OPTICAL I'I;MI>I.I;TM.S 



CUYSTALS 



203 



planes of (In- two einertfin; rays found as before ff and ee' will 

 both have components in t lie direction of the vibration planes of the 

 nicol-. and the point R, dark before, will now be illuminated. In tin- 

 same way. if all points in the 

 Held are toted and the dark 

 areas plotted, the dark areas 

 would have the form of hyper- 

 bolas, as shown in the photo- 

 graph, Fig. 351, with the optic 

 at the poles and the plane 

 of the optic axes bisecting the 

 curved shadow. The acute bi- 

 sectrix is located on the con- 

 ide of the curved shadow 

 midway between the two. 



Tln movement of these 

 shadows should be carefully FIG. 353. Interference Figure of Topea. 

 observed on revolving the sec- 

 tion, as their paths and curves help, very materially, to locate 

 the "acute bisectrix and the direction of the plane of the optic axes 

 when but a small portion of the interference figure is within the 



field of the microscope or 

 when the section is inclined 

 to the acute bisectrix, causing 

 the figure to lie eccentric in 

 the field of view. Fig. 352 

 is a photograph of the inter- 

 ference figure of aragonite, re- 

 volved just a little, showing 

 how the cross breaks up into 

 the two hyperbolas. Often 

 the angle between the optic 

 axes is so large that the optic 

 axes emerge out of the field 

 of the microscope ; but when 

 the section is perpendicular 

 to the acute bisectrix, the 

 be symmetrical, as illustrated in 



Fu;. 354. Interference Figure of B ante, 

 section nearly perpendicular to the Optic 

 Axis. 



interference figure will still 

 Fig. 353. 



When the section is cut perpendicular to an optic axis, the curve 

 or color areas are circles around the optic axis, as in uniaxial crys- 



