The times. [volume] (Richmond, Va.) 1890-1903, 1895-12-22
Telescope Lenses.

The great lenses, forty inches in clear
aperture for the Yerkes telescope, are
now complete in the workshop Of Avian
G.. Clark al Cambridgeport, Mass.
An observatory, to be under the control
Of Chicago University has been equipped
for the reception Of the great telescope
and II will soon be in use. The tube when
is Of steel and sixty-three feet long. was
made in time for exhibition at Chicaso
during tho Columbian Exposition and IS
said 10 be equal 10 all the demands for
strength, rightly. and easy movement
The two lenses composing the objective
are of the simplest form. one Is of crown
and the other Of flint glass, each belns
20 10 inches in diameter. An inch ana a
half is cut off in mounting giving a clear
aperture OF 30 inches. The crown tense
is double convex. three-quarters or an
inch thick at the edge and Two and three
quarters Of an inch thick at the centre.
Being well supported about the circumier-
ence this thickness gives sufficient rigid
itv although the weight is nearly 20
mounds The flint or negative lens Is
nIanoconcave. two inches thick on the
edge and about an inch and quarter IN
the centre, weighing 300 pounds Ihese
lenses have been tested Tor months py
Mr.. Clark. and local imperfections nave
been corrected in the most careiW man
ner. ,., ~,
class was a labor OF great difficulty ana
tho final success of the makers, IN IaflS
marks a great advance in the manulac-
tura If optfcal glass One OF the DISKS
was completed and delivered at 'aiA-
bridgeport long before the other Wa per
fected. and il was necessary TO await inc
production of both before the laDoI ol
grinding and polishing could De uhue'-
taken. &C.,. ,t.i ,,,,.
win the simple but effective machinery
ernnloved for grinding and pohshins' '"o
tabor is not so difficult as might De I'M
aeined. The chief danger Is IN the manu-
ling of such heavy masses. ii the gas
- not HOMOGENEOUS there Is also danger
,I,,t ,,,ben the an equal strain is fully de-
,aloned by grinding away some portions
the lens will break or explode. we De
iieve that Mr.. Clark had such an expert
once with one Of the lenses, ana was
obliged to wait for another. Ihe large
surface Of the lenses aids in the proauc-
ton OF perfect spherical surfaces. bu0
af,er a nerfect spherical surface Is Sc
cured and well polished, the instrument
may still be OFFICIALLY imperfect OWNS
10 slight inequalities in the density ol
the glass. That Is, a pench OF hs"l -
bent more by passing thtON1H IN
lion Of the lenses than another, thus us
torting the image Of an object AT the
focus. Were it not for the discovery py
Mr. Clark OF an effective method OF local
correction to counterbalance Inequalities
of density, the making Of a goo" lens
forty or even twenty inches IN diameter
would be an impossibilty. lie has so iaI
perfected the work OF local correction
that we believe there Is absolutely TO
limit 10 the size of the lenses he can iurh-
ish. except that which is involved IN the
manufacture of suitable disks by the
makers of optical glass
IL would not be surprising II teie-
SCOPE Of forty-eight inches aperture were
constructed within the next ten years
and perhaps earlier The work ought 10
he attempted while Mr.. Clark Is IN Sood
health. or course. the possible saggins
~ the glass from its own weight may
become perceptible In glass OF hlty
inches. But the perfection OF the forty
inch glass shows that the limit mas ho'
been reached. and no one can tell wnethet
a properly proportioned glass OF iorty-
eight inches will sag or not until the ex-
nerinent be tried The opinion OF Air.
Clark would have great weight IN 'his
matter.