Materials Science of Vision Correction: Glasses and Sunglasses
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Materials Science of Vision Correction: Glasses and Sunglasses Glass lenses have been used to improve eyesight since at least the 13th century when Roger Bacon, an English philosopher and scientist, mentioned their use in a manuscript written in 1267. Concave lenses provide correction for nearsightedness, while convex lenses correct farsightedness. By the 17th century, opticians in London had developed an iron tool with convex and concave sides for grinding and polishing lenses of optical (clear) glass. A glass "blank" was glued temporarily to the convex side, then covered with an abrasive liquid such as emery and water and ground down by hand with the concave side. Final polishing was carried out with the concave side covered by a wool cloth and a powder of calcined tin. But lenses prepared by this technique were of uneven quality and had to be produced one at a time by skilled opticians. The Worshipful Company of Spectacle Makers, founded in London in 1629, was a guild to which most opticians belonged and set standards for the quality of lenses. John Marshall (71659-1725), who served as optician to King George I, revolutionized the manufacture of spectacles by developing an improved grinding process that could produce uniformly ground lenses in batches. Instead of using optical glass, he used "crystal-glass," also known as agate, a form of quartz with small grain size. Agate lenses proved to be harder than those made of optical glass. Marshall fashioned his grinding tools from brass rather than iron because brass could take a smoother polish and in turn produce higher quality polished lenses. The new tools held four blanks that could be ground simultaneously (still by hand) to identical focal length. As a result of these improvements, spectacles both increased in quality and became more widely available at lower cost. By the 19th century, a lathe was used to turn the grinding tool. Though many additional improvements have followed, the batch method is still in use in the modern era. Until the 18th century, spectacles were held to the face by a spring-loaded grip on the bridge of the nose. In French, spectacles are still known as "pince-nez," or pinch-nose. Alternatively, lenses were sometimes mounted in a frame that could be held up to the eyes when needed. Around 1725, British optician Edward Scarlatt invented temples, the hinged portion of modern eyeglasses that rest over the ears. Many people find they need different
MRS BULLETIN/JULY 1997
lenses for near and distant focus, particularly as they reach middle age. In the late 1700s, Benjamin Franklin became frustrated by his frequent need to switch glasses and tried slicing his reading lenses and distance lenses in half, then gluing half of each reading lens below half of each distance lens. The result was the first pair of bifocals. Further development took place early in the 20th century, when John Borsch, Jr. cut a segment out of a distance lens and filled it with a segment of a reading lens, then heated the two sections to join them together. Later it beca
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