[L. lens, a lentil seed, which is much like the shape of a convex lens.] A piece of glass or other transparent substance, which may be spherical on both sides, or one side may be spherical and the other a plane surface. There are many forms of lenses, their purpose being to refract the rays of light, causing them to converge to a point, to diverge, etc. Lenses are usually made of flat pieces of glass, and the greatest care is needed in grinding and polishing them, as the least unevenness in the surface would spoil them. They have many uses, but the most common is that of making eye glasses and spectacles - convex lenses being used for far-sighted and concave lenses for near-sighted persons. Lenses of various kinds are used in making opera-glasses, microscopes, stereoscopes, telescopes, and other instruments.