48 



TRANSACTIONS OF THE 



[NOV. IG, 



Fig. 2. 



Two little cubical vessels with the two opposite sides made of 

 glass are furnished and a suitable stand. 

 The principal uses of the instrument are : 



1. Determination of the extinction angles of all faces of a 

 zone with respect to some given direction. 



2. Orientation of the optical principal sections and finding the 

 axial plane. 



3. Measurement of the apparent angle between the optic axes 

 or direct measurement of the true angle. 



4. Determination of pleochroism. 



1 Determination of the extinction angles of all faces of a 



zone. 



The microscope is tipped into horizontal position, the nicols 

 set with their vibration directions horizontal and vertical and 

 the apparatus is clamped to the stage as described. The crystal 

 is then fastened to the carrier by wax and adjusted by the arcs 

 L, Z/j until the zone axis is vertical. Any face of the zone is 

 then brought normal to the axis, most simply by first setting it 

 parallel to the axis by sighting through the microscope and 

 then rotating through 90°. 



The little glass vessel, filled with liquid of approximately the 

 mean index of refraction of the crystal, (see p. 50) is adjusted 



