52 



TRANSACTIONS OF THE 



[Nov. 16, 



ported in the liquid^ and there given motions of rotation both 

 in its own plane and around a horizontal axis until the desired 

 orientation is obtained. The cover glass is always removed 

 from the section to prevent total reflection. 



The figure shows the apparatus in vertical section. The metal 

 plate G is clamped securely to the stage of the microscope, and 

 to it is screwed the metal vessel i?, which is somewhat more than 



Fig. 3. 



a half sphere. The light enters through the glass plate a. The 

 hollow cone h is firmly attached to 5, and supports the system 

 of axes and plates which constitute the carrier for the section. 



The carrier is circular, and consists of a narrow metal rim,^ 

 Ti, and a large circular disk of glass, S. T^ is firmly attached 

 to the horizontal axis c, and its rotation around this axis is 

 measured by the graduated circle T and the vernier n. A rota- 

 tion of the carrier in its own plane is also produced by the knob, 

 k, which turns the fine toothed wheel z in contact with the teeth 

 in the rim T^. 



1 ° The determination of the position of the axial j^lane. 



Adjust the apparatus carefully so that the centre of rotation 

 of the carrier is the axis of the microscope, bring the crystal 

 section into the field, then with the carrier horizontal and the 

 nicols crossed, revolve the carrier in its own plane until the field 

 is dark when the plane of the optic axes must be parallel to the 

 principal section of one of the nicols. Revolve the carrier about 

 its horizontal axis (for 70°-75° on each side), first with one ex- 

 tinction direction parallel this axis, then with the other until the 

 brightening of the field shows the emergence of an optic axis 

 and hence the plane of the optic axes; or, revolve the entire 

 apparatus so that the crystal is in a diagonal position with 



