KINETIC UK MECHANICAL VIEW OF NATCRE. 4:'. 



of refraction, such as conical refraction, were mathe- 

 matically foretold and experimentally verified.' The 

 real ]ihysical question, however, remained unanswered; 

 and it remains only partially answered up to the present 

 day." How is it tliat the luminiferous ether, wlien ex- 

 isting inside ponderable matter, like air permeating a 

 grove of trees — as Young put it — is so changed that its 

 waves travel with variously altered speeds, that in 

 different directions the rays acquire diftierent pro- 

 perties, are differently maintained or partially extin- 

 guished (absorbed) ? It was natural to suppose that 

 the particles of ponderable matter must in some way 

 affect the ether, changing its density or its rigidity, and 

 that they themselves are affected by the movements 

 of the ether which tills their interstices. The question 

 can only be exhaustively answered l)y a complete know- 



^ The subsequent sug2;estioii of 

 the phenomena of inner and outer 

 conical refraction, experimentally 

 verified by Hum]>Iirey Lloyd in 

 1833 (see his 'Miscellaneous Pajiers,' 

 No. 1, or Transactions, Royal Irish 

 Academy, vol. xvii.), was jjopularly 

 regarded as a complete proof of 

 the correctness of the wave-surface, 

 and of Fresnel's entire theory. But 

 as to the first point, Sir G. G. 

 Stokes showed (Brit. Assoc. Repoi-t 

 on Double Refraction, 1862, p. 

 270) that conical refraction " must 

 be a property of the wave-sur- 

 face resulting from any reasonable 

 theory." And as the wave-surface 

 itself can Ijo geometrically con- 

 structed without any reference to 

 the mechanical theory of the ether 

 (as Mr Fletcher has most exhaus- 

 tively shown), the prediction of 

 conical refraction cannot be re- 

 garded as a proof of Fresnel's 



theory. Todhunter- Pearson says : 

 " But for Cauchy's magnificent 

 molecular researches, it might have 

 been possible for Fresnel to com- 

 pletely sacriticc the infant theorj* 

 of elasticitj- to that fiimsy sujx?r- 

 stition, the mechanical dogma, ou 

 which he has endeavoured to base 

 his great discoveries in light. 

 Cauchy inspired Green, and Green 

 and his followers have done soiue- 

 thing, if not all, to reconcile Fres- 

 nel's I'e.sults with the now fully 

 developed theory of elasticity, the 

 growth of which his dogma at one 

 time seriou.sIy threatened to check " 

 (' Hist, of Elasticit}-,' vol. i. p. 

 167). 



- In 1862 Sir G. G. Stokes "ex- 

 pressed his belief that the true 

 dynamical theory of double refrac- 

 tion had yet to be found " (Report, 

 I-. 268). 



