L. Page — A Century's Progress in Physics. 333 



fluid, on the other hand, on account of its lack of 

 rigidity, cannot support a transverse vibration. Hence 

 it was natural that Green, in searching for a dynamical 

 explanation of the ether, should have proposed in a paper 

 read before the Cambridge Philosophical Society in 

 1837 that the ether has the elastic properties of a solid. 

 One great difficulty presented itself; disturbances 

 inside an elastic solid must give rise to compressional as 

 well as to transverse waves. But no such thing as a 

 compressional wave had been found in the experimental 

 study of light. Green attempted to overcome this diffi- 

 culty by attributing an infinite volume-elasticity to the 

 ether. The expression above shows that longitudinal 

 waves originating in such an incompressible medium 

 would be carried away with an infinite velocity, and it 

 may be shown that the energy associated with them 

 would be infinitesimal in amount. The next step was to 

 calculate the coefficients of transmission and reflection 

 for light passing from one material medium to another. 

 Here the elastic solid theory is not altogether successful. 

 If the ether is supposed to have different densities in the 

 two media, as in FresnePs theory, but the same rigidity, 

 certain of these coefficients fail to give the values 

 demanded by experiment, while if the densities are 

 assumed the same but the rigidities different, other of the 

 coefficients have discordant values. In connection with 

 the phenomena of double refraction even more serious 

 difficulties are encountered. 



Electromagnetic Theory. — It was beginning to be felt 

 that an ether must explain more than the phenomena of 

 light, for Faraday's conception of electromagnetic 

 action as carried on through the agency of a medium 

 had added greatly to its functions. Finally Maxwell's 

 demonstration that electromagnetic waves are propa- 

 gated with the velocity of light made the theory 

 of light into a subdivision of electrodynamics. Maxwell 

 himself did not apply electromagnetic theory to the 

 explanation of reflection and refraction. This defi- 

 ciency, however, was remedied by Lorentz in 1875. The 

 results obtained, as well as those for double refraction 

 (J. W. Gibbs, 23, 262, 1882 et seq.), and metallic reflec- 

 tion (L. P. Wheeler, 32, 85, 1911), provided a complete 

 vindication of the electromagnetic theory of light. This 

 is all the more significant when the extreme precision 



