Eyeglasses and contact lenses
Eyeglasses are corrective lenses mounted in frames that help those with vision problems see clearly. The lenses are shaped in order to bend light rays sothat they will focus at the back of the eye, the retina. Some people who cansee distant objects clearly but to whom near objects look blurry suffer fromhyperopia or farsightedness. As explained by Franciscus Donders (1818-1889),a Dutch physiologist, the cause of farsightedness is that the eyeball is tooshallow and that the image actually focuses beyond the eye. To correct hyperopia, convex corrective lenses are used to make the light rays converge or come together on the retina. Some people suffer from myopia or nearsightedness, in which the image is focused in front of the retina so that only nearobjects can be seen clearly. Concave lenses can be worn to diverge the lightrays and permit light from far away objects to focus directly on the retina.A condition called presbyopia occurs when the lens of the eye loses it elasticity and it can no longer change shape. The condition is usually associatedwith age and becomes evident after 40. Presbyopia causes people to be somewhat farsighted. Sometimes this is corrected by wearing bifocals, or eyeglassesthat have a second lens below the top lens. A person with presbyopia can lookthrough the bottom lens while reading and use the top lens for distant objects.
The invention of eyeglasses has a long and colorful history. It is said thatduring the days of the Roman Empire, the emperor Nero watched exhibitions inthe Coliseum holding a jewel with curved facets up to one eye, but this cannot be verified. However, Roger Bacon, an English scholar, is said to have suggested the use of eyeglasses in the 1200s. An Italian physicist, Salvino degliArmati probably invented eyeglasses in around 1285. He shared the design ofhis new device with an Italian monk, Allesandro della Spina, who made publicthe invention and is often given credit for inventing eyeglasses.
In the 14th century Venetian craftsmen, known for their work in glass, were making "disks for the eyes." The finely ground glass disks were given the namelenses by the Italians because of their similarity in shape to lentils. Forhundreds of years thereafter, lenses were called glass lentils. The earliestlenses were convex--that is they bulged outward in the middle and aided people who were far-sighted. Wearing spectacles became common enough so that a 1352 portrait of St. Hugh showed him wearing them, although he had died some hundred years before. By the fifteenth century, eyeglasses had found their way to China. But long before, Chinese judges had worn smoky quartz spectacles, but it is thought they were worn so the judges could remain impartial and not show expression in their eyes when they heard cases rather than correct vision.
In 1451, Nicholas of Cusa in Germany invented eyeglasses to correct nearsightedness using concave lenses. Rather than bulging in the middle like convex lenses, concave lenses are thinner at the center and thicker at the ends. PopeLeo X was one of the first to wear them.
Early eyeglasses had glass lenses mounted on heavy frames of wood, lead or copper. Natural materials of leather, bone and horn were later used and then lighter frames of steel were made by the early seventeenth century. Tortoiseshell frames came into use in the eighteenth century. In 1746, a French opticiannamed Thomin invented actual eyeglass frames that could be placed over the ears and nose.
In the United States, Benjamin Franklin, statesman and scientist, designed the first bifocals in 1760. In this way he could use the top lens to see distant objects and peer down into the bottom lens when he read without needing twopair of glasses. The two lenses were joined in a metal frame. In England in1827, Sir George Biddle Airy (1801-1892), an English astronomer and mathematician made the first glasses to correct astigmatism, a condition he himself had. Astigmatism is blurry vision caused by irregular curves in the cornea, the transparent covering of the eye. The irregular curvature makes it impossible for light rays to focus on a single point. To correct this, the exactarea of the irregularity of the cornea is located, and a corresponding areaon the eyeglass lens is ground to bring light rays passing through that areainto proper focus.
Today eyeglasses come in a wide array of styles and designs. Frames are generally made of metal or plastic, and lenses are made of glass or plastic. Polycarbonate is a type of very hard plastic used for lenses. Robust and lightweight, it is scratch-resistant and provides 100% protection from ultraviolet radiation that can contribute to cataract formation. Along with lightweight frames, such lenses are commonly used in sports glasses and childrens' glasses.
In 1955 the first unbreakable lenses were made and in 1971 a new lens came out which combined the properties of plastic with glass. During the 1950s the Varilux was invented, corrective lenses of variable strength that can be usedin place of bifocals. Testing the eyes for visual acuity and examining the eye with a retinoscope are routine before determining the strength, or refractive index, of the lenses prescribed.
Eyewear has been revolutionized with the invention of the contact lens, corrective lenses without the frames, which put a tiny corrective lens directly onthe cornea of the eye. However the idea dates back to Leonardo da Vinci whodescribed a way of correcting vision using a water-filled tube, and to a number of scientists who experimented with layers of gelatin to correct vision during the seventeenth and eighteenth centuries. Contact lenses were first madein Europe near the turn of the twentieth century using glass. In 1936 IG Farben, a German company, made the first contact lens from Plexiglas--still usedtoday for "hard" contact lenses. An American inventor named Tuohy began useof a lens that covers only the cornea and in 1964 a Czech named Wichterle made the first flexible or "soft" lens. Today there is an array of hard and softlenses, extended wear lenses, tinted lenses, lenses that can correct astigmatism, and lenses to correct color blindness.