of Light ; and a new Theory of Vismi. 437 



yvith any regularity, because it miist depend upon the mere acci- 

 dent of the rays coming down vvith one or the otlier side foremost. 

 The phccnomenon could not be constant in Iceland crystal, and 

 never in glass, if the cause did not in great measure reside in 

 the crystal ; and then there can be no necessity to suppose any 

 such diiference in the properties of light; for, if we admit that 

 there is one substance in Iceland crystal tiiat has the power of 

 turning the rays round, and another that has not (and we can 

 only account for the phenomenon upon his principle bv supposing 

 this), there can be no difficulty in supposing that there are tvvo 

 distinct substances in the crystal possessing a greater and less 

 power of refraction ; and as the rays strike upon one or other of 

 these substances, they are more or less refracted. 



Huygens accounted for this phaenomenon in a very different 

 way : Light, according to him, instead of being an effluvium of 

 particles propagated in straight lines from a luminous body, and 

 proceeding in all directions, consists in undulations of an ethereal 

 medium. " He supposes the ordinary refraction to be produced 

 by spherical undulations, propagated through the crystal, while 

 the extraordinary refraction arises horn spheroidal undulations*." 

 Here I must confess that I have never read any of the arguments 

 by whicli this theory of undulations is supported ; and therefore 

 I do not pretend to controvert them : but as I caimot form in 

 my mind any idea by which it can account for the phcenomena 

 of light, at least, witiiout the violation of some of the known laws 

 of nature, I shall propose certain objections to it, which, if the 

 hypothesis be founded upon true principles, can very easily be 

 answered : if they cannot, it must be evident that it rests upon 

 no other authority than tiie fancies of a lively imagination, and 

 will bear no comparison with an hypothesis that is founded upon 

 principles which are kriown to exist in nature, and which, as 

 far as I have met with them, is capable in itself of explaining 

 all the phaenomena of light, without borrowing from any other 

 system. 



I shall say nothing about these undulations of light being 

 spherical and spheroidal, just as it suits the convenience of the 

 J theorist, l)ecause I do not know in what way Huv^iens accounted 

 for it ; but it certainly is not saying much for a system, that the 

 inventor of it can only explain himself by the use of principles 

 which he denies to be in existence. Huvgens, in describing the 

 phenomena of hglit passing through Iceland crystal, does not 

 say a word about niululations, but speaks of the rai/s, being di- 

 vided and variously affected by the crystal, in the same terms 

 that Sir Isaac Newton would have used; and ypt, innnediatcly 

 afterwards, it apjjcars that tiiese rays have no existence, but that 



* Sec Edinburgh I'hilosophical Journal. 



light 



