Spectacles

I

Introduction

Spectacles, lenses or prisms worn in front of the eyes to compensate for various defects of vision. The most common form of spectacles 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 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 lightness of weight.

Other forms of spectacles 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 (rarely used today).

The lenses of spectacles are ground in the form of concave spherical lenses for near- or shortsightedness (myopia), convex spherical lenses for far- or longsightedness (hyperopia), cylindrical lenses for astigmatism (non-uniform curvature of the eye's lens), 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 centre lens for intermediate distance.

II

Contact Lenses

The inconvenience of conventional spectacles 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 moulding 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 moulds itself to the shape of the cornea. Extended-wear contact lenses should be used only following careful consultation with an ophthalmic optician.

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 doctors. A relatively painless procedure, conductive keratoplasty, which uses radio waves to reshape the eye without surgery, has also been developed and is increasingly popular.

III

Protective Spectacles

Glasses are worn to protect the eyes as well as to correct visual defects. The coloured 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 pilots and racing drivers wear goggles to shield their eyes from the wind. Watertight goggles permit divers and swimmers to see under water.

IV

History

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. Spectacles were first used in Europe in Italy, and some portraits dating from the Middle Ages depict people wearing glasses. With the invention of the printing press in the 15th century, the demand for glasses 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 in about 1760. In early times the only spectacles 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, an optometrist, or ophthalmic optician examines the eyes for defects of vision and prescribes corrective glasses or contact lenses. A dispensing optician is only qualified to fit glasses and dispense them. In contrast, the oculist, or ophthalmologist (M.D.), specializes in medical or surgical treatment of eye diseases or abnormalities of the eye.