The Scranton tribune. [volume] (Scranton, Pa.) 1891-1910, 1898-02-05
MOON WITHIN A MILE or EARTH
Power Of the Great Paris Exposition
Telescope.


From the New York Commercial Adver-
tiser.
The telescope of the great Paris eX-
position Of 1900 will bring the image Of
the moon within a mile of the earth,
The tube of this telescope will be $50
feet long. The image is to be received
upon a level mirror seventy-five inches
in diameter, and from that reflected
onto a screen so rigged up that 600 peo-
Pile can view it's startling revelations
at one time. This Paris telescope, with
its huge objective, will be peculiarly
fitted to vivify and enlarge indistinct
parts of large stars or planets or suns
The objective OF the Flower observa-
tory of the University of Pennsylvania
is only eighteen inches, that at Prince
ton twenty inches, that at Harvard,
twenty-four inches. that at the Uni
vcrsity of Virginia twenty-six inches,
that at the Lick observatory thirty-six
inches, that at Pulkowa, Russia, thir-
ty-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 1900 is fifty-one
inches in diameter. 1t Is a thin con-
1t was formerly thought that the
diameter of the object glass of tele-
scope could not be increased beyond a
| certain number of inches without fa-
tally diminishing it's effectiveness. OBJ
| iectives rest on their edge, of course,
| and the weight of very wide glass
| was so great that it increased the
| density of the lower part of the rim, on
which it's weight rested, and 50 modified
with the refraction of that part of the
glass. Refraction is the bend or change
in it's direction, which a ray of light
| undergoes upon passing through any
| denser BODY than the air.
| Formerly detectives were made very
thick in the centre OF their convexity,
so their focal distance, or the point at
which all the rays passing through
them crossed, was close to them. The
thicker an objective the shorter the
distance to where the focal point lies
back of it in the tube. With these
thick detectives and their short focus,
low power magnifyng eyepieces to
telescopes We're used-not over a 1000
diameter eyepiece in the largest instru-
| ment.
But lately Alvin Clark and the Jena
manufacturers have turned out very
much thinner objectives. Such object
ives produce focal point much further
back from them in the tube of the
telescope. And this change of focal
points has given rise to the employment
OF eyepeces Of 1000 or 5000 magnify
ing power. The nearer the image OF
the object to be magnified is brought
to an eyepiece, the higher must he its
magnifying power, and so the old limi-
i've has been overcome.
detectives of telescopes are duplex
consisting of a convexity of crown
glass fitted tightly into a one sided
concavity Of flint glass. This arrange.
ment is necessitated by the fact thai
rays of different colors have a different
angle of refraction in passing through
condex crown glass. But these colol
rays are intercepted by the flint glass
backing, and so the focal point is pre-.
served.
The glass of these detectives must bt
of equal thickness or opacity through.
out. Alvin Clark told DR.. George +
Barker, of the chair of physics of the
University of Pennsylvania, that tht
delicacy of his objectives was alto
gether owing to his care in going ovel
them personally, and wherever he dis
covered a comparatively opaque spo
he would dip his finger tip in roug,
powder and gently rub the spot bacl
and forth for a minute, so exceedingl!
susceptible TO this very light friction
are such highly finished and polished
pieces of manufactured glass. The cos
of great detectives is enormous. Th
Lick objective, which stands fourth II
the list in size, cost $50,0O0 Alvil
Clarks reputation as a maker of OBJ
; jectives stands first in the world, s.