LAWS OF DOUBLE REFRACTION. 71 



nary refraction of this crystal, to see if each phenomenon which is 

 deduced from theory, would agree with what is really observed. And 

 this being so, it is no slight proof of the truth of our suppositions and 

 principles ; but what I am going tc add here confirms them still mon 

 wonderfully ; that is, the different modes of cutting this crystal, in 

 which the surfaces produced give rise to refractions exactly such as 

 they ought to be, and as I had foreseen them, according to the pre- 

 ceding theory." 



Statements of this kind, coming from a philosopher like Huyghens, 

 were entitled to great confidence ; Newton, however, appears not to 

 have noticed, or to have disregarded them. In his Opticks, he gives a 

 rule for the extraordinary refraction of Iceland spar which is alto- 

 gether erroneous, without assigning any reason for rejecting the law 

 published by Huyghens ; and, so far as appears, without having made 

 any experiments of his own. The Huyghenian doctrine of double 

 refraction fell, along with his theory of undulations, into temporary 

 neglect, of which we shall have hereafter to speak. But in 1*788, 

 Haiiy showed that Huyghens's rule agreed much better than Newton's 

 with the phenomena: and in 1802, Wollaston, applying a method 

 of his own for measuring refraction, came to the same result. " He 

 made," says Young, 4 " a number of accurate experiments with an appa- 

 ratus singularly well calculated to examine the phenomena, but could 

 find no general principle to connect them, until the work of Huy- 

 ghens was pointed out to him." In 1808, the subject of double refrac- 

 tion was proposed as a prize-question by the French Institute ; and 

 Mains, whose Memoir obtained the prize, says, "I began by observing 

 and measuring a long series of phenomena on natural and artificial 

 faces of Iceland spar. Then, testing by means <5f these observations 

 the different laws proposed up to the present time by physical writers, 

 I was struck with the admirable agreement of the law of Huyghens 

 with the phenomena, and I was soon convinced that it is really the 

 law of nature." Pursuing the consequences of the law, he found that 

 it satisfied phenomena which Huysrhens himself had not observed. 



*- J O 



From this time, then, the truth of the Huyghenian law was universally 

 allowed, and soon afterwards, the theory by which it had been sug- 

 gested was generally received. 



The property of double refraction had been first studied only in Ice- 

 land spar, in whic 1 ! it is very obvious. The same property belongs. 



4 Quart. Rev. 1809, Nov. p. 333. 



