MISCELLANEOUS PAPERS. 



I. ON THE PHENOMENA PEESENTED BY LIGHT IN ITS 

 PASSAGE ALONG THE AXES OF BIAXAL CEYSTALS. 



Transactions of the Royal Irish Academy, Vol. XVII. 



IT is well known that when a ray of light is incident upon certain 

 crystals, such as Iceland spar and quartz, it is in general divided 

 into two pencils, of which one is refracted according to the known 

 law of the sines, while the direction of the other is determined by a 

 new and extraordinary law, first assigned by Huygens. 



These laws were long supposed to apply to all doubly -refracting 

 substances ; and it was not until the subject was examined by the 

 ablest advocate of the undulatory theory, that the problem of 

 double refraction was solved in all its generality. Setting out 

 from the hypothesis, that the elasticity of the vibrating medium 

 within the crystal is unequal in three rectangular directions, Fresnel 

 has shown that the surface of the wave is not, in general, either a 

 sphere or spheroid, as in the Huygenian law, but a surface of the 

 fourth order, consisting of two sheets ; and that the directions of 

 the two refracted rays are determined by tangent planes drawn to 

 these surfaces under known conditions. Such crystals have, in 

 general, two optic axes, and are thence denominated bia.rnL When 



