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



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- 

 logues to illustrate the phenomena of electromagnetism, 

 he was never enthusiastic over the speculations of the 

 advocates of a dynamical ether. The electrodynamic equa- 

 tions provided an accurate representation of the electric 

 and magnetic fields, and beyond that he felt it was need- 

 less to go. That Gibbs (23, 475, 1882) held the same 

 view is made evident by the closing paragraphs of a 

 paper in which he shows that the electromagnetic theory 

 of light accounts in minutest detail for the intricate phe- 

 nomena accompanying the passage of light through cir- 

 cularly polarizing media. He says : 



"The laws of the propagation of light in plane waves, which 

 have thus been derived from the single hypothesis that the dis- 

 turbance by which light is transmitted consists of solenoidal 

 electrical fluxes, . . . are essentially those which are received 

 as embodying the results of experiment. In no particular, so 

 far as the writer is aware, do they conflict with the results of 

 experiment, or require the aid of auxiliary and forced hypotheses 

 to bring them into harmony therewith. 



In this respect the electromagnetic theory of light stands in 

 marked contrast with that theory in which the properties of an 

 elastic solid are attributed to the ether, — a contrast which was 

 very distinct in Maxwell's derivation of Fresnel's laws from 

 electrical principles, but becomes more striking as we follow the 

 subject farther into its details, and take account of the want of 



