eyeglasses or spectacles, instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses lens, device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved.
..... Click the link for more information. 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 nearsightedness or myopia, defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the
..... Click the link for more information. , so that the rays of light are diverged. Convex lenses are used for farsightedness farsightedness or hyperopia, condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either
..... Click the link for more information. , so that the light rays are converged. Astigmatism astigmatism (əstĭg`mətĭz'əm)
..... Click the link for more information. 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 contact lens, thin plastic lens worn between the eye and eyelid that may be used instead of eyeglasses. Actors, models, and others wear them for appearance, and athletes use them for safety and convenience.
..... Click the link for more information. 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.

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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eyeglasses

eyeglasses