EPOCH OF YOUXG AND FRESXEL. 87 



aperture." lie rightly considers this observation as of the most essen- 

 tial value. " This," he says, " was not known by those who began to 

 consider the waves of light, among whom are Mr. Hooke in his Micro- 

 graph;/, and Father Pardics ; who, in a treatise 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 prin- 

 cipal foundation, which consists in the remark I have just made, was 

 wantino- in his demonstrations." 



O 



By the help of this view, Huyghens gave a perfectly, satisfactory 

 and correct explanation of the laws of reflection and retraction ; and 

 he also applied the same theory, as we have seen, to the double refrac- 

 tion of Iceland spar with great sagacity 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 dis- 

 posed with regard to the faces of the rhornbohedron, for to these faces 

 the optical phenomena are symmetrically related. He found 7 that the 

 position of the refracted ray, determined by such spheroidal undula- 

 tions, would give an oblique refraction, which would coincide in its 

 laws with the refraction observed in Iceland spar : and, as we have 

 stated, this coincidence was long after fully confirmed by other 

 observers. 



Since Huyghens, at this early period, expounded the undulatory 

 theory with so much distinctness, and applied it with so much skill, it 

 may be asked why we do not hold him up as the great Author of the 

 induction of undulations of light ; the person who marks the epoch 

 of the theory ? To this we reply, that though Huyghens discovered 

 strong presumptions in favor of the undulatory theory, it was not 

 established till a later era, when the fringes of shadows, rightly under- 

 stood, made the waves visible, and when the hypothesis which had 

 been assumed to account for double refraction, was found to contain 

 also an explanation of polarization. It is then that this theory of light 

 assumes its commanding form ; and the persons who gave it this form, 

 we must make the great names of our narrative ; without, however, 

 denying the genius and merit of Huyghens, who is, undoubtedly, the 

 leading character in the prelude to the discovery. 



The undulatory theory, from this time to our own, was unfortunate 

 vn its career. It was by no means destitute of defenders, but these were 

 not experimenters ; and none of them thought of applying it tc 



7 Tracts on Optics, 237 



