The weekly herald. [volume] (Cleveland, Tenn.) 1876-1888, 1877-12-06
Testing a Lens-The Most Delicate Work I
the Making of a Telescope.

Is spoken from gravel
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Testing a Lens-The Most Delicate Work In
the Making of a Telescope.
One of the most interesting and deli
cate operations incident to the making
of telescope, especially very large
one, is that of testing the glasses, after
they have been polished or adapted to
the instrument in hand. Some facts
descriptive of the process are thus given
in a recent letter from CambridgepOrt,
Mass:
For the purpose of testing a glass of
telescope it is necessary that every
ray of light be excluded in the opera
ton. For this purpose Mr.. Clark has 4
tunnel under the ground. The glass is
placed in portable receiver under
ground lin the basement) and the en-
trance to the same is securely closed
against daylight, only small hole be-
ing allowed through which the eye of
the operator can see the image of the
object which is formed at the focus.
The tunnel used by Mr.. Clark is 23O
feet in length and about a foot in diam
eter, and as one peers through it, the
raves of light, twinkling and blinking
at the opposite end, have the appear
ance of a star, so complete is the optical
delusiOn. The necessity of testing the
glasses in this manner is obvious, and
many long and anxious months are
often employed in perfecting them be-
fore finally adjusting them in their
proper place. The grinding and pol
ishing process, which in some respects
involves the secret of superior tele-
scope, is very slow and very intricate
process.
Mr.. Clark, Sr., once told me that he
could grind month on glass and
change its power perceptibly every day,
and probably would not remove tenth
part of grain in weight. The glasses
in a telescope, too, are very sensitive.
The vital Importance of securing equi-
librium of all parts of very large in
strument is best illustrated by the fact
that a pressure of the hand on the back
of a glass, no matter if it be six inches
in thickness, produces flexure suffi-
cient to distort the image of star.
Another part of the process, equally
interesting, is thus described by the
Hartford Times:
When the glass has been red2ced to
such a seemingly perfect outline as to
render it impossible to detect any i'm
perfection by either fine mathematical
appliances, or eye or touch, it is tested
not only in the dark tunnel, but in ac-
tual trial on the stars, and on a terres-
trial target, the face of which is lined
off in mathematical squares. If any
defect is discovered, its locality on the
glass is ascertained by sticking patches
of wet paper over the surface inch by
inch, till repeated trials show at length
that the defective spot has been cOv-
ered--and then the paper is removed,
inch by inch, till the decision of the
last piece is confirmed by that method.
Then, Mr.. Clark himself takes the
glass, and gently and slightly rubs the
offending spot with the naked ball of
his finger-using no power or anything
but the bare skin, which he keeps soft
and susceptible. 1t is surprising that
a few slight, soft touches with the ball
of the finger should remove defect in
the glass, but that is what is actually
done. If the defect is found, on trial,
not to be entirely corrected, the process
is repeated until perfection is attained.
MIR. Clark, who is p years old, and is
assisted by his two sons, is still blessed
with the keen eyesight and delicate
touch for manipulating an object glass
which distinguished him twenty years
ago. The 26-inch object glass which
he made for the Washington oLserva-
tory, and which cost alone about $685.
600, has already discovered what no
other glass, from Gallileo's down to
Proctor's, had ever seen, viz, the moons
of Mars.
A similar glass, which was ordered,
at similar price, some years ago by
Chicago Mccormick, of "reaper" fame,
has never yet been called for; but it is
not likely to wait long fora customer if
Mr.. McC. declines to take it. We be-
lieve 1t was while testing one of his
unequaled glasses that Mr.. Clark dis
covered the companion star to Sirius.