MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
55 
by leaving* out a brick. They are made to occur nearly opposite the spaces be- 
tween the supports seen when looking across the course of the flame, and are 
stopped by the insertion of loose bricks, and a piece of paper put before the 
place, which adheres from the pressure inwards of the atmosphere. These 
holes, being in the thickness of the walls of the furnace, are 17 inches long. 
The tiles which form the bottom of the chamber and top of the flue, are of 
Cornish ware (52. 53), or at least the one which constitutes the half nearest 
the fire is of that material ; but the other, which is not so highly heated, and 
never has to be moved, may be some other ware, and 2| inches in thickness. 
The tile nearest the fire has to transmit heat to the glass, and if of Cornish 
ware, and supported as described, is abundantly strong when f ths of an inch 
in thickness. It should be nicely adjusted by grinding (53), and when fitted 
in, the edges should be made close by a little fire lute. 
There is a part of the furnace not yet mentioned, which must be arranged as 
the structure is raised. This is the air tube (55). It is of glazed porcelain, and 
passes horizontally through the side of the furnace, so that its inner aperture is 
2 inches from the end of the glass chamber, and its lower edge level with the 
upper surface of the Cornish tile constituting the bottom, whilst the outer end 
is flush with the outside surface of the brickwork. Its length is 17 inches, its 
internal diameter fths of an inch. The short pieces of adjusting tube (55) are 
6, 7, and 8 inches in length, and iVths of an inch internal diameter : their ends 
are usually finished obliquely. 
All those parts of the furnace which are in contact with or near the fire, are 
of the best fire-bricks laid in loam ; but the sides of that part of the cavity 
already described, which form the glass chamber, are fire-tiles, and they rise 
about an inch above the neighbouring brickwork, forming a raised edge all 
round, which, at the same time that it better excludes dirt than if level with 
the rest of the work, also allows the covers of the glass chamber to apply more 
closely. These covers are three wrought-iron plates, each |th of an inch in 
thickness and 16 inches long; but their widths vary, and are 7? 10, and 12 
inches. These put side by side cover the mouth of the chamber, but varied in 
juxta-position, allow of more or less of the chamber being opened at once, 
according to whatever the experiment may require ; each has a short solid 
handle fixed to the middle of the upper surface. 
