KINETIC OR MECHANICAL VIEW OF NATURE. 43 



of refraction, such as conical refraction, were mathe- 

 matically foretold and experimentally verified. 1 The 

 real physical question, however, remained unanswered ; 

 and it remains only partially answered up to the present 

 day. 2 How is it that the luminiferous ether, when 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 different 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 fills their interstices. The question 

 can only be exhaustively answered by a complete know- 



1 The subsequent suggestion of 

 the phenomena of inner and outer 

 conical refraction, experimentally 

 verified by Humphrey Lloyd in 

 1833 (see his ' Miscellaneous Papers,' 

 No. 1, or Transactions, Royal Irish 

 Academy, vol. xvii.), was popularly 

 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. Report 

 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 be 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 Freanel's 



theory. Todhunter-Pearson says : 

 " But for Cauchy's magnificent 

 molecular researches, it might have 

 been possible for Fresnel to com- 

 pletely sacrifice the infant theory 

 of elasticity to that flimsy super- 

 stition, the mechanical dogma, on 

 which he has endeavoured to base 

 his great discoveries in light. 

 Cauchy inspired Green, and Green 

 and his followers have done some- 

 thing, if not all, to reconcile Fres- 

 nel's results with the now fully 

 developed theory of elasticity, the 

 growth of which his dogma at one 

 time seriously threatened to check " 

 ('Hist, of Elasticity,' vol. i. p. 

 167). 



2 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, 

 p. 268). 



