OPTICAL PROPERTIES OF CRYSTALS 



17:, 



FIG. 321. Diagram of the Optical Proper- 

 ties of Wollastonite. 



of the species. Tin- axial plane may hold one of two position-: 

 (I) parallel to the plane of -vimm try oi' the -y.-tem; and (2) at 

 ri.nht angles to it. 



Figure :\'2\ represents the 

 optical conditions in the 

 mineral wollastonitc. The 

 axial |)lane i> parallel to 010 

 (Ax. PI. = 010), with X as 

 the acute bisectrix (Bx a = X), 

 optically ( ). The anuje 

 between the acute bisectrix 



ami the axis c is 32 12' in 

 the acute angle {J, or cx- 

 .(1 (Bx :lA c = 32 12' be- 

 hind) ; 2 V = 40. 



In the triclinic system, 

 where, at most, there is only 

 a center of symmetry, there is no relation between the optical 

 ellipsoid and the crystallographical axes, but usually the plane of 

 the optic axes is fixed in any given mineral species. In the 

 description of the optical properties of the triclinic minerals the 

 plane of the optic axes is located by measuring the angle between 

 its trace and some convenient edge, or by any convenient method. 

 In the case of axinite, the acute bisectrix is normal to 111. The 

 trace of the plane of the optic axes on 111 makes an angle of 40 

 with the edge 111/110, and 24 40' with the edge Ill/Ill. 



POLARIZED LIGHT 



In ordinary light the vibrations are not restricted to any one 

 plane, as the plane of the paper, in Fig. 322, but take place in all 



possible planes inter- 

 secting in the ray as 

 an axis, thus the vibra- 

 tions of the ordinary 

 beam of light are very 

 complex. When such 

 a complex ray strikes 

 the polished surface of 

 a transparent sub- 

 stance, a portion of 

 both the reflected and 



