Lamoille newsdealer. [volume] (Hyde Park, Vt.) 1860-1877, 1873-11-05

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Making Telescope GLASSES.
In Professor NewcOmb's article on the
new telescope. al Washington, "a SerIbner's
for November. occurs this account Of tbs
process employed by tho manufacturers,
Messrs. Nlvan Clark & Sons. in grind
lug, polishing aud Perfecting the Immense
object glasses. The tools are very simple
round plates Of cast iron, about three
feet in diameter, Followed out to sait the
curves of the lens. They have somewhat
the appearance of huge shallow saucorsy
or more nearly still, Of the shallow iron
drinking cups attached to pumps or by
drants in some of our cities. Three OF
these tools we're necessary, one Lcarly
fat, for the inner surface ef the flint
glass, one convex, for its outer surface,
aud one coucave, for the crown glass. The
surface of the tool is covered with coarse
emery aud water, the glass is laid upon
it, and the grinding is carried Onby slid
ing the glass back aud 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 operators
continually move around in the other d.
rection, so that the strokes are made sue
cessiyey in every direction on the tool.
The result of these combined motions is
that every inquality, either on the glass
OF the tool, is gradually worn away and
both are reduced to portions of nearly
perfect spheres. When this result Is at
tanned, the grinding is continued with en-
cry Of continually greater degree Of fine
ness, until the surface becomes quite
smooth. ...... ...H.#-H
The next operation is that Of polishing,
The whole tool is covered with thin
coating Of pitch, which is pressed while
sill warm into the proper shape. 1t 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 tho fac grinding. Thus
each surface Of the two glasses is speedily
brought to a high polish.
The operations we have thus far de-
scribed required no extraordinary skill Oh
the part Of the workmeu. With little
patience almost any one can make him
better glass than any known before the
Invention of the achromatic telescope.
But, such & lass would BY no means
serve the purposes Of astronomy now, and
the chances of a glass Of any size turning
out exactly right on the first trial are
very Slight. The skill Of the optietisn is
now called into play to rectify the figure,
Two formidable defcultties have to be
overcome, to find what defects of tho
glass are, aml where they are situated.
and then to remove them without intro.
duciDg others. To find the defects, thc
glasses are put togethersr set upon edge,
facinga lumneusipointat a distance equal
to ten or fifteen times the local lergthg The
image Of the point formed in the foeus'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
optction can judge where the curves Of
the glasses are too great, aud who're too
small.
The glasses are new taken back to the
tool and tho polishing process is lscom-
meuced, only pressing upon those parts Of
the glass where it has to be ground away.
The glass is tken tried again, and again
returned to the polisher The defects in
asmall glass can thus bepolished 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 arc, and Of hapdling the glass
on the polisher 50 as to diminish them,
increase enormously. Ii is in this extern
sive process Of gradual working out every
defect of figure, and even in compcnsa-
ting defects in the uniformity of the glass
itself by suitable change of figure, that
the Clarks have showed their unrivsllcd
skill.
Joseph A. Cooke, of Newport, went
with his wife and family few weeks ago
to Providence. R. M n visit n b;c tfa