Ophthalmology => Eyeglasses
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.
The inconvenience of conventional eyeglasses has led to the development of plastic corrective lenses that can be worn under the eyelids, directly over the eyeball. Such contact lenses minimize the danger of breakage that is always present with ordinary glasses, because, like the eye, contact lenses are protected from injury by the shape of the skull. Present-day contact lenses cover only the cornea of the eye; a special molding process permits precision fitting to the curvature of the cornea to minimize irritation. So-called soft lenses, now in common use, are made from a soft plastic material that molds itself to the shape of the cornea. Extended-wear contact lenses should be used only following careful consultation with an eye doctor.
Research has been done with implanted lenses that reshape the cornea to correct focal defects. Another approach is the direct reshaping of the cornea through a surgical procedure called radial keratotomy. Although this operation is coming into increasing use, it can present problems and it has been criticized by a number of physicians.
Glasses are worn to protect the eyes as well as to correct visual defects. The colored glasses, or sunglasses, worn to protect the eyes against the rays of the sun are a familiar example. To protect their eyes from the actinic rays of welding flames, workers wear glasses of a deeper tint. Machinists and other factory workers wear glasses or goggles of great strength to shield their eyes from flying particles of metal, and aviators and racing drivers wear goggles to shield their eyes from the wind. Watertight goggles permit divers to see under water.
In 1268 the English philosopher Roger Bacon recorded the earliest statement about the optical use of lenses. Possibly as early as the 10th century, however, the Chinese had made use of magnifying glasses placed in frames. Eyeglasses were first used in Europe in Italy, and some portraits dating from the Middle Ages depict persons wearing eyeglasses. With the invention of the printing press in the 15th century, the demand for eyeglasses increased and by 1629 was large enough for a charter to be granted to a guild of spectacle makers in England. The first bifocal glasses were made for Benjamin Franklin at his suggestion about 1760. In early times the only eyeglasses having spherical lenses were manufactured to correct nearsightedness and farsightedness. Not until the end of the 19th century did the cylindrical lens for the correction of astigmatism come into common use.
Today, the doctor of optometry (O.D.), called an optometrist, examines the eyes for defects of vision and prescribes corrective eyeglasses or contact lenses. In Europe, this specialist is known as an optician or ophthalmic optician, but in the United States the optician is concerned with making lenses and spectacles. In contrast, the oculist, or ophthalmologist (M.D.), specializes in medical or surgical treatment of eye diseases or abnormalities of the eye.