SPECTACLES

An optical instrument, consisting of two lenses set in a frame, for a isting or correcting the defects of imperfect vision. The lenses are convex or concave, according to the nature of the defect to be remedied. In old nge the pupil of the eye becomes flat, and the rays of light are consequently not refracted sufficienth in pa ing through it to meet on the retina and produce distinct vision. This defect is remedied by a convex lens, which produce! a slight convergency of the rays before they enter the eye Short sighted people, on the contrary, require concave lenses; because, in their case, the indistinctne of vision proceeds from too great a curvature of the pupil, which causes the rays to meet in a point before they reach the retina — a defect which Is remedied by giving the rays a slight divergency before they enter the eye. Spectacles appear to have been first used nbout the latter end of the 13th century; but the date and author of the invention are not certainly known, and have been much disputed.It seems most probable that the first hint of their construction and use was taken either from the writings of Alhazen, who lived in the 12th century, or of Roger liacon, who died about 1292. A pa age in the Opus Majue of the latter renders it certain, at least, that he was acquainted 5-90.)with the use of crystalline lenses in magnifying minute or distant objects. (See Smith's Optics, "Remarks," art 8, SPECTRES, Spectra. A name given by Stoll and Latreille to a family of Orthopterous insects, comprehending those which have a linear and attenuated body, like the ghost of an insect. [s. 1159]