26 



SCIENTIFIC THOUGHT. 



quences from Fresnel's calculations : Fresnel was invited 

 to prove by experiment these astonishing results, and he 

 found them actually confirmed. So far as the phenomena 

 of diffraction erroneously termed inflection are con- 

 cerned, this work of Fresnel established the fact " that 

 the theory of undulations foretells the phenomena as 

 exactly as the theory of gravitation foretells the move- 

 ments of the heavenly bodies." l It was, however, 

 quite different if we consider that other larger class of 

 phenomena 2 which revealed the fact that rays of light 



1 See Schwerd, ' Die Beugungs- 

 erscheinuugenausdenFundamental- 

 gesetzen der Undulations - theorie 

 analytisch entwickelt ' (Mannheim, 

 1835), Preface, p. x. 



2 The history of the final estab- 

 lishment of the wave theory of 

 light has been written by Whewell 

 in the second volume of the 

 ' History of the Inductive Sciences. ' 

 The main sources which existed at 

 that time were the memoirs of 

 Young and Fresnel, and the ' Life 

 of Dr Young ' by Peacock. This 

 history has been written again with 

 ampler materials by M. Verdet as 

 an introduction to the edition of 

 the complete works of Fresne], pub- 

 lished in 1866. It is well to read 

 both accounts, as some points which 

 remain obscure in the earlier are 

 fully explained in the later. There 

 is no doubt that Young suggested 

 that the phenomena of " sided - 

 ness," which rays of light exhibit, 

 lead to the conception of a lateral 

 or transverse movement ; he also 

 hinted that in biaxial crystals the 

 shape of the wave might be that 

 of an almond or an amygdaloid 

 (article " Chromatics," reprinted in 

 Works, vol. i. pp. 317, 322), what we 

 now call an ellipsoid ; but M. Ver- 

 det is right in characterising Young's 

 suggestions as vague, and vindicat- 



ing for Fresnel the full merit of 

 having defined transverse vibrations 

 and of having introduced the ellip- 

 soid of elasticity as a geometrically 

 perfect means of finding by con- 

 struction the paths of rays in biaxial 

 crystals. The method was quite 

 independent of the theoretical views 

 regarding light which were con- 

 tained in the same memoir, the 

 consideration of which was referred 

 to a commission consisting of 

 Ampere, Arago, Fourier, and Pois- 

 son. Of these Ampere had sug- 

 gested transverse vibrations as a 

 means of explaining the phenomena 

 of polarisation ( ' CEuvres de Fresnel, ' 

 vol. i. p. 394). Arago, though a great 

 friend of Fresnel and a believer 

 in the wave theory, never to the 

 end of his life accepted the 

 hypothesis of transverse vibrations 

 (ibid., p. Iv.) Poisson, a supporter 

 of Laplace's molecular theory, re- 

 tired from the commission ; and 

 Arago, who composed the Report 

 to the Academy, confined himself 

 to pronouncing on the experimental 

 portion, which fully confirmed the 

 general law of double refraction an- 

 nounced by the author ; refraining 

 from the expression of any opinion as 

 to the theoretical portion, it being 

 premature to do so (see ' CEuvres 

 de Fresnel,' vol. ii. p. 463). Im- 



