DOUBLE REFRACTION. 67 



of the extraordinary ray likewise varies, the ratio diminishing 

 as the inclination increases, and being least when the ray is per- 

 pendicular to the axis. This least value of the ratio is called 

 the extraordinary index ; in Iceland spar it is 1/4S3. 



In the preceding cases, the plane of incidence contains 

 the optic axis, and the extraordinary ray continues in that 

 plane. This is generally true under the same circumstances, 

 whatever be the refracting surface. To see this, we have only 

 to look obliquely through a rhomb of Iceland spar at a point 

 on a sheet of paper : the extraordinary image will be observed 

 to revolve round the other, as the rhomb is turned, and will 

 twice arrive in the plane of incidence, namely, when that 

 plane contains the optic axis. The same coincidence of the 

 two planes occurs also when the plane of incidence is perpen- 

 dicular to the optic axis ; but in this case the ratio of the 

 sines of incidence and refraction of the extraordinary ray is 

 constant, so that this ray then satisfies both the laws of or- 

 dinary refraction. This constant ratio is the extraordinary 

 index already referred to ; it is best determined by means of 

 a prism of the crystal having its refracting edge parallel to 

 the optic axis. 



(87) The directions of the two refracted rays are given by 

 the following construction. 



Let AC be the incident ray, 

 and OF the section of the sur- 

 face of the crystal made by the 

 plane of incidence. Let the 

 incident ray be produced any- 

 where to B, and let BF be 

 drawn perpendicular to it, 

 meeting the surface in F. 

 Let CD : CB : : sine of refraction ; sine of incidence of the 



