898 
MR. A. E. TUTTON AN INSTRUMENT FOR GRINDING SECTION- 
froui underneath the base. The rotation of the grinding disc is thus brought about 
in a steady and almost frictionless manner upon turning the driving pulley by means 
of the handle. For each rotation of the driving pulley the grinding disc rotates 
twice, a gain of speed which is not too great to permit of careful watching of the 
progress of the grinding, and quite sufficient to enable the grinding to be achieved as 
rapidly as possible without the crystal becoming unduly heated, which, if it had no 
injurious effect upon the crystal itself, would soften the wax in which it was held, 
and thus bring about movement of the crystal. 
Tfie surface of the grinding plane may, of course, be ground with any desired 
degree of fineness. It is a great advantage to have two such planes, the. second one 
being fitted over the one just described in a manner which enables it to be readily 
removed and replaced as desired. The permanent one may then be ground so finely 
that it is all but perfectly transparent, and employed exclusively for giving a final 
polish to the surface of the crystal ground by the other plane ; the latter may be 
relatively much rougher, a surface similar to that of the finer varieties of photographic 
focussing screens being suitable. This second prinding plane is seen reared up 
against the base to the right in fig. 1. It consists of a thick disc of plate glass, both 
surfaces of which are ground to a true plane, and are truly parallel to each other, the 
upper surface having the texture just indicated. It is slightly larger than the fixed 
disc, and is mounted in a narrow but strong brass frame whicli carries three small 
projecting pieces corresponding to a similar three projecting horizontally from the 
permanent brass disc which supports the polishing plane. Through a tapped hole in 
the centre of each projecting piece carried by the frame of the grinding disc is 
screwed a short screw ; when this disc is laid upon the polishing plane, glass to glass, 
the narrow metal frame of the grinding disc lying outside the circle of the polishing 
disc, the three screws are arranged to pass easily through three holes in the projecting 
pieces of the polishing disc. By means of three small milled nuts, seen lying near 
the grinding disc in fig. 1, the two discs can be rigidly fixed together. Bemoval of 
the grinding disc can very rajjidly be effected by placing a little glass crystal¬ 
lising dish partly under it, bringing each screw over the dish in turn, and with one 
finger giving a good twist to the little nut, when it almost immediately drops into 
the dish. 
. After a few weeks’ use the rough grinding plane becomes smoothed down and ceases 
to giind with its former rapidity. It Avill be observed that, for obvious reasons, the 
grinding table is so arranged that the grinding will occur somewhat near the circum¬ 
ference of the disc. By making use of the centering motions above the crystal, the 
position of the latter may be varied upon the grinding plate ; as one part becomes 
smooth the other parts of greater and less radius may be used. When the width of the 
smoothed annulus becomes inconvenient the plate may be re-ground in a very simple 
manner. In front of the instrument in fig. 1, very slightly to the left, is seen a 
thick disc furnished with a central handle. The disc is one inch in diameter and its 
