EPOCH OF YOUNG AND FRESNEL. 429 



of the combat between the rival theories, that the 

 light will not be diffused beyond the rectilinear 

 space, when it passes through an aperture ; " for," 

 says he 6 , " although the partial waves, produced by 

 the particles comprized in the aperture, do diffuse 

 themselves beyond the rectilinear space, these waves 

 do not concur anywhere except in front of the 

 aperture." He rightly considers this observation as 

 of the most essential value. " This," he says, " was 

 not known by those who began to consider the 

 waves of light, among whom are Mr. Hooke in his 

 Micrography, and Father Pardies, who, in a trea- 

 tise of which he showed me a part, and which he 

 did not live to finish, had undertaken to prove, by 

 these waves, the effects of reflection and refraction. 

 But the principal foundation, which consists in the 

 remark I have just made, was wanting in his de- 

 monstrations." 



By the help of this view, Huyghens gave a per- 

 fectly satisfactory and correct explanation of the 

 laws of reflection and refraction ; and he also ap- 

 plied the same theory, as we have seen, to the 

 double refraction of Iceland spar with great saga- 

 city and success. He conceived that in this crystal, 

 besides the spherical waves, there might be others 

 of a spheroidal form, the axis of the spheroid being 

 symmetrically disposed with regard to the faces of 

 the rhombohedron, for to these faces the optical 

 phenomena are symmetrically related. He found 7 

 8 Tracts on Optics, p. 209. 7 Ib. p. 237- 



