The copper has first to be melted in a furnace, for the metal must be above a red heat before it will begin to run. Then the tin has to be carefully added, and the casting of the mirror is effected by pouring the molten metal into a flat mould. Thus the rough mirror is obtained, which in Herschel’s earlier telescopes seems to have been about six or seven inches in diameter, and nearly an inch thick. Though copper is such a tough substance, and though tin is also tough, yet when melted together to make speculum metal, as this alloy is called, they produce an exceedingly hard and brittle material. When we remember that we could never break a copper penny piece by throwing it down on the flags, it may seem strange that the “speculum metal” should be so exceedingly brittle. A piece the size of a penny would be more brittle than a bit of glass of the same dimensions, and when the speculum is cast, unless it is cooled very carefully, it will fly into pieces. Herein lay one of the difficulties that Herschel encountered. Speculum metal must be put into an oven as soon as the casting has become solid, and then the heat is gradually allowed to abate. When the speculum has been at last obtained, next follows the labor of giving it the true figure and polish. It is not only more fragile than glass, but it is also quite as hard, and therefore the grinding is a tedious operation. First the surface has to be ground with coarse sand, and then with emery, which is gradually made finer and finer until the true figure has been given (Fig. 69). The mirror is then somewhat basin-shaped, but the depression is very slight. For example, in a mirror six inches across the depression at the centre would perhaps be not more than the twentieth of an inch. Small though this depression is, yet it has to be made with exactness. In fact, if it were wrong at any point by so much as the tenth of the thickness of this sheet of paper, the telescope would not perform accurately. The tool that is used in grinding is made of cast iron, and has been turned in a lathe to the right shape. It is divided into squares in the manner shown in Fig. 70. After the grinding comes the polishing, and this is effected with a tool like the grinder in shape. This has to be covered over with little squares of pitch, so that when warmed and put down on the mirror it is soft enough to receive the right shape. Some rouge and water is spread over the mirror, and the polisher is worked backward and forward with the hand until a brilliant surface is obtained.