Delaware gazette and state journal. (Wilmington, Del.) 1883-1902, 1886-06-17
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some Notes About she biggest RE
tractor Ever Underfakcn.

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Ionae Notes About tho biggest RE-
tractor Ever Undertahcn.
loston Herald.
Messrs. Alvan Clark 4 Sons of Cam
bridge, hope to have the great telescope,
which they are now making for the Lick
Observatory In California, finished before
another winter. Passengers on the Cot
tage Farm horse-car line, ns they near |
the terminus, can notice a fine residence |
on tha left, near where is q brick build |
ing used for the manufacture of tele-
scopes. Between these two structures is
pyramidical tower. A huge sheetiron
tube, riveted together in sections, rises
obliquely from the ground, supported on
an iron axis set into the masonry of the
tower. At first II suggests TO the mind
Holland windmill divesied of one of its
sails. This tube is used for the purpose
Of trying the largest object glasses, like
the one now making, and the hardly less
magnificent one lately finished for the
Russian government.
The process Of grinding" the great
lens is n long one. An iron post, a fixture
in the floor three feet high, bears on its
top an iron concave plat. Into this plate
the huge convex lens is laid, previously
provided with emery and water, 10 be
ground by stendy, patient motion with
the hands. Backward nnd forward and
round and round il must go for many
months. This particular lens was cast in
Paris, no American artisan yet caring TO
risk making perfect glasses of such q
size. The smaller ones. however, are
moulded by the Messrs. Clark, the process
curiously suggesting the kneading of
bread. Taken out of the furnace, the
glass is worked over in its dough like
state till entirely free from bubbles, and
then rolled into shape with q rolling pin.
II is interesting to watch the grinding
of 30 large a lens as this three footer,"
now slowly nearing its glossy perfection.
The men stand around it ut equal dis
tances, their hands resting upon its edge,
like sailors at their windlass, while to and
fro they keep ii moving. In this manner
a uniform cut is given to the whole lower
convex surface. When ground sufficiently,
or as near as approximation can deter
mine, II is turned over and worked as
before.
Yet this is only one-half of the object
glass. All lenses are formed by the union
of two parts, one CONVEX, the other plano
concaye, both so accurately fitted at their
meeting surfaces that on the application
of transparent cement they adhere as if
one solid body. The reader can imagine
the skill and delicate adjustment neces-
sary 10 master this combination on so
large scale. Mr.. Alvan CLARK, ST., ex-
presses himself as not free from doubt re-
garding his success in completing this
glass, as it might prove "beyond the
limits of their slill.n The sons, never
theless, are very hopeful of the result.
After nearly six months of grinding
and long period of polishing on soft
wood blocks with water and ''crocus,n
the great object glass was tried upon
artificial stars with some degree of salts
faction. The slightest irregularity in m
object seen through it would betray II
blemish, or variation on one side of the
lens, and then the other side would have
to be retouched. The least variation in
thickness is mischievous, if not fatal, to
the value of the glass. II must be fault
lessly accurate and uniform. The last
process of polishing is performed by the
finger tips, with the finest powder, occu-
plying several weeks.
The great Lick telescope, when mounted
and swung against the sky, will have a
focus of So feet length and nearly 15 feet
larger than the largest one ever made. II
will be q refractor, which means that the
image is formed directly to the eye by
the object glass, as contradistinguished
from the Gregorian and Herschelian tele-
scopes. The largest instrument ever
known of the latter style was DIR. Her
schel's The tube lacked eight inches OF
being 40 feet in length.
Clark Il Sons received $50, 900 for the
object glass of the large instrument re,
Gently completed for the Russian Govern.
ment, and an additional award of a gold
medal, the latter received few weeks
ago. The cost of the Lick object glass
will be SOL 900. Its weight can hardl,
be less than 450 pounds. The tube and
gearing are TO be furnished by othel
parties. Thus California aspires to take
the lead in experimental astronomy. She
stands, the daughter of the sunset, look
ing into celestial space with range Of
vision that promises to outvie all com
petitors.
In implying that the lenses forming
the great objective are to be cemented to
gether the Herald is in error. Large OBJ
jectives are never and small ones Blu
rarely cemented now. The lenses com
posing them are frequently so accurately
finished that their inner surfaces fit a
neatly as if they were cemented and thi
is probably what the HeraldS informan
meant. In some lenses, however, tn
curves of the inner surfaces are designed,
made quite dissimilar and occasionall
the lenses are widely separated, as is tn
case with the objective of the Princeton
telescope, which was made by the Clarks
where there is distance of several inche
between the flint and crown glass lenses.