in dioptrics, a machine consisting of two lenses, set in silver, horn, &c. to assist the defects of the organ of sight.

Old people, and others who have flat eyes, use convex spectacles, which cause the rays of light to converge so as to fall upon the retina; whereas myopes, or short-sighted persons, use concave lenses for spectacles, which causing the rays to diverge, prevent their meeting ere they reach the retina. See Optics.