A CENTURY'S PROGRESS IN PHYSICS 367 



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 

 obtainable in optical experiments is taken into account. 

 For instance, Hastings (35, 60, 1888) has tested Huy- 

 gens' construction for double refraction in Iceland spar 

 and found that "the difference between a measured index 

 of refraction ... at an angle of 30 with the crystalline 

 axis, and the index calculated from Huygens' law and 

 the measured principal indices of refraction" is a matter 

 of only 4-5 units in the sixth decimal place. Since Max- 

 well's time the gamut of electromagnetic waves has been 

 steadily extended. The shortest Hertzian waves merge 

 almost imperceptibly into the longest heat waves of the 

 infra-red, and from there the known spectrum runs con- 

 tinuously through the visible region to the short waves 

 of the extreme ultra-violet recently disclosed by Lyman. 

 Here there is a short gap until soft X-rays are reached, 

 and finally the domain of radiation comes to an end with 

 gamma rays a billionth of a centimeter in length. 



Maxwell's ether was not a dynamical ether in the sense 

 of Green's elastic solid medium. In spite of the fact that 

 Maxwell was always active in devising mechanical ana- 



