Eyeglasses, lenses or prisms worn in front of the eyes to compensate for various defects of vision. The most common form of eyeglasses consists of a pair of glass lenses in a metal or plastic frame fitted to the bridge of the nose. The frame is held in place by spring pressure on the nose or by bows, or arms, that grip the head or hook around the ears. Spectacles with lenses made of a hard plastic are commonly used for reasons of increased safety and light weight.
Other forms of eyeglasses include those held in place by pressure on the nose, usually called pince-nez (French, “pinch nose”). Single lenses used to correct the sight of one eye, held in place by wedging in the orbit of the eye, are known as monocles. Glasses with a handle rather than bows, occasionally employed for reading, are called lorgnettes.
The lenses of eyeglasses are ground in the form of concave spherical lenses for nearsightedness (myopia), convex spherical lenses for farsightedness (hyperopia), cylindrical lenses for astigmatism, and prisms for defects of convergence. Frequently it is necessary to grind lenses in a combination of these forms to correct several anomalies at once. Bifocal lenses are used to give a different amount of correction for vision at a distance and for close work. The upper part of such lenses is ground for distant vision and the lower part for close vision, so that the user has merely to lower the eyes to read and raise them to look at distant objects. Trifocal glasses are bifocals that are ground with a center lens for intermediate distance.