70 DOUBLE REFRACTION. 



CB sin i CB sin i 



CO sinr CE sin/ 



Let CO, which is equal to the axis of revolution of the ellip- 

 soid, = b ; and CE, the equatorial axis, = a. Then, if CB be 

 taken= 1, there is 



n = - ; ri = -. 

 b a 



The numerical values of n and n' 9 for Iceland spar, for 

 the less refrangible rays, are 



= 1-653; ' = 1-484. 



(89) In order to verify these conclusions, we have only to 

 polish a prism of Iceland spar so that the refracting edge 

 may be parallel to the axis of the crystal. The plane of in- 

 cidence will then be perpendicular to the axis, and the two 

 refracted rays will be found to observe both the laws of 

 Descartes. The following are the indices of refraction of the 

 principal rays of the spectrum for this substance, as deter- 

 mined by Eudberg : 







Ray. . n. 



B . 1-65308 . ' 1-48391 



C . 1-65452 . 1-48455 



D . 1-65850 , 1-48635 



E "'," 1-66360 .. 1-48868 



F . 1-66802 . 1-49075 



G . 1-67617 . 1-49453 



H . 1-68330 . 1-49780 



(90) When the refracting surface contains the axis of 

 the crystal, and the plane of incidence passes through that 

 axis, the section of the ellipsoid by the plane of incidence 

 will be the ellipse MEM', whose lesser axis, MM 7 , is the 



