El Paso daily herald. (El Paso, Tex.) 1881-1901, 1897-10-06
POWER OF THE TELESCOPE.
L,mitati ton of the Lenses Which Bring the
Planets Close to Earth.

IL
Planets Close to Earth.
The telescope at the Paris exposi-
lion will bring the image OF the moon,
AS it were, within u mile OF the
earth.
Itwas formerly thought that the
diameter of tt OBJECT glass of a great
telescope could not be increased beyond
a certain number of inches without
fatally diminishing it's effectiveness
Object glasses rest on their edge, OF
course, and the weight of a very wide
ylass was so great that it increased
he density OF the lower part of its rim,
on which it's weight rested, and 30
modified the refraction of that part OF
the glass. RefractiOu is the bepd, or
change in its direction, which ray OF
light uudergoes upon passing through
,ny denser body than the air.
Formerly object glasses were made
very thick in the center OF their COG
veXity, and so their focal distance, or
The point at which all the rays pas ing
through them crossed, was very near
to the glass itself. The thicker an OBJ
iective, the shorter the distance to
where the focal Pc Mt lies b ack of It in
the tube. With these thicker object
YES and their short focus, very .ow
power magnifying eye-pieces to teles-
cope were used-no5 over a 1O0-diame-
ter eye-piece to the largest instru-
ments.
But lately Alvau Clark and the Jena
manufacturers have employed very
much thinner objectives. Such OBJ
jectives produce a focal point mrHh
'arsher back from them in the tube of
the telescope. And this chan e of
focal point has given rise to the use or
pieces of 1000 or 5000 magnifying
power. The nearer the object to be
magnified is brought to an eye-piece,
the higher must be its magnlty1ng
power.
And so the old limitations of size
diameter-in an Elective, have been
overcome. The objective of the Flow
sr Observatory in the university Of
Pennsylvania is Is inches in diameter:
that at Princeton 20 inches: that at Har-
ward "a inches: that at the University
If Virginia 81 inches: that at the Lick
Observatory SO inches: that at Pulko-
wa, Russia, 80 inches. while the
Yerkes objective is 40 inches in dia-
meter The glass which the Alvan
Clark company (Alvan and his son are
now both dead is now making at Cam
bridge for the Paris exposition OF 1900
is 81 inches IN diameter. L. Is thin
convex with a long focal distance.
Detectives of The telescope are
duplex, convexity of crown glass fitted
tightly into one-sided concavlty OF
flint glass. This arrangement is nee
essitated by the fact that rays of diff
ferent coiors have different angle of
refraction in passing through convex
crown glass. But these color rays are
intercepted by the flint glass backing
aud 90 the focal point is preserved.
The glass of these Objectives must be
of equal clearness or opacity through
out Alvau Clark told Prof. George
F. Barker, of the University of Penn
sylvania (physics), that the delicacy of
of his detectives was altogether owing
to his care in going over them person
ally; and whenever he discovered a
comparatively apaque spot, he would
dip his finer tip in rouge powder and
gently rub the spot back and forth for
a minute-so exceedingly susceptible
to this very slight friction are such
highly finished and polished pieces Of
nodded glass.
Tho COS: Of great obj ctives is enor-
nous. The Lick objective, which
stands fourth in the LIST, cost $50, 600.
In making these objectives molten
glass is poured into pot. When the
glass has cooled the pot is Clipped
away from its mas, and mi iway from
the bottom, the cry ,tal is sawed
thu ough twice-the section being
trihe thicker than the tn ckne-s of the
center Of the objective. Out of this
middle section the crown convex OBJ
jective is made. The middle of the
mold is most free from impurities and
most uniform in density.
The tube of the Paris tele cope will
be $50 feet long. The image of the
M HOn or p anet5 is to be received upon
a level mirror, IS inches in diameter,
AMI from that ref c,ed on to screen
so rigged up that 600 persons can view
the wonderful revelations at one time.
This Paris telescope, with its huge OBJ
jective, will be peculiarly fit ed to vivi-
fy and enlarge indistinct parts of large
stars or planes or suns.-Self Culture.