Los Angeles herald., 1890-09-21
FORTY INCHES ACROSS
is WHAT THE SPENCE TELESCOPE
GLASS MEASURES.
1t is Inspected BY Judge Widney at Cam
bridge-Ten Feet in Circumference and
Two and Half Inches Thick.

A special dispatch from Cambridge,
Massachusetts, under date of the 18th
instant, gives the following account of
an exhibition of the glass for the Spence
telescope
The precious glass that is to be used
in making the telescope that will eclipse
even the famous one now mounted at
the Lick Observatory was shown today
to large gathering in the cellar Of the
residence of Alvan G. Clarke, the famous
telesccpe-maker, and there were present
Judge R. Widney, president of the
University of Southern California
George Clark and Judge Widney's son
Robert.
The glass measures about ten feet in
circumference and three feet four inches
in diameter. Although two and one
half inches thick at the center and one
and one-half inches at the edge it was
as transparent as bit of thin plate
glass. When it was carefully raised on
its edge in the box Mr.. Clark's hands
fondled it as he would baby raised
out of its cradle, while he murmured: :
"What beauty it is. No one ever
saw it's like"
The lens represents about twelve
months work already, although two or
three years will be required to finish it,
When it is ready for it's position in the
big telescope it will represent value of
from 100000 to $4,000.
The first process that this new and so
far greatest lens ever attempted will have
to undergo will be that of grinding to a
proper curve. 1t will be placed on a
mill and made to revolve at slow and
uniform rate of speed. The finest Of
sharp steel instruments will cut out the
surplus glass, and a smoothing machine,
moving in a constantly changing curve,
will be, for months and months, passing
over its surface before the lens can be
utilized.
Each of the processes employed in the
operation is intricate and delicate. That
of getting the correct focal power may,
however, be considered the most inter-
esting. On being set before a mirror, a
little starlike ray of light will be made
to pass through the lens and reflected
back through it again from the mirror.
until all inequalities of density in the
glass are removed, the ray of light trans
mltted will not be perfect in shape.
Back and forth the lens will have to go
from the grinding and smoothing
machine to the testing apparatus, until
the tiny ray of light is as perfect as
when it first leaves the little lamp that
sends it forth.
The total length OF the telescope can
not be decided upon until the focal
power of the lens has been established,
but it is estimated that it will be be-
tween fifty-six and sixty feet. Besides
being an object glass, it is also a photo
graphic glass. The largest ASTRONOMERS
Photographic lens previously made has
been but twenty-four inches in diameter,
and it is expected that the present one
will accomplish unheard of triumphs in
revealing the wonders of the starry
armament.
Judge Windey said that through it
the moon will look as if only one hum
dred miles away, and If there are any
cities or large buildings on its surface
their presence will be revealed through
its aid. The glass, it is thought, will
also settle the question as to the sup
posed signals of light which the inhab-
itants OF Mars are understood to be
making to the people of the earth.