The Mankato free press. [volume] (Mankato, Blue Earth County, Minn.) 1879-1924, 1885-02-20
TELESCOPE.
Dificuities Encountered In Casting the OBJ
tect-(4Iass for the Lick Observatory.

In 1880 contract was made with
Messrs. Alvan Clark and Sons to fur
nish an objective Of thirty-six inches
clear aperture. This was six inches
greater than the glass they had just
arranged to make Tor the Russian gov
eminent. and thus the telescope would
fulfill the condition Of being the largest
and most powerful ever made. The re-
suit has proved the old rule, that the
larger the glass the more difficult 1t Is
TO make it. In this connection there is
a curious contrast between our present
experience and that Of the opticians in
the early party OF the century. At that
time the making OF the crown-glass for
the double lens offered comparatively
little difficulty; it was the fiint-glass
with which the trouble was found. The
latter contained lead. substance OF
great specific gravity, which persisted
In settling toward the bottom OF the
pot In which the glass was melted, and
thus producing difference between the
two sides of the glass which was fatal
TO its performance. But this difficulty
has been so completely overcome that
all the trouble now arises with the
crown-glass. The method OF making
the best flint was long supposed to be q
secret in the hands Of Swiss named
Guinand and his family; but il is now
believed that the supposed secret In
volved nothing more than the very sim
pie device OF continuously and vigor-
ously stirring the molten glass until it
became too cool and stiff TO permit the
heavier material to settle. However
this might be, Fell, of Paris. who has
been most successful.in making large
disks, supplied satisfactory flint in a
reasonable time. But so much delay
was met with in casting the crown
glass that il has not yet reached the
hands OF the optician. The cause OF
his failure is one so simple that one
can not but wonder that ii should offer
any trouble after being once detected.
We call to mind that when the founder
has succeeded in casting his lump OF
glass, weighing several hundred
pounds, the clay pot in which il is con-
tanned is broken away The outside
portions OF the glass itself. being i'm
pregnated with the clay and other inn
purities, have to be cut away. This is
most tedious process. n any ordi-
nary cutting tool were used. tho glass
would be apt to fly to pieces. 1t has to
be sawed by wire working in sand
and water The process Of cutting
away the outside IL one. therefore, in
volving weeks. II not months. OF labor.
When it is done, the mass must be
pressed into the shape OF disk, like a
very thin grindstone, and in order to
do this the lump must first be heated
nearly 10 the melting-point, so as to be-
come plastic. But when Fell began to
heat bis large mass it flew TO pieces.
In successive attempts he took more
and more time for the heating, but
broke dozen or more pieces before he
at last succeeded. In February, 1884,
he reported that glass was actually
moulded without having been broken.
and would soon be ready for shipment.
All this refers only TO the great OBJ
iect-glass OF the telescope, which,
though the one vital organ of the in
strument. is really very small portion
of the whole. The construction of the
delicate yet powerful machinery by
which the tube sixty feet long is to be-
pointed toward any region II the heav-
ems, and kept in motion BY 'ock-work,
has not yet been commenced. In fact,
the question who shall construct this
mounting" rs it is called, is a diff
cult one 10 decide. In size and weight
it is piece Of very heavy machinery,
and would naturally be made in some
great shop devoted to the construction
of seam and other engines of the larg-
est size. But the great masses of met
nj which form the axes and supports of
the instrument have to be moved by q
system of mechanism some parts of
which are rs delicate as those of
watch, The micrometer alone will, il
made in the most approved way, be
piece OF mechanism far more intricate
than an ordinary astronomical instru-
meni. H. all this an astronomical in
strument maker is required. Finally,
what is more important. multitude Of
provisions must be made for the hand
ling and pointing OF the instrument,
for illuminating the different portions,
and for enabling the observer TO read
off the the lines by which he knows at
each moment exactly at what star his
instrument may DO pointed. The diff
Guilty of this last problem is one that is
very slightly appreciated by those not
accustomed to the Use of telescopes.
When using power of one thousand.
the whole field of view OF tho telescope
is only little spot oi iii" leIvcns not
one-hundredth OF the apparent surface
of the sun or n'oon. Within this little
spot is contained all that the observer
sees when he looks into the telescope.
Yet by being magnified thousand
times IL seems to fill fourth of the sky.
Since the observer can not see any
thing outside of this little spot, he has
no knowledge which way to turn his
telescope by mere sight of the heavens.
He mnst therefore have delicate ar
rangement of circles by which he finds
out where he is looking, not by what
he sees, but by looking into micro
scopes attached to various parts OF tho
instrument itself. ALI this requires the
combined skill of the astronomer, the
astronomical mechanician, and the en-
gineer. .
After decision is reached, it will
probably require two years to complete
the instrument. The trustees are APP
parontly delaying the decision in order.
to be FREQ to choose the best course
when the completion of the object-glass
is assured. Meanwhile the erection Of
the buildings and the construction Of
the other instruments are being pushed
forward. - Simon Ne,oconb,in Harper's
Magazine for February.
Corne, the Italian.