The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1885-01-25
THE FEBRUARY MAGAZINES.
The Lick Observatory Telescope.
In Har M

The Lick Observatory Telescope.
simon Newcomb, in Harper's Magazine.
In 1880 contract was made with Messrs.
Alvan Clark 81 Sons to furnish an objective OF
thirty six inches clear aperture. This was SIX
inches greater than the glass they had just ar
thus the telescope would fulfill the condition OF
being the largest and most powerful ever made.
The result has proved the old rule, that the
larger the glass the more difficult it is to make
it, In this connection there is a curious contrast
between our present experience and that of the
opticians IN the early part of the century. Al
that time the making of the glass for the
double lens offered comparatively little DIM
Guilty; it was the fint-glass with which the
trouble was found. The latter contained lead, a
substance of great specific gravity, when per-
sisted in settling toward the bottom of the pot in
which the glass was melted, and thus producing
a 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 hong
supposed to be secret in the hands OF a SWISS
named Guinand and his family; but 1t is now
believed that the supposed secret involved
nothing more than the very simple device of con-
tinuously and vigorously stirring the molten
glass until it became too cool and stiff to per-
NIT 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 it has not yet reached the hands OF
the optician The cause of his failure is one
so simple that one cannot but wonder that 1t
should offer any trouble after being once detected
We call to mind that when the founder has
succeeded in casting his lump of glass, weigh
ing several hundred pounds the clay pot IN
which it is contained is broken away. The
outside portions of the glass ITSELF being impfeg
mated with the clay and other impurities have
to be cut away. This is most tedious process.
Ii any ordinary 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 pro-
cess of cutting away the outside is one, there
FORE involving weeks, if not months. of labor.
| When it is DONE the mass must be pressed into
the shape of a disk, like 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 at
tempts he took more and more time for the heat
| ing but broke dozen or more pieces before he
at last succeeded. In February, 1884, he re-
ported that glass was actually nodded with
| out having been broken, and would soon be
ready for shipment.
Ali this refers only to the great object-glass of
| the telescope, which, though the one vital organ
OF the instrument, 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 OF
| the heavens and kept in motion by clock work
| has not yet been commenced. In fact. the ques-
| ton who shall construct this mounting" as 1T
lis called, is difficult one to 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 steam and other
engines of the largest size But the great masses
of metal which form the axes and supports OF
| the instrument have to be moved by 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 piece Of
mechanism far more intricate than an ordinary
astronomical instrument. In all this astronom-
| ical instrument makeris required. Finally, what
| Is more important. multitude OF provisions
must be made for the handling and pointing OF
the instrument, for illuminating the different
portions and for enabling the observer to read OF
| the fine lines by which he knows at each moment
| exactly at what star his instrument may be
| pointed. The difficulty of this last problem is
that is very slightly appreciated by those not
accustomed to the use of telescopes. When as
| ing power of ], 000, the whole field of view of
the telescope is only little spot of the heavens
| not ne-hundreth 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 magnified thousand
| times it seems to fill fourth of the sky. Since
| the observer cannot anything 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 arrangement
of circles by which he finds out where he is look
ing not by what he sees, but by looking into MI
cfoScOpeS attached Gc various parts OF the inStfu-
| ment itself. All this requires the combined
| skill of the astronomer, the astronomical mech
| ancian and the engineer.
| After decision is reached it will probably
| require two years to complete the instrument
| The trustees are apparently delaying the deei-
sion In order to be free to choose the best
when the completion of the object-glass is as
| sured. Meanwhile the erection of the buildings
| and the construction of the other instruments
| are being pushed forward.