204 



MINERALOGY 



tals, or possibly a little elongated at the margin, as indicated in Fig. 

 354, in the direction of the other optic axis. On revolving such a 

 section the curved shadow revolves around the optic axis as a center, 

 counter to the revolution of the section and always with its convex 

 side toward the other optic axis. The trace of the plane of the 

 optic axis will pass through the pole of the curve or the optic axis, 

 bisecting the shadow symmetrically. 



The optical sign of biaxial crystals. The positive or negative 

 character of a crystal may be determined from its interference figure. 

 The section is placed between crossed nicols, with the plane of the 



optic axis 'at 45 to 

 the vibration planes 

 of the nicols; the 

 quartz wedge is then 

 inserted, with the vi- 

 bration plane of the 

 slow ray of the wedge 

 parallel to the axial 

 plane of the section, 

 as indicated in the 

 diagram, Fig. 355. 

 When the slow ray 

 of the section vibrates 

 in a plane parallel to 

 the slow ray of the 

 wedge, the circles 

 around the optic axis 



will contract from the center of the figure as they disappear 

 at the optic axes. Other color bands will contract along 

 the long axis of the wedge, until they meet at the acute bi- 

 sectrix, when they break, forming two circles, one around each 

 optic axis; all the color bands will continue to contract in this 

 manner as the wedge is advanced. The direction of this con- 

 traction is indicated by the arrows in the diagram. The effect 

 is that of thickening the section, and the sign of the section 

 is the same as that of quartz, or positive (+). The heads of 

 the arrows, indicating the direction of contraction, make a posi- 

 tive sign with the long axis of the wedge as usually mounted. 

 When the motion of the color bands is the reverse, or they expand 

 from the optic axes, the section has been thinned by the advance of 

 the wedge, and the section is the reverse of that of the quartz, or 



