Wood County reporter. [volume] (Grand Rapids [i.e. Wisconsin Rapids], Wis.) 1857-1923, 1873-11-27
How Telescope Glasses are Mado

The tools are very simpIe-round nIo4o.
OF cast iron, about three feet in iamoton
followed out to suit the curves If ~
lens. They have somewhat the annaan
ancc OF huge shallow saucers ar pno1
nearly still, of the shallow iron drin'-iInS
cups attached to pumps or hvdran4r !
some Of our cities. Three of fhesn tooI n
were necessary, one nearly fiat, for Tho; -
ner surface of the flint glass, one conva III
Tor It's outer surface, and one concavo e.,
the crown glass. The surface of the toni
IS covered with coarse emery and veto
the glass IS laid upon it, and the grindiog
IS carried on by sliding the glass back and
forth on the tool. While slidinc tho
glass is slowly turned round so that tho
grinding strokes occur successiveIv IN
every direction on the glass, while, At tLn
same time the operators continually NOV.
around IN the other direction, so thai the
strokes are made successfully in every ii,
rection on the tool. The result of these
combined motions is that every ineouaIitv
either on the glass or the tool, is gradual
~ worn away. and both are reduced 70
portions of nearly perfect spheres. When
this result is attained, the grinding is con.
tlnued with emery OF continually greater
degrees OF f~ineness, until the surface be.
comes quite smooth.
The next operation is that of polishing
The whole tool is covered with a thin
coating Of pitch, which is pressed while
still warm into the proper shape. II is
then covered with a layer of water and the
polishing rouge, and the glass is again
laid upon it, and kept in motion in the
same way as in the fine grinding. Thus
each surface Of the two glasses is speedily
brought to a high polish.
The operations we have thus far de.
scribed require no extraordinary skill on
the part of the workman. With a little
patience and practice almost any one can
make himself a better glass than any
known before the invention of the achr0-
matte telescope. But, such a glass would
by no means serve the purposes Of astron-
omy now, and the chances of a glass of
any size turning out exactly right on the
first trial are very slight. The skill 0fthe
optician is now called into play to rectify
the figure. Two formidable difficulties
have to be overcome, to find what the de
fects 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 a luminous point at a dis
lance 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 it.
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 a 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.-Scrlbner t0r Norem
hon