Year: 1879

How q Great TeleAcOpe Lens IL Made


NJ Struve, the dirictor of the Russian I'm
perial Observatory at Pulkowa, has awarded
the contract for making the great objective
lens, the largest in the world, for the new re-
fracting telescope ordered by the Russian
government, to Alvan Clark & Sons, the we'll
known telescope makers Of Cambridgeport,
Mass. The Boston Post tells how the great
lens will be made. In the first place, the
raw material is carefully selected and melted
in an ordinary glass pot This is allowed to
cool in the pot and then broken out. II is
once more melted and cast into rough plates.
These are placed in a mould and heated until
they become wax-lke in consistency. In this
condition they spread out and fill the mould
completely, making what are called the rough
discs. This is the shape in which they are
received by the Clarks, whose labor begins at
this point. These discs are first cut perfect
ly round by means of revolving circle of
iron, with the aid of wet sand They are
then roughly ground on an iron plate, fed
with sand and water, TO something approach
ing their shape when completed. Following
this is the fine grinding with emery, on
similar iron plate So much Of the process
will last from six weeks to two months, and
the rest of the time will be devoted to polish


ing, testing and correcting. The polishing is
done with rouge on an iron tool, covered
with very fine layer of tar or pitch, and
hard labor must be employed When the
glass has received its first polish it will be set
up and tested, and from this time on the
work is a constant succession of testing and
correcting, repeated until the desired result is
reached. In this operation every possible
precaution must be taken, for an inaccuracy
amounting to but hairs breadth in the
thickness of the glass at some particular point
would ba gt1fh fhpiqpt n in iura / gprlOuslV and
require perhaps months to bring it to its
former state of completion. If the glasses
were Of absolutely uniform density through
out it would be comparatively simple mat
ter, and the outline curves would be very
nearly portions of circular. But such
piece Of glass it is impossible to find, and
therefore, since every degree of density
makes a corresponding variation in retractive
power, TO equalize it, it must be corrected in
nearly every point of its surface, so that it
might be said that no two curves on its sur
face are alike, and when 1t is remembered
that the glass is tested after each correction,
it will be seen that an immense amount of pa
tient, careful labor is required. Again, the
conditions under which the glass is to be
used must be taken into account and also the
state of the atmosphere and the temperature
when the tests are made-conditions which
are never twice alike. When the sun has
been shining all day the instrument and glass
become heated At night, when the tempera-
ture falls the outside of the glass contracts
first, causing the edges of the lenses to sepa-
rate little, resulting in disarrangement of
the figure The contraction continues long
after the temperature has become stationary,
and, as telescopes are mostly used before
midnight, their objectives are corrected for
falling temperature. The curves about the
edges are sharper than elsewhere, as part of
the correction for spherical alteration. In
fact. 20 many are the corrections to be made,
and so delicate withal, that, as one of the
Clarks said, glass which would be called in
fair condition might receive several months
labor before being what they would consider
perfect, and yet in that time the amount of
material removed would not equal one-tenth
of grain in weight.