POLARIZATION TRANSVERSAL VIBRATIONS. 81 



The phenomenon of polarization seems to have had much 

 weight with Newton in forcing him to reject the theory proposed 

 by Huygens. "It is difficult," he says, " to conceive how the 

 rays of light, unless they be bodies, can have a permanent virtue 

 in two of their sides, which is not in their other sides, and this 

 without any regard to their position to the space or medium 

 through which they pass."* " Are not all hypotheses erroneous," 

 he adds in another place, " in which light is supposed to consist in 

 pression or motion, propagated through a fluid medium ? . . . . 

 Pressions or motions, propagated from a shining body through an 

 uniform medium, must be on all sides alike ; whereas by those 

 experiments it appears that the rays of light have different proper- 

 ties in their different sides."f In this objection Newton seems to 

 have fixed his thoughts upon that species of undulatory propaga- 

 tion whose laws he himself had so sagaciously divined. When 

 sound is propagated through air or water, the vibrations of the 

 particles of the fluid are performed in the direction in which the 

 wave advances ; and if the vibrations of the ether, which are sup- 

 posed to constitute light, were of the same kind, the objection 

 would seem to be insuperable. But the case is altered, if, as is now- 

 assumed, the vibrations of the ethereal particles be transverse to the 

 direction of the ray's progress. And though we were unable to 

 render any account of this hypothesis, or even to show that it is 

 consistent with mechanical principles ; yet the numerous classes of 

 phenomena which it has explained, and the striking and exact 

 manner in which its predictions have been verified on trial, com- 

 pel us to admit, that if the law to which we have thus reduced so 

 various and such complicated facts be not itself a law of nature, it 

 is at least coordinate with it, in such a sense that we may take it 

 as the representative of actual existence, and reason from it as we 

 would from an established physical law. 



The hypothesis of transversal vibrations first occurred to Dr. 

 Thomas Young, who illustrated it by the propagation of undu- 

 lations along a stretched cord agitated at one of its extremities. 

 Young seems to have been led to this principle while considering 

 the results arrived at by Sir David Brewster, in his researches 

 on the laws of double refraction in biaxal crystals. The principle 

 was soon after raised above the rank of a mere hypothesis, and 



* Optics, book iii. Query 29. t Query 28. 



