Collection: Polishing

Fine grinding, or smoothing, is done using carefully graded emery flour as an abrasive. A number of fine-ground lens blanks are then mounted with pitch on a block so that they can be polished together. The polishing tool is covered with a thin layer of pitch, wax, or even coarse cloth, and wet rouge or certain other mineral oxides are also used as polishing agents. The polishing of glass is a slow process, requiring lenses to be oscillated back and forth, sometimes for hours, against the rotating polisher. After both sides have been polished, the lens is ground around the edge to center it and give it the correct diameter. If a compound objective is being made, several single lenses must be mounted together in a precise coaxial arrangement, and their thicknesses, separations, and centring must be kept very close to the prescribed values, or the aberration corrections laboriously determined by the lens designer will not be realized. The finished lens resolving power is then tested by using the lens to form an image of a point source or other suitable test object. Sophisticated variants of this basic testing procedure have been developed using photoelectric cells or interferometers to obtain greater measuring accuracy.