Evening star., 1899-03-25

would always be in danger of bending its
tubes and deforming its glasses. Its foun-
dationS would have to be something extra
ordinary. 1t was not to be thought of. The
Grande Lunette could not be pointed at the
sky.
SO this new telescope will lie fiat on the
earth. 1t will never need to be moved
about to draw a bead on any star. Instead
the image of the star or planet will be
caught by the seven-foot mirror that is now
the triumph of the Jeumont glass works
and reflected to the object glass in front of
it.
How 1t viii Work.
The great objection to these stationary
telescopes has heretofore been the apparent
movement of the stars, which made impos-
sible many precise calculations. In the
present construction the micrometer with
the eyepiece is furnished a rotary move
ment which preserves the images in their
\ apparent places. For the rest it is the great
seven-foot mirror that keeps up with the
rotation of the earth by means of clock
work. According TO the angle given to it
1T the beginning of the observation, it can
catch any part of the sky and hold it, The
mass of glass alone weighs 1000 pounds.
Its holder and neutrality-suspicious. the sta
tionary frame, which is, of course, much
heavier-weighs 10000 pounds. This weight,
in order to be absolutely stable floats in a
bath of mercury.
Every one mas ntdTQ OF lne UlInculty met
with in the casting of the great lenses. The
casting and polishing of this seven-foot
mirror was something absolutely novel.
Now they have succeeded they will try
to make a ten-foOt mirror The mere words
ten feet and seven feet do not speak
to the imagination as they should. Should
Il circular mirror seven feet in diameter be
placed against a wall 1t would Of course.
be higher than any man who might stand
| beside it. But stand a ten-foot mirror up
behind it. and-such is the relation Of the
circumference to the diameter of a circle
IL will appear to be fully twice as large.
The same is true of lenses The object
glass of the Yerkes telescope is three feet
five inches in diameter. The obje3t glasses
of the Grande Lunette are each four feet in
diameter. Yet these extra seven inches
make immense difference in their size
and weight and in the difficulty of casting
and polishing them. lense of four inches
in diameter costs So. a lense If twenty
inches in diameter does not cost five times
as much, but one hundred times as much
$50.
Making the Glass.
On account Of the high temperatures nee
essary to bring about a complete purifica-
tion of such masses Of glass as compose the
modern great lenses. the matter can only
! be treated in extra-refractory crucibles.
| They are placed in the furnaces and heated
gradually until they are white-red. The
Yitreous matter Is then put into them
- AitueLHyr III SHldll linen the
| hning must commence The glass is not
fined until the least tiny particles of air
! have been cooked from the mass For
twenty hours the glass boils In a tempera-
tIre of 500 degrees eenugrade. The cru
cibles themselves run risk of melting a
sample quantity of glass is taken out
cooled and minutely examined with a mag
| nifying glass. If 1t shows the slightest
speck a new heating is necessitated. If it
is perfect the mass is allowed to gradually
cool, process that takes several weeks
The block taken from the crucible s sawed
In half. to form two disks. which may be
lenses some day Again they are examined
most minutely with a microscope each
hundredth of an inch being gone over
nothing being overlooked. Each fault dis
| covered leads to other heatings which
| bring II closer and closer to the surface So
| the Terkes object glass, weighing 30
pounds, was commenced by NJ Mantois in
1877, and was only completed In 1889, in
| time to be shown at the Paris exposition
| of that year.
One of the two fiints of the Grande Lu-
nette is already quite finished. 1t weighs
| 800 pounds and cost 200. Each of the
object glass crowns will weigh 800 pounds.
These disks together have $4,000 worth Of
| glass in them. Polished, they will be worth
$4,000.
| Polishing the Mirror
The polishing of the great mirror could
| not be accomplished by hand in the old
way. Without entering into the details of
| the newly contrived polishing machinery, it
| may be said that the disk is supported by g.
! moving steel table. Above it hangs a NOV
\ ing polisher. The table revolves SLOWLY
| The polIsher moves backward and forward
| slowly But the polisher itself must never
come in contact with the glass A mixture
| of water and emery powder Is always be-
| tween them. AS the mirror flattens the
! more perfectly, the finer is the emery DOW
\ der and the less OF II mixed with the water,
until the polishing is done alone with dis
| titled water. At the last account after
| seven months of incessant movement, the
! polisher was within one-dftieth Of miyli-