Lens

a transparent body, usually of glass, bounded by two curved surfaces, or by a curved surface and a plane. The name is derived from the lentil, which is shaped like a double convex lens. Lenses are of two sorts. Those that bring rays of light to an actual focus are called converging or convex lenses. They form a real image. Lenses that cause rays of light to diverge, as though they proceeded from a point, are called diverging or concave lenses. The former are bounded by convex surfaces and give a magnified image. They are known also as magnifying glasses. The origin of lenses is not clear. The British Museum has a ground rock crystal found by Layard in excavations at Nimrud. A simple magnifying glass is described by an Arabic writer of 1052. The invention of spectacles is credited to an Italian of about 1255. The combination of lenses to form a compound microscope is credited to an optician named Janssen of Holland, dating about 1590. Galileo invented the telescope in 1610. The largest lenses known are those made for modern telescopes. A large lens was made for the Carnegie Solar Observatory on Mount Wilson, California. The block of glass was cast in a village near Paris. It was transported to Pasadena. An optician spent three years in grinding and polishing it. When complete, it measured sixty inches across, it was eight inches thick at the middle point, and weighed an even 20,000 pounds. The surface was so delicate, so polished, that the touch of a visitor's finger would have meant ruin. The finished lens was packed securely in soft material and was carted up the mountain side by a road constructed for the purpose. See Lentil; Spectacles; Thallium; Telescope; Clark