284 On the Manufacture of Optical Glass. [1829. 



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 exceed- 

 ingly 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 

 8J 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 bottom as is not appro- 

 priated 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 covering tile to 

 the grate. The other part is covered by an iron plate, 17- 

 inches long, 13 inches wide, and |ths 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 



