauncey and their associates; 

 for it was early suspected that the electrons responsible for the Comp- 

 ton shift are not altogether free, but very loosely bound to atoms; it 

 was assumed that the incident quanta must spend energy enough to 

 break the bonds as well to set the electrons into motion, and efforts 

 were made to disclose this breaking of the bonds.^" 



It is the third of the foregoing formulations of the principle which 

 I wish to stress in closing — the principle from the viewpoint of the 

 quantum, the authorization of the quantum to give up part of its 

 energy and retain the rest. To the unprejudiced mind this must 

 seem very natural. We have accepted for years the principle that an 

 electron may give up part of its energy and keep the rest — that the 

 life-history of an electron is an endless sequence of gains and losses 

 of kinetic energy, of speedings-up and slowings-down, during which 

 the identity of the electron is never lost. Why should we not have 

 thought likewise about the quantum ? Yet it has been almost an article 



^^ I am told that Kramers tried vainly to persuade a number of experimental 

 physicists to look tor the effect. At present they must be feeHng Hke the astronomers 

 whom Adams vainly pressed to make haste in looking for the planet Neptune, until 

 finally someone else discovered it. 



^" Jauncey and Compton anticipated in 1927 the idea that atoms in a lattice may 

 acquire energy of vibration from incident quanta, and discovered an important 

 restriction which should be noted; apparently the lattice or some third particle 

 must be involved in the impact. 



