1S29.] On the Manufacture of Optical Glass. 



259 



its tray was to be placed, whilst the other end of the tube 

 reached to and was flush with the outside of the furnace. A 

 loose piece of tube, similar in kind but smaller in diameter, 

 being laid upon the bottom of the chamber, and applied at its 

 end to the orifice of the larger one, served as a continuation 

 of it until the inner extremity reached to and was under the 

 cover of the glass experiment. When the furnace was hot, 

 there was always a draft inwards through this tube; but 

 the quantity of air admitted was regulated by a valve (70). 

 The air, by first passing through the hot sides of the 

 furnace, and then through the shorter ignited tube serving for 

 connexion, was transmitted in a thoroughly heated state to 

 the place where its presence was required, without producing 

 any serious cooling effect ; it there maintained a continually oxy- 

 genating atmosphere, and, judging from the effects, prevented 

 the draught inwards of any vapours from the fire to the space 

 beneath the glass covers. 



56. The next point of importance, in the preparation of the 

 glass, is the arrangement of the tray in the furnace just 

 described. To understand this, it will be necessary to say 

 that the glass-chamber is 25 inches long, 13 inches wide, and 

 8 inches deep, and that the fire being at one end, the flue is 

 at the other. Plates of glass 7 inches square have been made 

 in it ; but it would probably require a larger furnace to make 

 much larger pieces. 



57. The bottom of the chamber being perfectly level and 



clean, the gauge-board, on which the tray was formed (35), 



