172 



MINERALOGY 



two rays, one of which has a constant index of refraction within the 

 plane and is therefore an ordinary ray ; the other, represented by the 

 ellipse, is variable in its velocity, and is an extraordinary ray. In Fig. 

 317 the radius of the large circle represents the maximum velocity 



-, and the inner ellipse the variable ray with its two limiting values 

 a 



^ and - ; Fig. 318 represents the conditions in the plane of sym- 

 metry at right angles to a, in which the inner circle is the minimum 

 velocity - and in which "y is the ordinary ray. The diameters of the 



ellipse represent the variable rays ; in Fig. 316 the conditions in the 

 third plane of symmetry are represented, or the intermediate value 



represents the velocity of the constant ray, the circle, and the el- 

 P 



lipse represents the maximum velocity - and the minimum velocity 



- in the crystal ; in this section the ellipse and circle cut each other 



at the four points P, P', P", P'", which represent the de- 

 pressions in the wave surface and are the four points at which 



the inner and outer surfaces are con- 

 tinuous. 



The two directions PP" and P'P'" 

 are the two optic axes. Parallel to 

 these two directions there is no double 

 refraction, and the section of the wave 

 surface perpendicular to the optic axis 

 in each case is a circle, as was also the 

 condition in uniaxial crystals. Strictly 

 these two lines PP" and p'p'" are the 

 secondary optic axes, but the true optic 

 axes are so near them as not to be 

 separable from them in practice. In 

 any section of the wave front other than 

 in the three planes of symmetry and 

 parallel to an optic axis, light will travel with two velocities or 

 rays, neither of which will be an ordinary ray, but both will vary 

 in speed with the direction of transmission. 



It is seen, Fig. 316, that the two optic axes, the direction of the 

 greatest velocity, and at right angles to it the direction of the least 



FIG. 318. 



