SPECTACLES. An optical instrument consisting of two lenses set in alight 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 counter act 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 -each the retina, which defect is remedied by glass of a suitable concavity.
A patent for improvements in spectacles was taken out in 1826, by Mr. A. A. de la Court, of Great Winchester-street, London, which are of so peculiar dascr;ption as to demand a brief notice from us. The invention consists in fixi.ig to the joints or bows of common spectacles a pair of mirrors or reflectors, PO jointed and posited as to enable the wearer to see sideways, and even behind him, as he may be inclined, without turning his head. In another form of these patent spectacles the ordinary glasses are omitted, and the mirrors only introduced as exhibited in the prefixed illustration of the patent, extracted from a scientific journal, in which the uses and abuses of this singular invention (that gives to the wearer all the boasted powers of the god Argus,) are humo rously detailed.