888 
MR. A. E. TUTTON ON AN INSTRUMENT FOR GRINDING SECTION- 
The first surface of a section-plate is usually ground by holding the crystal firmly 
between the finger and thumb, and moving it gently to and fro over the surface of a 
finely-ground and slightly-convex glass plate, employing as lubricant either oil or a 
slow solvent for the crystal, endeavouring to avoid movement of the wrist, which 
would cause tlie ground surface to be more or less convex. If the crystal is tolerably 
hard, and not brittle, a case which but rarely happens, a holder may, perhaps, be 
safely improvised out of the two halves of a split cork, but, in most cases, grinding 
between the finger and thumb has to be resorted to. Having thus ground one face, 
it is polished upon a piece of silk fabric, and tested as to its planeness, and whether 
it is approximately true to the desired direction by adjusting it upon the goniometer, 
observing the character of its reflection of the signal of the collimator, and actually 
measuring at least two angles which it makes with developed faces of the crystal. If 
the results are not satisfactory, grinding must be resumed and continued until upon 
similarly testing the indications are satisfactory. A second face is then to be ground 
parallel to it in a similar manner, until a plate is obtained sufficiently but not too 
thin to exhibit (in the polariscope of the axial angle goniometer, which is to be 
employed for measuring the separation of the optic axes, supposing the crystal to be 
biaxial) the interference figures with inner rings of very small size, when the hyper¬ 
bolic brushes, whose separation is to be measured, are best defined. Before grinding 
the second face it is usually found most convenient to mount the crystal by the first 
ground surface upon a small glass plate by means of Canada balsam. The plate is 
more easily held during the grinding, the chance of breaking is diminished, and, if 
the crystal is strongly doubly-refracting so that a very thin section is required, 
approximate parallelism is more easily attained. When the crystals are not very 
small, the second surface may be ground more truly parallel to the first by employing 
the small apparatus made by Fuess, of Berlin. The crystal is cemented by its first- 
ground face upon one end of a closed white metal cylinder, two and a-half inches 
long, and a little over an inch in diameter, the ends of which are plane and fixed as 
nearly as possible perpendicularly to the axis. The cylinder slides vertically, with 
the crystal downwards, in an outer tube of brass from the lower end of which radiate 
horizontally three arms carrying levelling screws with fine threads; these are 
adjusted, by use of a graduated wedge, so that the ends of the arms are at the same 
height above the surface of the grinding-plate, when the cylinder will be perpen¬ 
dicular to the latter. By moving the apparatus to and fro over the lubricated 
grinding-plate, exerting at the same time a gentle pressure uporr the cyliirder, a 
second surface of the crystal is ground parallel to the first. This nrode of grinding 
the second surface is rrot found convenient in the case of small crystals. 
The grlirding of the first face of a prism from a small artificial crystal is carried out 
I)y hand in the same manner as the first surface of a section-plate. Tlie second face 
is naturally more difficult to obtain true to the desired direction ; it is usually, also, 
accomplished by hand. 
