eyeglasses

Lenses set in frames to wear in front of the eyes to aid vision or correct vision defects (see ophthalmology, optometry).

Their use for farsightedness and nearsightedness has been known since the late Middle Ages. In 1784 Benjamin Franklin invented bifocals, with divided lenses for distant and near vision. Eyeglasses can also correct astigmatism. Most lenses are made of glass or plastic (lighter and more shatterproof than glass but easily scratched). Sunglass lenses are tinted to reduce glare and often treated to reduce ultraviolet light exposure. See also contact lens.

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▪ optics

also called  glasses , or  spectacles 

      lenses set in frames for wearing in front of the eyes to aid vision or to correct such defects of vision as myopia, hyperopia, and astigmatism. In 1268 Roger Bacon made the earliest recorded comment on the use of lenses for optical purposes, but magnifying lenses inserted in frames were used for reading both in Europe and China at this time, and it is a matter of controversy whether the West learned from the East or vice versa. In Europe eyeglasses first appeared in Italy, their introduction being attributed to Alessandro di Spina of Florence. The first portrait to show eyeglasses is that of Hugh of Provence by Tommaso da Modena, painted in 1352. In 1480 Domenico Ghirlandaio painted St. Jerome (Jerome, Saint) at a desk from which dangled eyeglasses; as a result, St. Jerome became the patron saint of the spectacle-makers' guild. The earliest glasses had convex lenses to aid farsightedness. A concave lens for myopia, or nearsightedness, is first evident in the portrait of Pope Leo X painted by Raphael in 1517.

      In 1784 Benjamin Franklin (Franklin, Benjamin) invented bifocals, dividing his lenses for distant and near vision, the split parts being held together by the frame. Cemented bifocals were invented in 1884, and the fused and one-piece types followed in 1908 and 1910, respectively. Trifocals and new designs in bifocals were later introduced, including the Franklin bifocal revived in one-piece form.

      Originally, lenses were made of transparent quartz and beryl, but increased demand led to the adoption of optical glass, for which Venice and Nürnberg were the chief centres of production. Ernst Abbe (Abbe, Ernst) and Otto Schott in 1885 demonstrated that the incorporation of new elements into the glass melt led to many desirable variations in refractive index and dispersive power. In the modern process, glass for lenses is first rolled into plate form. Most lenses are made from clear crown glass of refractive index 1.523. In high myopic corrections, a cosmetic improvement is effected if the lenses are made of dense flint glass (refractive index 1.69) and coated with a film of magnesium fluoride to nullify the surface reflections. Flint glass, or barium crown, which has less dispersive power, is used in fused bifocals. Plastic lenses have become increasingly popular, particularly if the weight of the lenses is a problem, and plastic lenses are more shatterproof than glass ones. In sunglasses, the lenses are tinted to reduce light transmission and avoid glare. See also contact lens; lens.

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