194 



MINERALOGY 



FIG. 342. Interference Figure of Brucite. 



placed in the field; but when the section is slightly inclined, 

 the optic axis will not appear in the center of the field, but 



to one side; and when the 

 section is revolved the point 

 where the shadows cross will 

 describe a circle around the 

 axis of the microscope, the di- 

 ameter of which will depend 

 upon the inclination of the 

 section. When the inclination 

 is large, the optic axis may 

 fall without the field of the 

 microscope and only a portion 

 or segment of the colored cir- 

 cles will show; but still the 

 radii of these quadrants will 

 point to the optic axis. The 

 dark shadows of the cross will move across the field, on revolving 

 the section, holding parallel positions. 



Use of the one quarter wave mica plate in determining the opti- 

 cal sign of a section. The quarter-wave mica plate is a cleavage 

 piece of mica of such a thickness as to yield an interference color 

 between crossed nicols of gray or light blue-gray. That is, on 

 emerging, one ray is retarded 1/4 X behind the other. Mica is 

 monoclinic with the acute bisectrix nearly parallel to the vertical 

 axis ; the two rays emerging in such a section are Y or the slow 

 ray and (3 or the intermediate ray. The cleavage piece is mounted 

 with the vibration plane of \ parallel to the long edge of the slide, 

 or the direction of the vibrations of the slow ray is marked on the 

 slide as was the case in the quartz wedge. The mounted section 

 slips in the slot in the tube of the microscope, between the ob- 

 jective and the analyzer, with its planes of vibration at 45 to 

 those of the nicols. 



Fig. 343 is a diagram of the interference figure to be tested with 

 the 1/4 X mica plate inserted, with the slow ray X vibrating parallel 

 to the arrow and across the quadrants 1 and 3. The faster ray will 

 vibrate across the quadrants 4 and 2 at right angles to the slow 

 ray. Let the section under observation be one of quartz, cut 

 perpendicular to the optic axis, an optically (+) mineral. In 

 this case the extraordinary ray is the slow ray ; its vibrations 

 at any point in the section are directed toward the optic axis c 



