Wichita eagle. [volume] (Wichita, Kan.) 1886-1890, 1888-05-05
TESTING A GREAT LENS.
VISIT TO THE WORKSHOP OF THE
LATE ALVAN CLARK.

Finishing the Object Glass of q Great
Observation-Disclosures with the Palm of
Story.
In 1882 called to see Alvan Clark, the
distinguished constructor of telescopes, who
has Just died. found him at his home In
Cambridgeport, Mass. His sons were busy
putting the finishing touches to the great
telescope object glass then being made for
the Russian Astronomical observatory at
Pulkova The Pulkova objective was to be
placed in temporary mounting that very
evening to be tested for the first time on
celestial objects.
1t is in the after correction and perfecting
of the objective which calls for the special
skill which has made Avian Clark's name so
famous The large detectives, such as the
Pulkova and Lick glasses, are ground and
polished by machinery. Two levers work in
opposite directions, and are so arranged that
any part Of the glass surface may be reached,
doing the work. The final polish, which is Of
the most delicate nature imaginable, is done
with the surface of the hand Mr. Clark went
so far as to say that one revolution of the
hand i'll excess would change the correction.
On a table was the finished Pulkova lens.
which weighed $50 pounds, and consisted OF
two lenses each thirty inches in diameter.
Generally these lenses are made to accurately
fit, and are joined together with Canada bal
sam, but in such large glasses as the Pulkova
and the Lick, they are fitted in a metal frame
with an adjustment so that they can be made
to approach each other, or otherwise.
When the evening was sufficiently advanced
the great Pulkova glass was placed in its tenn
porary fitting in the garden. There was no
moon and the darkness was intense The glass
was brought out on four wheel hand truck
and lifted into the tube by five men and fixed
by revolving it IL the screw fitting The tube
was forty five feet long and weighed with the
attending fittings about seven tons. Twopiles
of brickwork supported the whole. There
was no clockwork movement and the rough
est apparatus was employed, the telescope was
raised and moved by a guide rope, the mo-
ton of an equatorial movement being imi-
tated by using q common windlass. As the
motion of the earth caused the object to pass
across the field of the telescope, the observer
gave the order follow" when a slight turn
of the windlass kept the object in view. Such
were the rough appliances used to test this
$4,000 lens.
VIEW OF A FIXED STAR
The planets had all set, and had to be sat
isfed with a view of a fixed star, which is an
excellent object for testing the optical proper
ties Of lens, but very uninteresting other
wise, as the largest telescope can make little
impression on a fixed star; no disk can be
seen, merely speck of light The star So
lected was a small one, and barely visible as
pale, minute object. On looking at it with
this magnificent instrument its wonderful
light gathering powers were at once evident,
for the star shone with the luster and brill
fancy of an electric light. 1t was an object
which brought out all the imperfections of
the glass, and to the eyes Of Mr.. Clark and
his sons many were evident and, it was said,
two months work was necessary to correct
them During the trial the lens We're lowered
and five men revOlved the glass in its fitting.
On its being placed in position again one of
the sons was about to make another test when
the old man shouted: wait, boys, let her
cool"
l was curious to know what this could
mean, and Alva Clark explained that the cor
rection was so delicate that the heat from the
hands of the five men holding the metal case
of the objective would change the correction,
SO it had to cool
In conversation with Mr.. Clark on the
advantages of immense telescopes such as this
one, be admitted that the telescopes of mod
crate aperture, say from ten to fifteen
inches, were preferable for general use even
for the highest purposes. He spoke with
pride Of such an instrument he had made,
with which he had seen ali that could be seen
even with colossal telescopes. Only one dis
coyery has been made with the Washington
telescope of twenty six inNles diameter during
its many years of use. The Russian Puikova
instrument, delivered by the Clarks in 1859,
has never been heard from, and those vino
expect immediate and extraordinary results
from the Lick thirty-six inch objective viii
probably be disappointed These huge tele-
scopes are great light gathcrers and useful
for Photographic and spectroscopic work, but
their definitions of the surface of the moon
and the planets are disappointing.
A story is told Of French astronomer vhc
Travelled hundreds Of miles 70 look through
the great Ross telescope. When asked what
he would like to see he called for the planet
Saturn, which was then in good position. On
looking through the telescope and finding the
wretcheGi definition OF tie object be said:
You tell me have looked AZ the planet
Saturn 50 must believe have done SO. but
l was not aware Of tie fact. John Aicbels
in New York Sun