Public ledger. [volume] (Memphis, Tenn.) 1865-1893, 1873-10-25

In Professor NeWCOmD AIIClC OH tnc
new telescope at Washington in Scrib-
ners for November occurs this account
of the process employed by the manu-
facturers, Messrs. Alvan Clark & Sons,
in grinding, polishing and perfecting the
immense object glasses: The tools arc
very simple round plates Of cast iron,
about three feet in diameter, followed
out 10 suit the curves of the lens. They
have somewhat the appearance of huge
shallow saucers, or more nearly still, OF
the shallow iron drinking cups attached
to pumps or hydrants in some of our
cities. Three of these tools were neces-
sary, one nearly fiat, for the inner sur
face of the flint glass, one convex, for its
outer surface, and one concaye for the
crown glass. The surface of the tool is
covered with coarse emery and water, the
glass is laid upon it, aud the grinding is
carried on by sliding the glass back
and forth on the tool. While sliding,
the glass is slowly turned round
so that the grinding strokes occur
successfully in every direction on the
glass, while at the same time the oper-
ators continually move around in the
other direction, so that the strokes are
made successfully in every direction on
the tool The result of these combined
motions is that every inequality, either
on the glass or the tool, is gradually
worn away, and both are reduced to por-
tons of nearly perfect spheres. When
this result is attained, the grinding is
continued with emery of continually
greater degrees OF fneness, until the
surface becomes quite smooth.
The next operation is that Of polish
lag. The whole tool is covered with a
thin coating of pitch, which is pressed
while still warm into the proper shape.
1t is then covered with 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 fRe grinding
Thus each surface Of the two glasses is
speedily brought to 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 better glass
than any known before the invention of
the achromatic telescope. But, such a
a glass would by no means serve the pur-
poses of astronomy now, and the chances
of a glass of any size turning out exact
ly right on the first trial are very slight.
The skill of the optician is now called
into play to rectify the figure. Two for
midable difficulties have to be overcome,
to find what the defects of the glass are,
aud where they are situated, and then
remove them without introducing
others. To find the defects, the glasses
are put together, set up on edge, facing
a luminous point at n distance 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
placed exactly in the focus, and the as
pect sc 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 arc too
rreat. and where too small.
The glasses arc now taken back to the
tool aud the polishing process is recon
menced, only pressing upon those parts
of the glass where il has TO be ground
away. The glass is then tried again, and
araiB returned to the polishcr. The de-
fects in a small glass can be thus pol
ished away in few days, but as the size
;s Increased the process becomes more
and more tedious, aud the difficulties Of
iudeinc what the defects are, and OF
handling the glass on the polisher so as
40 diminish them, increase enormously.
n IL in this tentative process Of gradu-
allv working out every defect Of figure,
and even in compensatiug defects in the
uniformity of the glass itself by suita'
HE change of figure, that the Clsrks
-. .,b,h;ted their unriy1lled skill