If the pieces required are _rectangular_, no other apparatus will be required save a diamond and a flat rule; but if _circles_ are wanted, a machine for that purpose should be used (of which no description is necessary here). There are, however, other contrivances which answer tolerably well. One method is, to cut out from a thick piece of cardboard a circle rather larger than the size wanted. Dr. Carpenter recommends metal rings with a piece of wire soldered on either side; and this, perhaps, is the best, as cardboard is apt to become rough at the edge when much used. A friend of mine uses thin brass plates with circles of various sizes “turned” through them, and a small raised handle placed at one end. The diamond must be passed round the inner edge, and so managed as to meet again in the same line, in order that the circle may be true, after which they may be readily disengaged. The sizes usually kept in stock by the dealers are one-half, five-eighths, and three-quarters inch diameter; but other sizes may be had to order.
For the information of the beginner it may be mentioned here that the price of the circles is a little more than of the squares; but this is modified in some degree by the circle being rather lighter. If appearance, however, is cared for at all, the circles look much neater upon the slides when not covered with the ornamental papers; but if these last are used (as will shortly be described) the squares are equally serviceable.
As before mentioned, the thin glass is made of various thicknesses, and the beginner will wish to know which to use. For objects requiring no higher power than the one-inch object-glass, the thicker kinds serve well enough; for the half-inch the medium thickness will be required; while, for higher powers, the thinnest covers must be used. The “test-objects” for the highest powers require to be brought so near to the object-glass that they admit of the very thinnest covering only, and are usually mounted betwixt glasses which a beginner would not be able to use without frequent breakage; but if these objects were mounted with the common covers, they would be really worthless with the powers which they require to show them satisfactorily.