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Glasses, or spectacles, lenses mounted in a frame and worn in front of the eyes to correct defects of vision. Converging lenses have been worn to correct farsightedness (hyperopia) since the late 13th century and diverging lenses for nearsightedness (myopia) since the 16th. Glasses with cylindrical lenses are used to correct astigmatism, and those having bifocal lenses (2 different powers in the upper and lower areas of each lens) or even trifocals (3 powers) may be worn for presbyopia (loss of elasticity of the eye's lens).

See also: Lens.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Ghibellines to Grand Prix