The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1897-12-04
MOON WITHIN MILE OF EARTH.
Power of the Great Paris Exposition
Telescope Now Being Constructed.

New York Commercial Advertiser.
The telescope at the great Paris exposi-
ton of 1900 will bring the image or the moon ta
telescope will be $50 feet long. The image
is to be received upon a level mirror seven
ty-five inches in diameter and from that
reflected on to a screen so rugged up tnat
six hundred people can view It's startlnS
revelations at one time This Paris tele-
scope with its huge objective will be pc
culiarly fitted to witty and enlarge indis-
tlnct parts OF large stars or plahets or
suns.
of the University of Pennsylvania is only
eighteen inches in diameter; that at Prince
ton twenty inches that at Harvard twenty
four inches: that at the University OF N1r-
ginia twenty-six inches; that at the Lick
Observatory thirty-six inches; that at Pull
kowa, Russia, thirty-eight inches; while the
Yerkes glass is forty inches in diameter
The glass which the Alvin Clark Company
is now making at the Cambridge works for
the. Paris exposition Of 1909 is hfty-one
inches in diameter. 1t is thin convex,
with a long focal distance.
1t was formerly thought that the diam.
eter of the object glass OF a telescope could
not be increased beyond certain number
of inches without fatally diminishing it's eff
fectiveness. Detectives rest on their edge,
of course. and the weight of very wide
glass was great that 1t increased the
density of the lower part of its rim, on
which it's weight rested, and so mod
flied the refraction of that part OF the
glass. RefractiOn is the bend or change
in it's direction which ray of light I'M
dergoes upon passing through any denser
body that the air.
in the center of their convexlty, so their
focal distance. or the point at which all
was close to them. The thicker an object
i've the shorter the distance to where the
focal point lies back of it in the tube. With
these thick detectives and their short TO
Gus. low power magnifying eyepieces TO
telescopes were used-not over a one-thou-
sand diameter eyepiece in the largest in
strument.
But lately Alvin Clark and the Jena man.
ufacturers have turned out very much thin
ner detectives. Such objectiyes pro-
duce a focal point much further back from
them in the tube of the telescope. And this
change of focal point has given rise to
the employment of eyepieces of four thou
sand or five thousand maGnifying power
The nearer the image of the object 10 be
magnified is brought to an eyeplece the
higher must be its magnifying power, and
so the old limitation of size-diameter-in
an objective has been overcome.
ObjectiveS OF telescopes are duplex, con-
sisting of a cohVeXity of crown glass fitted
tightly into a one-sided concavity of flint
glass This arrangement is necessitated by
the fact that rays of different colors have
a different angie of refraction in passing
through complex crown glass. But these
color rays are intercepted by the fnt-glass
backing, and so the focal point is pre-
served.
The glass of these detectives must be of
equal thickness or opacity throughout
Alvin Clark told DR.. George F. Barker, of
the chair of physics in the University of
Pennsylvania, that the delicacy of his OBJ
jecuves was altogether owing to his care
in going over them personally, and wher-
ever he discovered a comparatively Opaque
spot, he would dip his finger tip in rouge
powder and gently rub the spot back and
forth for u minute. So exceedingly sus-
ceptible to this very slight friction are
such highly finished and polished pieces of
manufactured glass. The cost of great OBJ
jectiveS is enormous. The Lick objective
which stands fourth in the list in size cost
$4,000 Alvin Clarks reputation as a
maker of objectives stands first in the
world so that even Europe patronizes him
in preference TO it's home industries
In making these detectives molten glass
is poured into pot. When the glass has
cooled the pot is Clipped away from its
mass and midway from the bottom the
crystal is sawed through twice Out of this
made. The middle of the mold is most rree
from impurities and most uniform in
density.