The Helena independent. [volume] (Helena, Mont.) 1875-1943, 1892-06-05

brings the glass roughly into the shape of
the lens. Then, with eight courses of
emery, each course finer than the proceed
ing one, the disk is ground with an adjust
pressure may be increased or diminished
at any point. Thus the disk is brought
into approximately its final form.
This is oil comparatively coarse work.
It is really very fine work. In
making the measurements at this stage Mr..
Clark employs an instrument Ga homemade
affair, which looks as if it had been whit
tied out with a j~ackknife) that registers
one thirty-thousandth of an inch. But"
says he, this is used only for coarse meas
urements," No instrument can be made
by human hands for measuring the inini-
the final shaping Of one Of these great
lenses. The human eye, and q rarely keen
and long practissd one at that, il the only
instrument fit to make these measurements.
The final shaping and polishing are done
with beeswax and rouge. Think OF grind
from eighteen months to two years to do it.
Oh, the patience of it! And then the final
testing:-The lens is taken back into the
long. dark room. whence it was brought
two years before. Again it is set on edge
fetal star-a point of lights placed at the
lower end of the room and at the upper end,
precisely fifty feet from the lens, exactly
where the focus must be, an eyepiece is
fixed. Now the myriad rays of light from
the star must fall upon every point of sur
face of the forty-inch lens, and each must
fall at such an angle that they all will be
refracted TO \ common point just exactly
fifty feet away. You cannot oven imagine
a mathematical point-a point SO small
that it GOULD not possibly be smaller. Yet
Mr.. Clark's business is to make hat great
forty-inch lens SO perfect liz its curve
that every one of those countless rava
shall come to a mathematical point at
precisely fifty feet. If one single ray
falls the breadth of n spider strand
away from that point. the lens is defec-
tive and the lensmuker, with keen. blue eye
and his lifelong experience and his ~tireless
patience, must find where that particular
ray strikes the surface of the lens. and then,
with his lump of soft beeswax, or perhaps
with his thumb, he must lightly rub that
spot until this ray is turned into its proper
course; and this must be done SO deity, 30
sxquisitely, that meantime no other ray ia
disturbed. 1t makes ones head swim to
think of the tineness Of this work.
And with your bare thumb you can wear
down such glass as this?" | asked.
For answer Mr.. Clark took up an old
castaway disk and gave it less than n dozen
sharp rubs with the smooth. soft thick of
his thumb. "There," said he. "if this Ind
been a perfect lens that would have
chsngsd its shape enough to ruin it,"
wanted to accuse the man of playing upon
me. but his earnestness forbade. Anu then
there was that mathematical point staring !
me in the imagination. And rs | stood
wondering whether ought to be amused or
MIR, Clark, pointing at the twenty |
four inch compound lens he is making on
the order of Miss G.. W. Bruce, of New |
York, for the Photographic telescope to pe
given by her to Harvard univel sity, said:
when that lens is finished We can hide