CONTACT LENS

thin, artificial lens worn on the surface of the eye to correct refractive defects of vision. The first contact lens, made of glass, was developed by Adolf Fick in 1887 to correct irregular astigmatism. The early lenses, however, were uncomfortable and could not be worn for long. Until the development of optical instruments that could measure the curvature of the cornea (the transparent surface of the eye that covers the iris and the pupil), the contact lens was made by taking an impression of the eye and fashioning a lens on a mold. The modern contact lens, developed by Kevin Tuohy in 1948, is made of plastic and rests on a cushion of tears. It covers the cornea approximately over the iris and pupil. Contact lenses most effectively neutralize visual defects arising from irregular curvatures of the cornea; they are the preferred treatment for some varieties of astigmatism and aphakia (absence of the eye's crystalline lens). They also are cosmetically appealing substitutes for eyeglasses to treat myopia (near-sightedness) and other visual defects. Hard-plastic contact lenses have a limited wearing time because of potential irritation of the cornea and may require a period of adaptation when first worn. Both front and back surfaces of the hard contact lens are spherically curved, altering refractive properties by changing the shape of the tear film on the eye's surface, which conforms to the curve of the rear surface of the contact lens, and by a difference in curvature between the two surfaces of the lens itself. In the 1970s, larger soft lenses, made from a water-absorbing plastic gel for greater flexibility, were introduced, and in the 1980s extended-wear soft lenses, which can be worn without removal for several weeks at a time, were introduced. Soft contact lenses are usually more comfortable because they allow oxygen to penetrate to the eye's surface; their large size makes them more difficult to lose than the hard lenses. Their delicacy, however, makes them more subject to damage, and they require more careful maintenance. They are also less effective than hard lenses in treating astigmatism, because they reflect the corneal curvature more closely. In the late 1970s gas-permeable hard lenses (which allowed more oxygen to reach the eye, thus easing some of the relative discomfort) were also developed. Contact lenses have particular advantages in treating certain defects that can be corrected only partially by prescription eyeglasses; for example, contact lenses avoid the distortion of size that occurs with thick corrective lenses.