IV.

History

The earliest lenses, which were known to the ancient Greeks and Romans, were glass spheres filled with water. These water-filled lenses were used as burning glasses. True glass lenses were not known in classical times; they were probably first manufactured at the end of the 13th century in Europe. The processes used in lens manufacture have not changed essentially since the Middle Ages, except for the utilization of pitch as a polishing medium, introduced by Isaac Newton.

The recent development of plastics, and of special processes for moulding them, has led to their increasing use for the manufacture of lenses. Plastic lenses are cheaper, lighter, and less fragile than glass ones.