Spectacles

a pair of lenses used to aid and protect the eyes. They are said to be the invention of Roger Bacon in the thirteenth century. They were used in Florence about 1285. They are mounted usually in oval rims united by a bridge designed to sit astride the nose. Bows extend usually from the opposite ends of the frame to the temples or ears of the wearer. A short-sighted person requires concave lenses, i.e., lenses thinnest at the middle point. Farsightedness is corrected by convex lenses. As a matter of fact, the lenses of spectacles usually have one surface convex, the other concave. The desired effect is produced by having the curvature of one surface exceed that of the other. The optician buys his lenses in the rough from the glass-maker. He then grinds and polishes each surface. Frequently the lenses of a pair of spectacles are made to differ in power to suit differences in the two eyes.

Frames were formerly made of bone or tortoise shell, but now they are made of steel, silver, or gold. A transparent rock crystal, called pebble, is prized by the lensmaker. The Franklin or divided spectacles, much prized by elderly people, have the lower part of the frame filled with a half lens ground for reading and the upper part of the space filled by a half lens ground for looking at objects at a distance. Lenses of colored glass are worn to defend the eyes from the glare of the sun. Goggles have colored lenses surrounded by baskets of fine wire gauze that fit to the cheeks and are designed to keep cinders, dust, and chaff out of the eyes. Drivers of automobiles and workers about a threshing machine find goggles a protection.

Many birds are provided with natural spectacles, a transparent membrane called the third eyelid. This eyelid, when not in use, lies folded in the inner corner of the eye. Two muscles spread it over the cornea or fold it up again much more cleverly than a man can put on or take off his spectacles. But for its third eyelid, many a bird could not dive from a height.