( 63 



CHAPTEE Y. 



DOUBLE REFRACTION. 



(81) It has hitherto been assumed that, when a ray of 

 light is incident upon the surface of a transparent medium, 

 the intromitted portion pursues, in all cases, a single determi- 

 nate direction. This is, however, very far from the fact. In 

 many, indeed in most cases, the refracted ray is divided 

 into two distinct pencils, each of which pursues a separate 

 course, determined by a distinct law. 



This property is called double refraction. It was first dis- 

 covered by Erasmus Bartholinus, in the well-known mineral 

 called Iceland spar. After a long series of observations, he 

 found that one of the rays within the crystal followed the 

 known law of refraction, while the other was bent according 

 to a new and extraordinary law not hitherto noticed. An 

 account of these experiments was published at Copenhagen, 

 in the year 1669, under the title " Experimenta Crystalli 

 Islandici dis-diaclastici, quibus mira et insolita refractio dete- 

 gitur." 



A few years after the date of this publication, the subject 

 was taken up by Huygens. This distinguished philosopher 

 had already unfolded the theory which supposes light to 

 consist in the undulations of an ethereal fluid ; and from that 

 theory had derived, in the most lucid and elegant manner, 

 the laws of ordinary refraction (37). He was, therefore, 

 naturally anxious to examine whether the new properties of 

 light, discovered by Bartholinus, could be reconciled to the 

 same theory ; and, in his desire to assimilate the two classes 



