An optical instrument consisting of two lenses set in a light frame, the extremities of which are made elastic, so as to retain, by a slight pressure against the sides of the head, the instrument in its place, which is supported upon the nose of the wearer. The use of spectacles is to counteract some defects in the organs of vision; and as these differ in their nature, the lenses vary in their properties. Those with convex lenses serve to counteract the effects arising from the too great flatness of the eye, by giving the rays of light a degree of convergency sufficient to make them meet exactly at the retina, and are, therefore, generally proper for elderly persons. On the contrary, short-sighted people use concave lenses, to prevent the rays from converging so suddenly, - because the eyes of such persons being too round and protuberant, give too great a convergency to the rays, and cause them to meet before they reach the retina, which defect is remedied by glass of a suitable concavity.