DOG MU. A. E. TUTTON ON AN INSTRUMENT FOR GRINDINO SECTION- 
piiiacoidal faces. The basal plane is set parallel to the upper tangent screw, and the 
zone containing it and one pair of prism faces is adjusted perpendicularly in the 
manner previously described. Knowing the angle between the prism faces from a 
previous goniometrical measurement, the upper tangent screw is rotated in the proper 
direction until the segment has described an angle equal to half the goniometrical 
angle between (the normals to) one of the adjusted prism faces and an adjacent prism 
face not so adjusted, when the theoretical pinacoid will be parallel to the grinding- 
plane. If sufficient rotation cannot be effected by starting from the neighbourhood of 
zero, the preliminary adjustment of the zone is carried out with the segment rotated 
well over in the contrary direction, when an ample amount of rotation will be 
available. 
The case of a rhombic crystal exhibiting noue but dome forms—a rectangular 
pyramid—is to be similarly treated,, the only difference being that one of these faces 
is to be set parallel to the upper tangent screw instead of the basal plane or pinacoid 
in the simple case just considered, and after adjusting perpendicularly the zone con¬ 
taining this face and an adjacent one, or, in other words, the edge between these two 
faces, and rotating both segments by means of the tangent screws for the calculated 
number of degrees, the plane containing the two crystallographical axes parallel to 
which it is desired to grind a surface will be parallel to the grinding disc. 
4. For the case of a monoclinic crystal which does not exhibit the clinopinacoid 
(the symmetry plane), but only prism faces in. the principal zone, the special form of 
adjusting apparatus represented in fig. 3 will be found useful. The usual course of 
grinding a section parallel to the symmetry plane can first be carried out, by simply 
adjusting parallel to the axis of rotation of the instrument the zone of faces perpen¬ 
dicular to the symmetry plane containing the basal plane and orthodomes. The 
results of the stauroscopical observations with this section will, of course, reveal the 
positions of the axes of optical elasticity. Suppose it is, then, desired to grind a 
section perpendicular to that axis of optical elasticity which is inclined at an angle 
less than 45° to the vertical axis of the crystal. If the clinopinacoid were developed 
this could readily be carried out by the method of Case 2 ; if the orthopinacoid were 
present there would also be no difficulty, for if that were set parallel to the lower 
tangent screw, and the zone of prism and orthopinacoid faces adjusted perpendicular 
to the grinding plane, the direction of the axis of optical elasticity could be brought 
vertical by rotation of the upper tangent screw, ^vhich is set at right angles to the 
lower one, and therefore parallel to the symmetry plane. As, however, there are 
only prism faces present in the vertical zone, symmetrically inclined to the symmetry 
plane, it is evident that the two circular motions fixed at right angles will not 
directly enable the axis of optical elasticity to be brought vertical. But the necessary 
rotation of the symmetry plane in its own plane can evidently be effected by two 
equal motions in planes equally inclined to the symmetry plane. The special 
adjusting apparatus, in which the plane of the lower circular motion can be set at 
