The clarion. [volume] (Jackson, Miss.) 1883-1888, 1885-04-15
TELESCOPIC.
Difficulties Encountered In Casting the OBJ
ject-Gisss for the Lick Observatory

ject-Giass for the Lick Observatory
In 1880 a contract was made with
Messrs. Avian 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
eminent. aud thus the telescope would
fulfill the condition OF being the largest
ana most powerful ever made he Pc
-ult has proved the old rnle. that the
larger the glass the more difficult 1t l.
10 make it. In this CONNECTION there i
4 curious contrast between our present
experience and that of the opticians ia
the early party of the century. AT That
time the making of the crown-glass for
the double lens offered comparatively
little difficulty: ii was the hlnt-giass
with which the trouble was found The
latter contained lead. a 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 heen so completely overcome that
crown-class. The method of making
the best flint was long supposed TO be a
secret in the hands of q Swiss named
Guinand and his family; but 1t Is now
believed that the supposed secret IN
yolved nothing more than the very SIM
pie device of continuously and vlgor-
ously stirring the molten glass until it
became too cool and stiff 30 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 net 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 ca ting 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 l.
Il
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
pressed into the shape of a disk, like a
very thin grindstone, and in order tc
do this the lump must first be heated
nearly to the melting-point, so as to be-
come plastic. But when Fell began tc
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 a 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 tn,
delicate yet powerful machinery by
which the tube sixty feet long is to b.
| pointed toward any region of the heav
| y
| has not vet been commenced. In fact
the question who shall construct thi
| mounting" as it is called, is a diff
! cult one to decide. In size and weigh
it is a piece of very heavy machinery
and would naturally be made in som
| 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 O
| the instrument have to be moved by
| system of mechanism some parts c
| which are as delicate as those of
watch The micrometer alone will,
made in the most approved way, be
| piece OF mechanism far more intricat
than an ordinary astronomical instru
ment. Ia all this an astronomical i'll
strument maker is required Finally
what is more importaus. a multitude c
| provisions must be made Tor The hand
ling and pointing of The Instrument
for illuminating the different portions
and for enabling the observer to rea,
off the fine lines by which re knows a
each moment exactly At what star hi
instrument may be pointed. The diff
Guilty Sf this last problem is one that i
very slightly appreciated by those no
accustomed to the use OF telescopes
When using n power of one thousand
the whole field of view OF the telescop
is only little spot of the heavens no
one-hundredth of the apparent surfac
of the sun or moon. Within this littl
spot is contained all that the observe
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 ha
no knowledge which way to turn hi
telescope by mere sight of the heavens
He must therefore have a delicate ar
rangement of circles by which he find
out where he is looking, not by wha
he sees, but by looking into micro
scopes attached to various parts of tn
instrument itself All this requires thu
combined skill of the astronomer, tn
astronomical mechanician, and the en-
gineer.
After decision is reached, it vii
probably require two years to complet
the instrument. The trustees are APP
parently delaying the decision in orde
to be free to choose the best cours
when the completion of the object-glas
is assured. Meanwhile the erection 0
the buildings and the construction o
the other instruments are being pushes
forward. Simon NeWcomb, in Harper
Magazine for February.
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