In rough grinding, use flour emery until all the pits left by theeoarse grains are worked out. The surface of the speculum will then appear quite uniform when examined with a magnifying lens of about 1-in. focus. The fine grinding is then commenced. A good way of judging when the process of fine grinding is Completed is to hold the speculum in such a position that a gas flame is seen by oblique reflection in it; then graduall increase the angle of incidence until the image grows dim. By slightly tilting the mirror the image may be made to pass over every part, and should appear equally though feebly illuminated in every position. Another method is to hold the mirror horizontally, about level with the eye; then, when Looking along it at the window, no greyness should be visible. The glass is now ready for polishing. The more thoroughly the fine grinding is done, the less polishing will be necessary and the less injury will be done to the figureinthe polishing. What is known as the shadow test is applied continually when figuring and polishing.

Sometimes the speculum becomes tightly Locked with tool during the process of fine grinding, and this may be explained as follows: - As the upper disc moves along the lower one there is a tendency to heap up the emery towards the centre and away from the edges. This makes the convex tool slightly flat, and the concave speculum slightly hollow in the middle, thus leaving an empty space between them. After a time the film of moisture between them becomes exceedingly thin, a partial vacuum forms in the centre, and the two discs become locked together. The chances of such mishaps may be lessened by dividing the surface of the tool into parallel grooves l in. apart and at right angles to one another. To make these grooves, first mark them out with an American glass-cutter and then deepen them with a steel graver, keeping the part wet with turpentine.