KINETIC OR MECHANICAL VIEW OF NATUEE. 27 



have sides, the phenomena of " lateral! ty " (misleadingly 

 called polarisation). The believers in the emission theory 

 studied them with predilection, Biot at their head. Al- 

 though to Young their explanations were unconvincing, 

 their results were so perplexing that he wrote to Brewster 

 in September 1815, "With respect to my own funda- 

 mental hypotheses respecting the nature of light, I be- 

 come less and less fond of dwelling on them, as I learn 

 more and more facts like those which Mr Malus dis- 

 covered ; because, although they may not be incompat- 

 ible with these facts, they certainly give us no assistance 

 in explaining them." * When Young wrote this, Fresnel 

 had not yet presented his first memoir on Diffraction to 

 the Institute ; his own labours on that matter were more 

 than ten years old ; the phenomena of polarisation had 

 meantime absorbed the attention of opticians. In the 

 summer of 1816 Arago and Gay-Lussac paid a visit to 



mediately after the reading of 

 Arago's report, Laplace, " who had 

 thought for a long time that his 

 analysis had made the phenomena 

 of double refraction depend on his 

 emission theory," proclaimed the 

 great importance of the memoir, 

 and declared that he placed these 

 researches above anything that had 

 for a long time been communicated 

 to the Academy ( ' (Euvres de Fres- 

 nel', vol. i. p. Ixxxvi., and vol. ii. 

 p. 459). We are indebted to M. 

 Verdet for having shown that the 

 discovery of this law by Fresnel is 

 independent of the theoretical con- 

 siderations by which he tried 

 synthetically to prove it. On this 

 point he says : " En re've'lant la 

 8e"rie de generalisations et de con- 

 jectures par lesquelles Fresnel est 

 arrive peu a peu a la de"couverte 

 des lois generates de la double re"- 



fraction, ils font disparaitre une 

 difficult^ qui ne pouvait manquer de 

 resulter de toute etude tant soit peu 

 approfondie de ses Merits imprints. 

 . . . On a vu au contraire que cette 

 loi s'est manifeste a Fresnel cornme 

 le resultat d'une generalisation toute 

 semblable aux generalisations qui 

 ont amene la plupart des grandes 

 decouvertes. Lorsqu'il a voulu 

 ensuite se rendre compte de la loi 

 par une theorie mecanique, il n'est 

 pas etonnant qu'il ait, peut-etre a 

 son insu, conduit cette theorie vers 

 le but qu'il connaissait d'avance, 

 et qu'il ait ete determine, dans le 

 choix des hypotheses auxiliaires 

 moins par leur vraisemblance intrin- 

 seque que par leur accord avec ce 

 qu'il etait en droit de considerer 

 comme la verite " (ibid., vol. ii. p. 

 327. Cf. vol. i. p. Ixxxiv.) 

 1 Works, vol. i. p. 361. 



