Springfield globe-republic. (Springfield, Ohio) 1884-1887, 1885-02-10
TELESCOPE.
Difficulties Encountered in Casting the Oh
Ject-Giass for the Lick Observatory.


Ject-Giass 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 for the Russian gov
| ernment. 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 tlint-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 adherence between the
two sides of the glass which was fatal
| to its performance. But this difficulty
has been so completely overcome that
| ali the trouble now arises with the
| crown-giass. The method of making
the best flint was long supposed to be a
| secret in the hands of a Swiss named
| Guinand and his family; but it 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
J this might be. Fell, Of Paris, who has
been most successful in making large
disks, supplied satisfactory flint in
| reasonable time. But so much delay
was met with in casting the crown
glass that it 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 it 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 it is con-
tanned is broken away. The outside
portions of the glass itself. being i'm
pregnated with the clay and other i'm
purities, have to be cut away. This is
a most tedious process. If any ordi
nary cutting tool were used, the 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 is one, therefore, in
| volving weeks, if not months, of labor.
| When it is done, the mass must be
/ ;-t1 tba eh Of Iict lilo n
very thin grindstone, and in order to
do this the lump must first be heated
nearly to the melting-point, so as to be-
come plastic. But when Fell began to
heat his 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
| ject-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 vet powerful machinery by
which the tube sixty feet long is to be
| pointed toward any region OF the heav-
ems, and kept in motion by clock-work,
has not yet been commenced. In fact.
the question who shall construct this
mounting" as it is called, is diff
| cult one TO decide. In size and weight
| it is a piece of very heavy machinery,
| and would naturally be made in some
great shop devoted to the construction
| of steam and other engines of the larg-
| est size. But the great masses of met
| al which form the axes and supports of
the instrument have to be moved by q
| system of mechanism some parts of
| which are as delicate as those of a
watch. The micrometer alone will. if
made in the most approved way, be a
| piece of mechanism far more intricate
than an ordinary astronomical instru-
ment. In all this an astronomical in
strument maker is required. Finally,
what is more important. a multitude OF
| provisions Giust 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 fine lines by which he knows at
each moment exactly at what star his
instrument may be 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 a power of one thousand,
the whole field of view of the telescope
is only little spot of the heavens not
one-hundredth of the apparent surface
of the sun or moon. Within this little
spot is contained all that the observer
sees when he looks into the telescope.
| Yet by being magnitied a thousand
times it 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 must 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 the
instrument itself. All 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 ap-
parently delaying the decision in order
to be free 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 NeUcomb,in IN Harper 5
Magazine for February.
|