904 
MR. A. E. TUTTON ON AN INSTRUMENT FOR GRINDING SECTION- 
for this length of time usually furnishes a surface almost like that of ordinary glass, 
and is of great value, as it enables measurements of the optic axial angle to be made 
without the use of cemented cover-glasses. 
It IS so easy to give a last glance at the images from the adjusted zone after 
])olishing that it should always be done, for one is then absolutely certain that the 
desired surface has been obtained. It may be remarked that the use of a solvent for 
tlie crystal as lubricating liquid is to be deprecated, as it destroys the faces of the 
crystal, and so prevents the possibility of thus checking the adjustment. 
2. As the second typical case, a monoclinic crystal may be considered, for which 
determinations of extinction (for sodium light) in the symmetry plane, which i 
considered to be developed as a prominent face, and an examination of the same plane 
in convergent polarised light, have been carried out. These, it may be supposed, 
reveal the fact that one of the median lines perpendicular to which we desire to grind 
a surface, is inclined at a certain angle smaller than 45° to the intersection (edge) of 
the symmetry plane with either a prism, orthopinacoid, or dome face, or the basal 
plane. Four operations are necessary in order to adjust such a crystal so that this 
known direction of the median line shall be perpendicular to the grinding plane. The 
c]'ystal must first be cemented upon the holder in such a manner that the zone of 
faces parallel to the edge just mentioned is approximately perpendicular to the 
grinding plane; suppose, for instance, it is the prism zone of faces parallel to the 
vertical axis, containing the symmetry plane (clinopinacoid), the orthopinacoid, and 
several prismatic forms. The symmetry plane must, in the second place, be made 
exactly parallel to the upper tangent screw of the adjusting apparatus. The whole 
zone should, in the third place, be exactly adjusted perpendicular to the grinding 
plane. It then only remains to carry out the fourth operation of rotating the tangent 
scj-ew so as to move the segment round the required number of degrees to bring the 
direction of the median line exactly perpendicular to the grinding plane ; for, as the 
symmetry plane is j)arallel to the screw, and hence to the circle of motion, it remains 
perpendicular as a plane, and we only require to rotate it until the desired direction 
in it is perpendicular to the grinding plane. 
For use in all cases in which it is required to adjust any crystal face parallel to a 
tangent screw a special crystal holder is provided, which permits of nearly 90° of 
rotation of the crystal after placing in its socket, and subsequent fixing in any position. 
The two parts of this holder are seen in fig. 1, to the left of the larger ordinary holder, 
recognized by its cross grooves ; it is also shown in position in fig. 3. It consists of a 
grooved steel rod, similar to those of the other holders, carrying below a small solid 
brass cylinder. The latter fits closely into an outer hollow cylinder, closed below; the 
outer side of the end is cross-grooved like the discs of the other holders, for the more 
secure holding of the wax with which the crystal is to be cemented on to it. This 
hollow cylinder is pierced by two horizontal slots of slightly more than 90° extent, 
on opposite sides of the cylinder, and at different heights, for reasons of strength. The 
