The news and herald. (Winnsboro, S.C.) 1877-1900, 1879-11-11
Great Telescope c.lassos.

The glass for tho now rcfracting tclc-
scope ordered by tho Russian Government,
when completed, will be the largest in the
world. Its extreme diameter will be from
thirty ono and one-half to thirty-two inches,
with n clear aperture Of thirty inches
Three years and one-half are allowed for
its completion-two years to procure the
rough disks and eighteen months for grind
ing, polishing, correcting, Sc, with an cX-
tension of time, provided good and sufficient
reasons arc given for the failure to finish
within the specified period. When finished
the glass will be mounted in Hamburg.
The cost of the glass alone will be $32, 900.
The material Tor the glass will probably be
furnished by French manufacturers, the
Clarks finding their disks to be most trust
worthy. AS the procuring of glass of n
density as nearly uniform rs possible, and
free from flaws, is Of the utmost Importance,
Mr.. Alvan G. Clark left for Europe to as-
certain tho prospect OF careful completion
of that part of tho contract. He was ac-
companicd by Mr. Struvc, the Director of
tho Russian Imperial Observatory at Pull
kowa, who made tho award to Clark &
Sons, and will return In about six weeks
The method of making glass of this size is
very interesting. The raw material is
carefully selected and melted in an ordinary
glass pot, allowed to cool and then broken
out. It is one cmore melted and cast into
rough plates. These arc placed in mould
and heated until they arc waXlil1c in con-
sstency. Then they spread out and fill the
mould, making what are called the rough
disks. This is the shape in which they arc
received by the Clnrks, whose labor begins
at this point. The disks arc first made
perfectly circular by means Of revolving
disk of iron, with the aid of wet sand.
They arc then roughly ground on an iron
plate, fed with sand and water, to some
thing approaching the shape they will have
when completed. Then comes the fine
grinding with emery Oh another IRON plate.
This part of the process lasts from six
weeks to two months, and the rest Of the
time Is devoted to polishing, testing and
correcting The polishing is done with
rouge on an iron tool, covered ith q fine
layer of tar and pitch, and hand .;tbor must
be employed. When the glass has received
Its first polish it is set up and tested, and
from this time on the work Is Il succession
of testing and correcting, .epratcd until the
desired result is reached. In this operation
great carcmust he taken, for an inaccuracy
amounting to but r'sbroadth in the thick
ness OF the glass at some particular point
would DO sufficient to seriously iniurc it and
't''AN ii'o'Aeii w ~ w no lcilliCl
OF completion. If the glasses were Of ab-
solutely uniform density throughout, the
operation would re comparatively simple.
But such piece of glass it is impossible to
find, and rs each variation in rcfractivc
power must be cquaizcd, tho glass needs
to he corrected in nearly every point of its
surface AS the glass is tested after each
correction, all immense amount Of patient,
careful labor Is required. Again, the con
ditions under which the glass is to be used
mnst be taken into account, and also the
state of the atmosphere and tho temperature
when tho tests arc made, conditions which
arc never twice alike. AS telescopes are
most used before midnight, their ob1cctivcs
arc corrected for falling temperature.
One OF the Clarks said that glass which
would be called in fair condition might re-
ccivc several months labor before being
what they would consider perfect, and yet
In that timo the amount Of material removed
would not equal onc-tcnth of grain in
weight. The largest glasses now in exist
ence are tho objective at Washington and
one made by the Clarks for Mccormick,
but still in their possession, each of which
Is twenty six inches in diameter. Whom
this and the Russian glass arc completed, of
the four largest glasses in the world, three
will have been made by Alvan Clark & Sons.
The contract made with the Russian Gov
crnmont with the Clarks was concluded
September l. 1t Is provided that the def
milton OF tho glass shall not be inferior to
that OF the telescope III tho Naval Obscrya-
tory, and the amount OF light shall be
greater in proportion to the increased area
of the objective, allowance being made for
the absorption of light by the glass. When
the rough disks are ready for figurinr
NJ. Struvc is to receive for his approval the
plan for fixing the objective glasses III their
cells. M. Struvo has the right 40 designate
the ratio Of focal length of the objective to
lts aperture. said ratio being not less than
IL to l, nor greater than 20 to 1, and tho
Clarks must make the focal length conform
TO this ratio within one-forticth of its whole
amount. II M. Slruve dcsignates no ratio
the Clarks shall give such focal length that
the ratio shall be 1G to l, within the limit
OF one-torlicth. M. Struvo may also des.
lgnatc what two rays or parts Of the solar
spectrum, shall bo brought to one focus Nv
the combined actions of the lenses, and if i
1all to 10 this, the makers shall fashion the
curves of the glass so rs to give the most
perfect achromatism. 1t is provided that
the Clarks shall, about the time of tho COM
plction OF the objective, construct rough,
reversible equatorial mounting for it, inciu-
ding tube Of such size, form and thickness
rs tho Russian Government may direct, tho
whole to be so arranged that the objective
may DO safely and conveniently tested by
observations of the stars at least one hour
on each sido of tho meridian, The pay
mcnts for tho objective wcro agreed to 6o
paid as follows: $4,000 in advance, SO, 600
when the rough disks arc ready for figuring
and $4,000 when tho glass is accepted.
For the rough mounting 81 900 will be
allowed.