MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
19 
i. e. such as a pin might pass through, and smaller. The patches are always to 
be applied on that surface of the plate which is to constitute the outside of the 
tray ; and therefore, before the soldering begins, the two surfaces should be ex- 
amined, and the most polished and perfect selected as that intended for the 
inside. The patches are valuable in their use far beyond what the mere appli- 
cation of gold to the hole would be ; for the heat afterwards applied to the tray 
when charged with glass, is abundantly sufficient to melt gold ; in which case, if 
unsupported by the platina patch, the weight of glass and the action of stirring 
would probably force the gold out of the hole and cause the tray to run ; 
whereas the patch of platina, although the gold holding it to the plate is liquid, 
still adheres by so strong a capillary action as to be sufficient to retain its 
place, and being outside is not disturbed by the motion of the stirrer. Besides, 
after a long application of heat, the gold and platina combine so perfectly as 
to become one piece of white alloy, infusible at the heat applied. 
35 . The plate is now to be folded into a tray, preparatory to which, a piece 
of thin board is to be provided as a guage, which in the present instance must 
be 7 inches square. This laid upon the plate and held tightly down, directs 
the foldings of the sides, and would, if placed in the middle, leave sufficient 
for edges one inch and a half high all round ; but as the plate should serve for 
use several times, it is advantageous to apply the guage a little eccentric ; for 
then, when used for a second and third operation, its place may be shifted, 
and the folds not occurring where they did before, there is less chance of holes 
being broken through the platina. The folds necessary at the corners of the 
tray are especially likely to render the same parts unable to bear a second 
and third bending ; but the necessity of having them in the same place may be 
usefully obviated by placing the guage oblique to the sides in one direction 
and in another, on different occasions, and moreover gives other advantages in 
finishing the folding of the corners ( 36 ). These attentions, tending to the 
preservation of the platina for repeated service, are very needful, in conse- 
quence of the great expense of the material : the value of the plate in question 
is about 6/. 10s., and when worn out, it may be sold for about half that sum. 
Whether it be used therefore once, twice, thrice, or four times, makes con- 
siderable difference in the expense of the resulting plates of glass. 
36 . When the guage is properly placed on the platina, the sides are raised 
d 2 
