The Democratic advocate., 1878-12-28
Testing a Lens.

One of the most interesting and deli
cate operations incident to the making o
a telescope, especially very large one, i
that of testing the glasses, after they hav'
been polished or adapted to the instrumen
in hand. Some facts descriptive of the
process are thus given in a recent lettel
from Cambridgeport, Massachusetts
For tie purpose of testing glass of
telescope it is necessary that every ray Of
light be excluded in the operation. Fol
this purpose Mr.. Clark has a tunnel under
the ground. The e lass is placed in por-
table receiver under ground Gin the base
ment) and the entrance to the same is Sc
curely closed against daylight, only small
hole being 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 230
feet in length and about foot in dame
ter, and as one peers through it, the rays
of light, twinkling and blinking at the op-
posite end, have the appearance of a star,
so complete is the optical delusion. The
necessity of testing tho glasses in this
manner is obvious, and many long and
anxious months are often employed in per-
fecting them before finally adjusting them
in their proper place. The grinding and
polishing process, which, in some respects,
involves the secret of superior telescope,
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 prob
ably would not remove the tenth part of
grain in weight. The glasses in tele-
scope, too, are very sensitive. The vital
Importance of securing equilibrium of all
parts of very large instrument is best
illustrated by the fact that pressure of
the hand on the back of the glass, no mat
ter if it be six inches in thickness, pro-
duces o 1exure sufficient to distort the
image of star
Another part of the process, equally in
teresting, is thus described by the Hart
ford Times:
When the glass has been reduced to
such a seemingly perfect outline as to
render it impossible to detect any imper-
fection by either fine mathematical appli-
ances, or eye or touch, it is tested not
only in the dark tunnel, but in actual trial
on the stars, and on a terrestrial target,
the face of which is lined off in mathema-
tical squares IF any defect is discovered,
its locality on the glass is ascertained by
sticking patches of wet paper over the sur
face inch by inch, till repeated trials show
at length that the defective spot has been
covered-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.
It is surprising that few slight, soft
touches with the ball of the finger should
remove the glass, but that is what is ac-
tually done. If the defect is found, on
trial, not to be entirely corrected, the pro-
cess is repeated until perfection is at
tanned.
Mr.. Clark, who is II 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 dis
tinguished him twenty years ago. The
26-inch object glass which he made for
the Washington observatory, and which
cost alone about S33s 900, has already dis
covered what no other glass, from Gallio's
down to Proctor's, had ever seen, the
moons of Mars.
A similar glass, which was ordered, at
similar price, some years ago by Chicago
Mccormick, of 'treaper" fame, has never
yet been called for; but it is not likely to
wait long for customer if Mr.. Nc, de-
clines to take it. We believe it was while
testing one of his unequalled glasses that
Mr.. Clark discovered the companion star
to Sirius.