Lens Story: 8 of 28
40-inch Yerkes refractor built in 1897 and
Making a Great Lens
The making of a great lens presents a task
of unparalleled difficulty. It requires the highest skill and the
utmost patience. From the selection of the raw materials for the melt
to the removal of the last blemish from the surface of the lens and the
final test of its optical qualities, it is a work of extreme scientific
precision and accuracy.
GREAT MIRROR USED IN CANADIAN REFLECTOR|
Produced by the John A. Brashear Company, Pittsburgh
The big crucible in which the glass is melted
has been made of the purest clays and by specially trained craftsmen.
Previously to receiving the "batch" materials it is heated very gradually
for a week to a high temperature. The melting and stirring, under perfect
temperature and physical control, proceed for days. Then, when the
master workman knows that the glass is ready, the crucible is lifted from
the furnace and its molten contents of dazzling brilliancy are poured
into an iron mould lined with sand. The mould is covered with an iron
plate and lifted into an annealing furnace. There is remains for nearly
a month, very gradually cooling from its high initial temperature down
to ordinary temperature. And right here is one of the critical stages
of the process, for if the annealing is not properly done, the glass
will be subject to strains and
inequalities that render it useless for optical purposes.
When cold the glass passes through its first grinding
and polishing stage, preliminary to the very careful examination for
BENDING OF A RAY OF|
LIGHT IN PASSING
THROUGH A TRIANGULAR
Even this preliminary
grinding requires several weeks. For the inspection of the glass the
camera, the microscope and an instrument called the polariscope are used.
With the camera a perfect piece of glass yields a pure white picture,
whereas strains in the glass will produce dark colored spots at those
points. When viewed with the polariscope the glass must be perfectly
bright and free from irregularities and figures. Some bubbles may pass,
but veins, indicating incomplete fusion of the ingredients, strains
and spots of unequal density are not permissible. If these defects are
found the glass must be returned to the furnace and the process repeated.
And this frequently must be done many times. The big 36-inch refractor for
the Lick Observatory was poured twenty times at the Paris glass works,
with one month consumed for annealing after each pouring.
When a piece of glass has passed this very
rigid examination it is ready for the skilled lens grinder, who
polishes its sides with the utmost care and in accordance with the
curvatures calculated by the mathematician. The man who first applied
pure mathematics to the calculation of the correct curvatures of lenses,