400 



CHAPTER V. 

 DISCOVERY OF THE LAWS OF DOUBLE REFRACTION. 



THE laws of refraction which we have hitherto 

 described, were simple and uniform, and had a 

 symmetrical reference to the surface of the refract- 

 ing medium. It appeared strange to men, when 

 their attention was drawn to a class of phenomena 

 in which this symmetry was wanting, and in which 

 a refraction took place which was not even in the 

 plane of incidence. The subject was not unworthy 

 the notice and admiration it attracted ; for the pro- 

 secution of it ended in the discovery of the general 

 laws of light. The phenomena of which I now 

 speak, are those exhibited by various kinds of 

 crystalline bodies ; but observed for a long time in 

 one kind only, namely, the rhombohedral calc-spar; 

 or, as it was usually termed, from the country 

 which supplied the largest and clearest crystals, 

 Iceland spar. These rhombohedral crystals are 

 usually very smooth and transparent, and often of 

 considerable size ; and it was observed, on looking 

 through them, that all objects appeared double. 

 The phenomena, even as early as 1669, had been 

 considered so curious, that Erasmus Bartholin pub- 

 lished a work upon them at Copenhagen 1 , (Experi- 



1 Priestley's Optics, p. 550. 



