Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1892-11-20

telescope, through which visitors to the Ex-
position will be allowed to gaze at what
ever is most interesting in the heavens both
by night and by day. 1t is also intended
to show picture of the sun on q large
scale, a pencil of rays being thrown through
a lens by a mirror 40 feet into dark room.
In this camera obscura huge image of the
orb of day will appear on a screen, showing
the tremendous flames which leap 1000
miles above its surface and also the So
called ''spots," which are fiery chasms cap
able of swallowing up hundreds of such
planets as the earth at q gulp. At noon each
day the astronomers in Washington will
drop time-ball five feet in diameter on top
of the main building at the Fair.
1t has been suggested that the newly dis
covered fifth satellite of Jupiter ought to
be secured for exhibition at Chicago. 1t is
! such very little one that the star-pazers
| suspect it to be merely captured asteroid.
They entertain like surmise respecting
,, the moons of Mars, which were originally v
found by the big glass of the equatorial
telescope at tine Naval Observatory here.
Possibly the earth may capture an asteroid
or two some day. There doesn't seem to be
any reason why such a thing might not
happen.
The great telescope of the Lick Observa-
tory, by which Jupiter's new satellite
was discovered, has an object glass Sc inches
In diameter 1t is expected that the Chi
cago University will buy the 4O-inch lens
which now lies in the rough at the Clark
factory in Cambridgeport, Mass
The Limit of Size.
This mighty glass was ordered for the
University of Southern California, but that
institution has iailed to pay for it. Clark
thinks that he can make telescope lens As
inches in diameter. A limit OF size exists
by reason of the fact that lenses bend of
their own weight, and thus their very big
ness may cause them to distort the images
of objects seen through them Obviously it
is not possible to put anything beneath to
hold them up. All of these telescopic
glasses are cast in Paris bv a concern which
keeps the details of Its processes secret.
Each one actually consists of two lenses.
one of flint glass and the other of crown
glass. The two together so neutrallze all
colored rays that the light which comes to
the eye of the observer is perfectly white,
The big glass disks are shipped in the
rough to the factory at Cambridgeport
Alvan G. Clark is the only man living who
understands to perfection the art Of pre-
pairing and finishing them. First they are
sfouuu uonu W Tue proper snape with re-
volving concave tools, after which the sur
faces are carefully smoothed and polished
with fine rouge. Then the lenses are ad-
justed in big tube like that of a telescope
1or trial. The tube is pointed at star, the
image of which should appear perfectly
round when seen through the glasses
The Genius of Mr.. Clark.
But in reality the image is sure to be
more or less out Of shape, because of inev-
liable defects. 1t is in remedying these de-
fects that the genius of Mr. Clark is chiefly
exercised. He goes over the lenses with a
bit of rouge on his thumb, and rubs the sur
faces here and there, polishing away an in
hnitesimal thickness in this spot and that,
until at length all tie rays passed through
the obiect-glass are concentrated at q sin
gle point Now the image of the star is
perfectly round. and the lenses are fin.
ished. Even the finest of them contain
many little bubbles of air, bnt they do no
harm.
The astronomers of the Naval Observatory
have looked all over the world for spiders
webs Such gossamer flaments spun by
industrious arachnids are utilized in tele-
scopes tor cross-lines extended at right
angles with each other ACROSS the field OF