Wright: a new crystal-grinding goniometer 39 



a given crystal and only an actual test can show which powder 

 is the best for a particular crystal. By use of the counter weight 

 Z (fig. 2) the pressure of the grinding disk against the crystal can 

 be regulated. 



(d) The attachment for the orientation of optical sections is 

 the optical system of Wiilfing's axial angle apparatus mounted 

 on a tripod (fig. 3b) which fits in place on three hardened steel 

 points. The crystal or fragment from which a plane-parallel, 

 optically oriented plate is to be ground is mounted on a special 

 holder (fig. 3d) by means of which the fragment is first strapped 

 into place by a copper wire or belt and then cemented. On the 

 goniometer the crystal is immersed in a hquid of its refractive 

 index (8; this eliminates in large measure the disturbing reflections 

 at the irregular surfaces of the crystal grain. In this form the 

 grinding goniometer serves all the functions of an axial angle 

 apparatus and has the additional advantage that on it the angle 

 which a given optical direction includes with a given crystallo- 

 graphical direction can be obtained directly from the position- 

 angles of the two directions. After the crystal has been properly 

 set so that the desired optical direction is observed, the axial 

 angle apparatus is removed, the arm of the goniometer is rotated 

 90° on the horizontal axis Z), and the grinding disk is swung into 

 place. 



Adjustment of the goniometer. The goniometer is in adjustment 

 when: (1) the horizontal and vertical axes of rotation (D D and 

 P S, fig. 2) are at right angles and intersect at a point; (2) the 

 axis of the telescope is vertical and intersects the above point; 

 (3) the axis of the grinding plate is vertical; (4) the axis of the 

 optical system of the axial angle attachment is horizontal, but 

 normal to the horizontal axis of the goniometer, and intersects 

 the above point. 



In the adjustment of the goniometer it is essential that a 

 definite plan, like that outlined below, be followed so that each 

 successive adjustment does not interfere with those preceding. 

 The adjustment is facilitated by tke use of two small attach- 

 ments: (a) a mounted minute glass bead obtained by melting 

 the end of a fine glass thread, and (b) a plane parallel plate 



