The Mentor · The Story of the Lens · Page 6

40-inch Yerkes refractor built in 1897 and still unsurpassed. 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.
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

BENDING OF A RAY OF
LIGHT IN PASSING
THROUGH A TRIANGULAR
GLASS PRISM

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 imperfections.
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, both for