MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
29 
59. When the first set of packing pieces is properly adjusted, 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 platina tray, but 
also to sustain the glass covers, and likewise, by retaining the heat upon the 
bottom of the chamber, prevent much of the inconvenience that would other- 
wise occur at the times of stirring the glass. 
60. The glass covers have, up to this period, consisted of inverted evapora- 
ting basins, suspended at pleasure, in the manner before described, by platina 
wires (26). When the platina trays used have been sufficiently small to ad- 
mit of the arrangement in our present furnace, two, and even three covers have 
been used simultaneously, each prepared with its own platina suspension ; but 
of such size, that the larger could be placed over and inclose the smaller, with- 
out 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 inclose the platina tray and its charge, not only for the purpose of accu- 
mulating heat and confining an oxygenating atmosphere within, but also shel- 
tering 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 strength, and no injurious degree of elasticity, are made taper at 
one extremity, and slightly curved there. This end is easily introduced be- 
neath the platina suspension wire, and as easily withdrawn when the cover is 
removed. 
62. All these matters being preliminarily arranged, the final disposition of 
