OITK AI. ri;<>n.KTii;s <>i- CRYSTAL- 





with a velocity of ox and an index of refraction of oz. Such an 

 ellipsoid is known as the indicatrix of Fletcher. 



Any ray wha.tever, as the ray entering the crystal at R, Fig. 335, 

 will in general he transmitted as 

 two rays. The indices of refrac- 

 tion will be represented by the 

 radii vert ores of the elliptical 

 section of the indicatrix, passing 

 through the point o and perpen- 

 dicular to the direction of the 

 ray. as the ellipse bab', which in 

 uniaxial crystals contains one 

 diameter aa', representing the 

 ordinary ray; this diameter is 

 constant in all sections of the 

 indicatrix passing through the 

 point o. The two planes Rcob 

 and Raa', at right angles to the 

 elliptical section aba' and con- 

 taining the two diameters bb' and 

 aa', are the planes of vibration of the two rays ; of these the 

 extraordinary ray vibrates in the plane containing the optic axis 

 cc' and the direction of the ray Ro, and termed the principal optic 



section. The extraor- 

 dinary ray vibrates 

 in the principal optic 

 section and is polarized 

 in the plane ROaa' at 

 right angles to it. 

 There is one direction 

 in which the two diam- 

 eters of the elliptical 

 section are equal, that 

 at 90 to the optic axis, 

 or the ellipse becomes a 

 circle and the two rays 

 are transmitted with 

 the same velocity and with no fixed plane of vibration; they 

 e not polarized. 



Angle of extinction. When a section of a uniaxial crystal, or in 

 fact any double refracting substance with plane parallel faces, is 



