254 DISCOURSE ON THE STUDY 



cautious how we employ, in stating physical laws 

 derived from experiment, language which involves 

 any thing in the slightest degree theoretical, if we 

 would present the laws themselves in a form which 

 no future research shall modify or subvert. 



(279.) A third class of optical phenomena, which 

 were likewise discovered while Newton was yet 

 engaged in his optical researches, was that exhibited 

 by doubly refracting crystals. In what the pheno- 

 menon of double refraction consists, we have already 

 had occasion to explain. The fact itself was first 

 noticed by Erasmus Bartolin in the crystal called 

 Iceland spar; and was studied with attention by 

 Huyghens, who ascertained its laws, and referred it 

 with remarkable ingenuity and success to his theory 

 of light, by the additional hypothesis of such a con- 

 stitution of his ethereal medium within the crystal 

 as should enable it to convey an impulse faster in 

 one direction than another : as if, for. example's 

 sake, we should suppose a sound conveyed through 

 the air with different degrees of rapidity in a ver- 

 tical and horizontal direction. 



(280.) Some remarkable facts accompanying the 

 double refraction produced by Iceland spar, which 

 Bartolin, Huyghens, and Newton, had observed, led 

 the latter to conceive the singular idea that a ray of 

 light after its emergence from such a crystal acquires 

 sides, that is to say, distinct relations to surrounding 

 space, which it carries with it through its whole 

 subsequent course, and which give rise to all those 

 curious and complicated phenomena which are now 

 known under the name of the polarization of light. 

 These results, however, appeared so extraordinary, 



