MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
17 
it should be watched for cracks and casualties, that the running of the glass 
into the furnace may be prevented, and, if necessary, another vessel taken. 
30. The rough glass thus prepared is in the next operation to be converted 
into an annealed and finished plate. The size must therefore be determined 
upon, and we will assume it as 7 inches square, and 8 tenths of an inch 
thick, that being the dimension of the largest plate as yet made. For the 
purpose of making a competent platina vessel, a plate of that metal will be 
required at least 10 inches square ; but if larger, it should not be cut, but 
either made into a tray with higher sides than is absolutely needful, or else 
used first in the manufacture of a larger plate of glass than the one to be de- 
scribed. It should be of such thickness as to weigh at least 17‘5 grains to the 
square inch ; and it is important that in its preparation a good ingot or the good 
part of an ingot of platina has been selected, and that it has been rolled very 
gradually and carefully without the formation of any holes by the adhesion of 
dirt or hard particles, or by the dragging of the metal in the mills. The de- 
sired perfection is, I understand, best obtained by rolling the platina between 
two clean plates of good copper. 
3 1 . The plate, being laid upon clean paper or a cloth on a smooth table, is 
to be cleansed with a cloth and a little water or alcohol, and then to be ignited 
at every part by a large spirit lamp. It must next be carefully examined as 
to its state and the occurrence of places upon its surface where holes are likely 
to exist. If the metal seems dragged in any place, an effect indicated by a 
roughness upon the surface, or by short lines parallel to each other but per- 
pendicular to the course of rolling, such place should be noted or marked, 
for which purpose a dot of ink will be convenient. If a scale appears, 
or a small portion is apparently folded over, it should also be marked ; and 
if a black spot is visible, (and they are sometimes formed by the adhesion of a 
particle of dirt or grit,) it should be examined, and removed by the point 
of a knife, if necessary, and its place also marked. All these places and the 
whole surface of the plate should then be examined for holes by a still stronger 
test, namely, by holding the sheet of metal before and close to a bright light, 
as a candle or lamp, in a dark room, and every hole observed marked. In 
making this examination, it must be done carefully and minutely, holding the 
plate in different directions to the light (for sometimes the holes are oblique), 
MDCCCXXX. 
D 
