284 BELL SYSTEM TECHNICAL JOURNAL 



this entrance to the path is closed, one must reason in the inverse 

 sense from the wa\e-motion to the qualities of the shining body. 

 Inevitably, it was assumed that when the path should at last be 

 successfully retraced, the shining body would be found in the sem- 

 blance of a viljrator. 



For a few years at the end of the nineteenth century and the begin- 

 ning of the twentieth, it seemed that the desired vibrator had been 

 found. Apparently it was the electron, the little corpuscle of nega- 

 tive electricity, of which the charge and the mass were rather roughl>- 

 estimated in the late nineties, although Millikan's definite measure- 

 ments were not to come for a decade yet. Maxwell had not con- 

 cei\'ed of particles of electricity, his conception of the "electric fluid" 

 was indeed so sublimated and highly formal that it gave point to 

 the celebrated jest (I think a French one) about the man who read 

 the whole of his "Electricity and Magnetism" and understood it all 

 except that he was never able to find out what an electrified body 

 was. H. A. Lorentz incorporated the electron into Maxwell's theory. 

 Conceiving it as a spherule of negative electricity, and assuming 

 that in an atom one or more of these spherules are held in equilibrium- 

 positions, to which restoring-forces varying proportionally to displace- 

 ment draw them back when they are displaced, Lorentz showed that 

 these "bound" electrons are remarkably well adapted to serve as 

 sources and as absorbents for electromagnetic radiation. Displaced 

 from its position of equilibrium by some transitory impulse, and then 

 left to itself, the bound electron would execute damped oscillations in 

 one dimension or in two, emitting radiation of the desired kind at a 

 calculable rate. Or, if a beam of radiation streamed over an atom 

 containing a bound electron, there would be a "resonance" like an 

 acoustic resonance — the bound electron would vibrate in tune with 

 the radiation, absorbing energy' from the beam and scattering it in 

 all directions, or quite conceivably deli%ering it over in some way or 

 other to its atom or the environing atoms. There were numerical 

 agreements between this theory and experience, some of them very 

 striking.' Apparently the one thing still needful was to produce a 

 plausible theory of these binding-forces which control the response 

 of the "bound" electron to disturbances of all kinds. Once these 

 were properK- de.scribed, the wa\es of light would be supplied with 



' Nolal>ly, the trend of the dispersion-curves for certain tran.sparcnt substances, 

 recently extended by Bergen Davis and his collaborators to the range of X-ray fre- 

 quencies; the normal Zeenian effect; Wicn's observations on the exponential dying- 

 down of the luminosity of a canal-ray beam, interpreted as the exponential decline 

 in the vibration-amplitudes of the Ixjund electrons in the Hying atoms; the de|)en- 

 dencc of X-ray scattering on the number of electrons in the atom. 



