stated of them now would probably require correction from future experiments. Up to this period the attention has been devoted, as it still must be for a while, to the establishment of a process which, competent to produce with certainty a glass fitted for optical purposes, may have the philosophy and practice of every part so fully ascertained, as to be capable of description in a manner sufficiently clear to enable any other person, with moderate care, to obtain the same results without the labour of long and tedious investigation.
Appendix.
Rough-glass furnace.—The only furnace for making rough glass which has been constructed, answers its purpose exceedingly well; and though if a second were to be made, it should be upon a larger scale, yet I think it better to describe the tried one accurately, than to direct alterations which have not been experimentally approved of; especially as there seems to be nothing which, in principle, need differ in a larger furnace. An iron box (Plate II.) 30 inches long, 14 inches wide, and 8½ inches deep, forms the principal part of the exterior: it is open entirely at the top, and at the bottom also, in the fore part, where a fire-grate is to be placed. It has a common iron furnace door in front, the aperture of which is 8 inches wide by 6 inches high; and at the opposite end, or back of the furnace, a flanched aperture, 6; inches by 4½ for a piece of funnel pipe to connect the furnace with a powerful flue. The sides of this box, and such part of the bottom as is not appropriated for the fire-grate, are lined with fire-stone 1½ inch in thickness, except in the fire-place, where it is 2⅝. The grate is 12 inches long by 8 wide; and the part above it is closed by a fire-tile 2 inches thick and 12 inches square, which resting on the edges of the lining, finishes the portion intended for the coal fire, leaving it 5½ inches in depth from the covering tile to the grate. The other part is covered by an iron plate, 17; inches long, 13 inches wide, and ⅝ths of an inch thick, which, resting upon the edges of the lining, encloses a space of 16 inches long, 10 inches wide, and 5 inches deep, for the reception of
