170 



MINERALOGY 



-c r 

 FIG. 314. Negative. 



section of the spherical wave front, in the plane of the paper. The 

 extraordinary ray travels with a velocity which varies with the 

 direction; the minimum value of which, let it be supposed, is in 



the direction of the c 

 axis and is equal in 

 this direction to that 

 of the ordinary ray ox. 

 They will both arrive 

 at p and p' on the c 

 axis simultaneously, or 

 for light traveling 

 through .the crystal 

 parallel to the c axis 

 there is only one in- 

 dex of refraction. 

 Crystals of the tetrag- 

 onal and hexagonal 

 systems are isotropic 

 in the direction of their c axis only; such crystals are optically 

 uniaxial. The direction in which the extraordinary ray travels 

 with a maximum velocity is at right angles to the c axis or paral- 

 lel to it according to the 

 optical sign. Let it be sup- 

 posed the crystal is calcite 

 ( ), the maximum velocity 

 will therefore be in the plane 

 of the lateral axes, or the basal 

 plane ; this is true for any 

 direction in this plane from o. 

 Let this maximum value be 

 represented by oa. 



The cross section of the 

 wave front parallel to the 

 basal plane is a circle. Inter- 

 mediate values between po 

 and oa, as in the direction of 



od or of, when plotted on the plane of the paper, form an ellipse, 

 which is similar for all plane sections containing the c axis. 

 The whole wave front of the extraordinary ray is an ellipsoid of 

 revolution, the axis of which is the c axis, or is parallel to the optic 

 axis. The ellipsoid of revolution or wave front of the extraordinary 



FIG. 315. Positive. 



