instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses
mounted in a frame to hold them in position before the eyes. The first device of this kind was probably invented by Roger Bacon in the 13th cent. although similar devices are believed to have existed in ancient times in China and in the Mediterranean civilizations. Early forms were crude and clumsy and were not improved until the 18th cent. when the grinding of lenses was first based upon the principles of light refraction. Lenses are made of clear or rock crystal glass or plastic ground to suit the defect of the eye. Concave glass is used for nearsightedness
, so that the rays of light are diverged. Convex lenses are used for farsightedness
, so that the light rays are converged. Astigmatism
is remedied by cylindrical lenses. Bifocal lenses, with a lower part for viewing objects near at hand (as in reading), were first devised by Benjamin Franklin. Telescopic lenses are used by the near-blind. A contact lens
is shaped to fit the eye and is worn under the eyelid. Incorrect eyeglasses may do harm, and lenses should be prescribed by an ophthalmologist or optometrist and fitted by a skilled optician. Eyeglasses to protect the eyes from glare or from foreign bodies are made of tinted or polarized glass and of wire mesh. The Eskimo make and use wooden eyeglasses that have only narrow slits for eyepieces to protect the eyes from glare reflected by ice and snow.
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