76 DOUBLE REFRACTION. 



between them will be proportional to the thickness of the 

 crystal. But if the surfaces be inclined, so as to form a prism, 

 the deviation of the two rays will be different, and they will 

 emerge inclined to one another ; consequently the separation 

 will increase with the distance. 



Such a separation is of use in many experiments. In order 

 to render the divergence of the emergent pencils greatest, the 

 prism should be cut with its edge parallel to the optic axis ; 

 so that the refraction may take place in a plane perpendicular 

 to the axis. In this case the ordinary and extraordinary 

 refractions differ by the greatest amount, and therefore the 

 difference of the deviations jof the two pencils is greatest. A 

 double-refracting prism, so cut, is usually achromatized by 

 a prism of glass, with its refracting angle turned in the oppo- 

 site way. 



A better arrangement has been suggested by Wollas- 

 ton. Two prisms of the same substance, and of equal refract- 

 ing angles, are cut in such a manner, that in one the refracting 

 edge is parallel to the optic axis, and in the other perpendicu- 

 lar to it. They are then united, with their refracting angles 

 turned in opposite directions, so as to form a parallelepiped. 

 The resulting separation of the two rays, in this compound 

 prism, is the sum of the separations produced by each sepa- 

 rately. Let 0j and e t be the deviations of the ordinary and 

 extraordinary rays produced by the first prism ; o 2 and e 2 the 

 corresponding angles for the second. On account of the po- 

 sition of the axes, the ray which is refracted in the ordinary 

 manner by the first prism will be refracted extraordinarily 

 by the second ; and its angle of deviation will be QI e.y. 

 In like manner, the ray which is refracted extraordinarily by 

 the first prism will be refracted ordinarily by the second ; and 

 its deviation will be e l - o 2 . Consequently the resulting an- 

 gular separation of the two rays will be 



