MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
51 
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 (see fig. 1. & 2.) 30 inches long, 14 inches wide, and 8^ 
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 4^ for a piece of funnel pipe to connect the 
furnace with a powerful flue. The sides of this box, and such part of the bot- 
tom as is not appropriated for the fire-grate, are lined with fire-stone 1|- inch 
in thickness, except in the fire-place, where it is 2f. 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 5|- inches in depth from the cover- 
ing tile to the grate. The other part is covered by an iron plate 17^ inches 
long, 13 inches wide, and fths of an inch thick, which, resting upon the edges 
of the lining, incloses a space of 16 inches long, 10 inches wide, and 5 inches 
deep, for the reception of crucibles. This plate is formed with circular holes 
about 3 inches, or rather more, in diameter, arranged as in the engraving, that 
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