Year: 1873

The Ools are very simple-round plates
of cast ron, about three feet in diameter,
hollowd out to suit the curves of the
lens. hey have somewhat the appear
ance { huge shallow saucers,. or more
nearly till, of the shallow iron drinking
cups attached to pumps or hydrants in
some o our cities. Three of these tools
were necessary, one nearly fiat, for tn in
ner Surace of the flint glass, one convex,
for its otter surface, and one concave, for
the cTotn glass. The surface of the tool
is coveed with coarse emery and water,
the glas is laid upon it, and the grinding
is calrid on by sliding the glass back and
forth a the tool. While sliding, the
glass iSlowly turned round so that the
grindin strokes occur successfully in
every d.eetion on the glass, while, at the
same the the operators continually move
aroundn the other direction, so that the
strokeslre made successfully in every di-
rectiomn the tool. The result of these
combird motions is that every inequality,
either the glass or the tool, is gradual
ly VOL away. and both are reduced to
portion of nearly perfect spheres. When
this reilt is attained, the grinding is con-
tinuedvith emery of continually greater
degreeof tineness, until the surface be-
comes ute smooth.


The ext operation is that of polishing.
The wole tool is covered with thin
coatiniof pitch, which is pressed while


still WIN into the proper shape 1t is
then cyered with layer of water and the
polismg rouge, and the glass is again


laid upn it, and kept in motion in the


same NY as in the fine grinding. Thus
each Stace of the two glasses is speedily


brougl to high polish.


ThepeIations we have thus far de-
scribe require no extraordinary skill on


the pa of the workman. With a little


patiem and practice almost any one can


make himself a better glass than any


knowibefore the invention of the achro-


Kaleidoscope But, such a glass would


by no leans serve the purposes of astron-
ony NV, and the chances of a glass of
any sir turning out exactly right on the
fist trial are very slight. The skill of the
Qticin is now called into play TO rectify
he figure. Two formidable difficulties
lave to be overcome, TO find what the de-
ects of the glass are, and where they are
situated, and then to remove them with
out introducing others. To find the de-
tects, the glasses are put together, set up
on edge, facing luminous point at dis
tance equal to ten or fifteen times the
focal length. The image Of the point
formed in the focus of the glass is then
examined with an eye piece of high
power, or the eye is placed exactly in the
focus, and the aspect of the glass noted as
the light from the point passes through 1t.
By these means the skilled optician can
judge where the curves of the glasses are
too great, and where too small.


The glasses are now taken back to the
tool and the polishing process is recon-.
menced, only pressing upon those parts OF
the glass where it has to be ground away.
The glass is then tried again, and again
returned TO the polisher. The defects in
a small glass can be thus polished away
in few days, but as the size is increased
the process becomes more and more tedi
Gus, and the difficulties of judging what
the defects are, and of handling the glass
on the polisher so as to diminish them, in
crease enormously.-Seribro7' for NoUena
ber .. - .H