430 HISTORY OF OPTICS. 



that the position of the refracted ray, determined 

 by such spheroidal undulations, would give an ob- 

 lique 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 b$ 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 pre- 

 sumptions in favour of the undulatory theory, it was 

 not established till a later era, when the fringes of 

 shadows, rightly understood, made the waves visible, 

 and when the hypothesis which had been assumed 

 to account for double refraction, was found to con- 

 tain 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 ; with- 

 out, however, denying the genius and merit of Huy- 

 ghens, who is, undoubtedly, the leading character in 

 the prelude to the discovery. 



The undulatory theory, from this time to our 

 own, was unfortunate in its career. It was by no 

 means destitute of defenders, but these were not 

 experimenters ; and none of them thought of ap- 

 plying it to Grimaldi's experiments on fringes, of 



