New-York tribune., 1882-08-13
NOTABLE OBJECT-GLASSES.

SCOPES.
lFRoM AS OCCASIONAL CORRESPONDENT OF TUE IhibuNt.
RosroS. Aug. 1O-A few days ago paid VISIT
70 AIvan Clark of Cambridgeport, Mass. to gather
some news of the great telescope objcctg glass now
eino prepared for the Russian Government
found that had arrived on fortunate day. lot
pre- paratons were in progress to make tetnPorarN
mounting of the objective that very evening loI
its first test n celestial objects.
Mr. AIvan Clark is genial intelligent old man
Of sevcnfy-eight wears who has rrlinquished all
mammal labor IN the business. but is still hale and
hearty and personally directs the WORK. lie re-
lated the story of many of his past achievements
and successes and of his visit to England "here
he met Sir John Iierschel and Lord RosSe VINO
built the great reflector in Ireland. Inc piactcai'
work of this important telescope manulac'oiy. Is
now carried on by Ar Clark's two sons Oeo'tc -
Clark and Alvan G. Clark the latter is now grind
ing the great Pulkoya objective. The CLARK lamily
reside in two handsome houses, at the rearoi when
stands the worksho a three-sory brick building
in which steam engine lathes and machinery are
arranged without much order or attempt at neat
ness. In the upper story found The telescope or
dered for Professor Lewis Swift, or RochesteI'
n Y. the cost of which Is to be paid by MI
Warner The objective for this telescope mas
a diameter of sixteen inches In the basement con
be witnessed the operation OF grinding anu p"hSu
ing these noted object glasses which appears 10 Do
q very simple matter in the early stages. An
assistant was rubbing with both hands disc OF
glass abont eight inches in diameter upon pre-
pared surface covered with a red polishing
material The employment seemed monotohous
one. and | snould think easily acquired by an
amateur desirous of making his own objective Mr
Alvan c. Clark told me that he had seen very
creditable obectives which bad been maze by
amateurs ignorant Of the business; in TACT that
he had been puzzled to tell how some had oeeh
ground and polished without previous instruction
in technical details The immense objectives lol
which the house is famous are ground ana poiShcu
by aid of machinery two levrls working in oppo-
site directions, which can be arranged to move SO
as to polish any part of the surface OF the glass
On sholf stood the great disc Of glass now
thirtv-elght inches in diameter and two lntncs
thick which will be used for the Lick Observatory
in California 1t has been roughly polished on
both sides. and is fortunately perfect When
ground and polished it will be reduced III diameter
to tbirtv-six inches. and when finished will be the
LARGEST telescope objective In the world. On Il
table was placed the Pulkova objective, dlNlu7d
into two parts each thirty inches III diameter; LOT
all acnromatic glasses are composed OF TWO lenses
consisting Of two materials, the front one being
made of crown glass and double convex in shape,
and the back lens made of flint generally nearly
plano-convex. The two fit accurately together anu
are arranged in metal ring case, the whole iorm-
ing an achromatic objective. II a single lens were
used the object would be seen irIngrd WIN
varions artificial colors and other defects duo to
spherical aberration but Dy the simple artlhce OF
usng two kinds Of glass these defects are cured to
great extent, and nearly peIlecV iul"so VI V'0 ""
ject is secured.
Althouuh one of the reasons for using telescope
Is 10 magnify an object, the most important use of
these great telescopes is their hght-gatherlng
mower. There appears to be- no limit to the human
vision so far as distance is concerned provided inc
object il Sufficiently Illuminated. Thus hxed
star being a very bright object can be seen by the
naked eye, although situated many millions OF miles
distant from the observer How these large aper-
ture glasses gather light and render minute objects
visible can be understood by Il very simple eNplana-
ton When the unaided eye looks at an OBJECT
the reina can only receive as many rays as iai on
the pupil of the eye. The pupil OF the human eye
has a diameter of about one-ifth OF an inch, but by
the use of telescope this is virtually increased III
surface IN the ratio OF the square OF the diameter of
fine objective to the square of One-tth of an inch.
The diameter OF the object glass now being made
bv the Clarks is thirty inches. AS an example,
with "A two-inch aperture to a telescope the num-
her of rays collected is one hudGcd times as great
~ that collected by the naked eye. With the Wash
inpton telescope of twentyslX inches aperture, The
ratio is 5000 to l. Thus with the Pulkova tele-
scope which has an aperture Of thirty inches stars
can be examined which are 50 small that it would
reonire thousands of them to be clustered together
TO make them even visible to the naked eye.
The Pulkoya objective which weighs 30 ponnds,
having been placed in position in the telescope
made to receive it temporarily was now ready Tor
use: and was surprised to find with what perfect
ease the pondarous tube, iorty-Gve feet in length
and weighing with It's attending fittings
about seven tons, was swung TO its proper ceva-
ton. The roughest apparatus was used, the tcie-
scope being raised BY guide rope, and the motion
Of an equatorial mounting initiated by uspg
common windIass. AS the motion of the earth
caused the object TO pass across the field OF the
telescope the observer gave the order follow"
when slight tuIn OF the windlass would bring the
object once more in view. Such were the rough
appliances used to test this, the largest telescope
objective as yet made, and costing $4,000.
The planets were all set and the moon was TOO
low down to be seen with the telescope a an eie-
walton which was practicable, 40 had to be con-
tented with n view OF n fixed star, which Is all
essential for testing he optical properties Of an Oh
jectve. The moon was shining brightly and tho
star to which the telescope was directed was barely
visible rs pale minute ObJect Vh iuohlng
through this magnificent lens Us wonderful light
gathering powers were at once evident. Tor the
star now shone with the lusre and brIiianOy of an
electric light at short distance. 1t was an object
which would develop al the impcrfectons ot tie
glass and to the technical eye, of the Clarks they
were very evident. and will take two moatns TO
eradlcate. 1t is this knowledge Of correction
which has secured the reputation Of tniS house. 10
grind lens is not difficult, but to correct 10 errors
perfectly requires all tho art AMI science which
the OpticIan can bring to bear Oh his wOIk. lhe
general verdict after The trial was that the GAS.
had behaved well on tins It's hrst test IN the rough
and forcshadOwed the promise Of doing the best
work that can be accomplished with an objective
~. .Ll..L.n m LIaa ~