II.

Lens Manufacture

Most lenses are made from special types of high-quality glass, known as optical glass, that are free from internal strains, bubbles, and other imperfections. The process of making a lens from a block of optical glass involves several operations. The first step is to saw a lens blank from the glass block. For this purpose, the glass is held against a thin, revolving, circular plate of metal, the edge of which is charged with diamond dust. The blank is next brought roughly to shape by grinding on a flat cast-iron plate covered with a mixture of abrasive grains and water. To form the rounded surface of the lens, the glass is then ground on concave or convex iron tools charged with abrasive. A convex lens surface is formed by a concave tool, and a concave surface by a convex tool. Commonly, two or more tools are employed in this grinding process, using successively finer grades of abrasive. The final process of finishing the lens surface is polishing, which is accomplished on a pitch-covered iron tool coated with jeweller’s rouge and water. After polishing, the lens is “edged” by grinding the edge until the physical centre and the optical centre of the lens coincide. (The optical centre is a point such that any ray of light passing through it is not deviated.) In this process, the lens is mounted in the headstock of a lathe, so that its optical centre is on the axis of revolution, and the edges are made true with a strip of brass charged with abrasive.