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 age the pupil of the eye becomes flat, and the rays of light are consequently not refracted sufficiently in pa ing through it to meet on the retina and produce distinct vision. This defect is remedied by a convex lens, which produces 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 about the latter end of the 13th centurj^; but the date and author )f the invention are not certainly known, and have been much disputed. It seems most probable that the first.lint of their construction and use was taken either from;hc writings of Alhazen, who lived in the 12th century, jr of Roger Bacon, who died about 1292. A pa age in;he Opus Majus of the latter renders it certain, at least, -hat he was acquainted with the use of crystalline lenses n magnifying minute or distant objects. (See Smith's Optics, " Remarks," art. 8. 5—90.) SPECTRES, Spectra. A name given by StoU and [s. 1150]