As optical glass is now made, the material is constantly stirred
with an iron rod during all the time it is melting in the furnace,
and after it has begun to cool, until it becomes so stiff that the
stirring has to cease. It is then placed, pot and all, in the
annealing furnace, where it is kept nearly at a melting heat for
three weeks or more, according to the size of the pot. When the
furnace has cooled off, the glass is taken out, and the pot is
broken from around it, leaving only the central mass of glass.
Having such a mass, there is no trouble in breaking it up into
pieces of all desirable purity, and sufficiently large for
moderate-sized telescopes. But when a great telescope of two feet
aperture or upward is to be constructed, very delicate and
laborious operations have to be undertaken. The outside of the
glass has first to be chipped off, because it is filled with
impurities from the material of the pot itself. But this is not
all. Veins of unequal density are always found extending through
the interior of the mass, no way of avoiding them having yet been
discovered. They are supposed to arise from the materials of the
pot and stirring rod, which become mixed in with the glass in
consequence of the intense heat to which all are subjected.
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