1829.] On the Manufacture of Optical Glass. 261 



it touches the tray, to prevent adhesion by the glaze when cold 

 The general disposition of the tray, the tube, and the packings, 

 may be seen in the sketch (57). 



59. When the first set of packing pieces is properly ad- 

 justed, a second series is to be arranged over them; but these 

 are to be removed backward from the tray about the third or 

 half of an inch all round, that accidental contact with its edges 

 may be avoided. Their thickness should be sufficient to raise 

 them level with, or rather above, the edges of the tray. All 

 these adjusting pieces are to be rendered perfectly clean and 

 free from dust before they are applied. Their use is not only 

 to afford support and assistance to the platinum tray, but also 

 to sustain the glass covers, and likewise, by retaining the Heat 

 upon the bottom of the chamber, prevent much of the incon- 

 venience that would otherwise occur at the times of stirring 

 the glass. 



60. The tray-covers have, up to this period, consisted of 

 inverted evaporating basins, suspended at pleasure, in the 

 manner before described, by platina wires (26). When the 

 platinum trays used have been sufficiently small to admit of the 

 arrangement in our present furnace, two, and even three covers 

 have been used simultaneously, each prepared with its own 

 platinum suspension ; but of such size, that the larger could be 

 placed over and enclose the smaller, without touching it. In 

 such cases the temperature of the glass, after being lowered 

 by stirring, or in any other way, rose very rapidly ; but with the 

 large plate of 7 inches square, the furnace would admit of but 

 one glass cover of sufficient size, and the only additional 

 assistance which could be obtained was that which was given 

 by putting a similar but smaller cover on the outside and 

 above the principal one. 



61. The first and important cover is to be selected of such 

 dimensions, that when in its place and resting by its edges 

 upon the packing pieces, it shall fully enclose the platinum tray 

 and its charge, not only for the purpose of accumulating heat 

 and confining an oxygenating atmosphere within, but also 

 sheltering the glass, and preventing any oxide of iron from the 

 chamber covers, or dirt from other sources, falling into it. 

 These covers, when hot, are raised and removed by means of 

 clean iron rods, which being sufficiently thick to have abundant 



