SPECTACLES, 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 converge so as to fall upon the retina: whereas myopes, or short-sighted people, use concave lenses for spectacles, which causes the rays to diverge, and prevent their meeting ere they reach the retina. See OPTICS, n° 66.