has hardened for several days it is ready for use. Most of the mirrors
are now made by the quicker and cheaper process of painting the plate of
polished glass with a preparation of silver. They are known as "red backs."
The common looking-glasses for bureaus, etc., millions of feet of which are
imported yearly, are known in commerce as German mirror plates. A German
family will take home a box of ordinary window-glass, the mother and
children will polish the surface of each light with rouge, and when it is
done, take the glass back to the maker of the looking-glasses, and get
another box.
For optical instruments the glass must be as dense
as possible, as the refractive power increases with its weight. The sand
is therefore mixed with large quantities of lead and potash. The
melting-pot is covered with a dome roof to exclude smoke and gases. The
fusing material is stirred with a fire-clay cylinder until the melting is
complete, then the furnace heat is lowered, and the pots rest for a couple
of hours to permit all the bubbles to rise. The gummy mass is then constantly
stirred, while the temperature declines so low that at last stirring becomes
very difficult. Then the cylinder is withdrawn, all the openings of the
furnace are stopped, and the crucible and glass gradually cool. This
requires a week. The pot is taken out and carefully broken away from the
great lump of flint-glass. Parallel faces on its sides are ground and
polished to locate the interior blemishes, which determine how the glass
shall be cut to the best advantage. It is then tediously cut, ground, and
polished. For large lenses the glass is cast into a round flat plate.
Repeated trials are necessary before a piece perfectly clear can be
obtained for telescope lenses. These are made almost entirely in France.
The typical method of preparation is to carefully select a lump of high
specific gravity, and placing it in a clay disk mould, slowly flatten it
down by heat into the desired shape. Sometimes the glass is delivered
to the lens-maker in rectangles, which are cut into disks by an annular
saw.
The famous Alvan Clark establishment in Cambridge,
which has furnished the Pulkowa, Washington, Lick, and other great
telescopes with objectives, polishes with infinite pain the slabs
received from France. In this modest workshop the most efficient
instruments of astronomy have been equipped.
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How delicate
its results are may be judged from the fact that a finger touch upon a
lens swells it sufficiently to create a prominent spot in the tests
applied. The 36-inch objective of the Lick telescope, the largest yet made,
would seem to be a sufficient triumph, but the Clark brothers are confident
of their ability to make one 40 inches in diameter. The cutting is done by
cast-iron sand, which, by a rapidly rotating machine, gives the general
curvature. Then the patient polishing is done on an iron lap coated with
pitch and fed by water and rouge. There are eight manufactories of fine
lenses in this country, but none west of Rochester, which is the main
centre for microscope, camera, and eye-glass lenses. The glass is now
furnished to our manufactories in plates six to nine inches square and an
inch thick. Being made only abroad, it enters without duty, but is worth $10
a pound in the rough. An annular saw cuts it into disks. These are sawn by
the help of emery into thin pieces, which, cemented to sticks, are ground
into concave or convex circles, and then ground oval for their frames.
Besides the enormous range of uses in which glass
familiarly achieves a unique purpose, it does many strange services, and
every year adds to the catalogue of its unsuspected virtues. From the
material that produces Prince Rupert's drops, combining in one bead the
toughness of iron with the explosiveness of powder, we may expect
anything. A favorite amusement of glass-workers is to reel out fine
threads quickly drawn from a molten batch, making a mineral silk to spin
into incombustible cloth or to fashion into the plumage or hair of animals.
Especially in Austrian factories the glass is woven into fabrics, sometimes
with a warp of silk, to made into collars, neckties, chains, brushes,
lamp-wicks, etc.* Recently a mineral cotton has been made from the slag
refuse of iron smelting. The crude
* One of the most wonderful specimens of
glass in the world is to be seen in the Conservatoire of the Arts and Trades
in Paris. It is the life-size figure of a lion in the act of stifling a
serpent. Every part is marvellously natural, and it is made entirely of
glass. It cost the artist, M. Lambourg, thirty years of work, and was
conspicuous in the Universal Exposition of 1855. At the Paris Exposition
in 1878 there was exhibited a bonnet with feather, ribbons, and lining
made entirely of spun glass, as well as cloaks and other articles of wear.
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