The human body has evolved in a rough and ready response to environmental demands. The eye is no exception to this rule, for very few people have perfect vision. The many with imperfect vision fall into two general groups, the farsighted who have trouble focusing on near objects (presbyopes) and the nearsighted who have poor vision beyond a very short distance (myopes). Presbyopes require spectacles with convex lenses that curve outwards on both surfaces, myopes need lenses that are concave, or curve inwards. Attempts to correct human vision are as old as human society. The oldest known lens, of polished rock crystal, was found in the ruins of ancient Nineveh. The classical Roman writer Seneca is said to have read all the books in Rome by using a glass globe of water to enlarge the handwritten letters. However, spectacles in some way shaped to sit on the face, (as opposed to eyeglasses, which are held to the eye but are not on the face for long periods), seem to have been an invention of medieval Europe. Over the period from 1450 to 1700 they helped to change the way in which Europeans both perceived the world and operated within it.
The first spectacles contained convex lenses and were only effective for presbyopia or farsightedness, which normally occurs around the age of 40 as the crystalline lens of the eye hardens. They may have developed from "reading stones," segments of glass spheres used by presbyopic monks to read manuscripts by holding the glass against the letters. Convex spectacles seem to have evolved by chance, not through optical theory, although medieval Europe had acquired some scientific knowledge of optics from the Muslim world. The Muslim mathematician and natural philosopher Ibn al-Haitham (c.965-1039), called Alhazen by Europeans, wrote about the properties of lenses in a work translated from Arabic into Latin in 1266. In 1267 the English monk and scientist Roger Bacon (c. 1214-1294) wrote about his experiments in using convex lenses to correct vision, advocating their use to help old people.
Convex spectacles seem to have been invented around 1285. The first reference to spectacles dates from 1289, in a manuscript written about the Popozo family from Tuscany, Italy. The Dominican Friar Giordano da Rivalto of Pisa, in a sermon delivered in Florence in 1305 or 1306, claimed that spectacles had been invented nearly 20 years earlier, and that he had met the unnamed inventor. Historians differ over whether Florence or Pisa was the site of this invention, though it has been considered so important that for centuries patriotic historians of Italian cities have altered manuscripts and invented evidence to claim the prestige of the invention for their city. Venice became an early center of the mass production of lenses. The glass blowers of the Venetian suburb of Murano produced thicker and clearer glass, better for grinding high-quality lenses, than elsewhere in Europe, just as they had produced the best "reading stones." The Venetian crystalworkers' guild laid down regulations for producing "glass discs for the eyes" in 1301, and by c. 1320 a guild of spectacle-makers existed there.
These earliest spectacles had no sidepieces, or temples, the invention of which in 1725-1730 is attributed to Edward Scarlatt of London. Medieval spectacles were riveted at the center and had leather grips to hold on to the bridge of the nose. Some pictures show readers holding spectacles on the face by hand, and some frames were made of leather to reduce their weight. By the 1360s the early Renaissance writer Petrarch could refer to spectacles for the elderly as if they were commonplace in Florence, and in paintings of this period and the fifteenth century they are often included in portraits of saints and scholars to signify piety and learning. By the late fifteenth century their use had spread so far outside the elite that artists increasingly used them to signify folly or senility.
Making concave lensesconcave lenses was more difficult than convex ones, which evolved naturally from the magnification observed through convex reading stones. Any convex lens will magnify, and even inaccurate medieval lenses helped presbyopes. However, concave lenses have an important relationship with literacy, since they enable shortsighted myopes to read even small letters and to write more clearly themselves. The invention of concave lenses is sometimes attributed to Nicholas Krebs (1400?-1464), better known as Nicholas of Cusa, a Cardinal, senior politician, and diplomat of the Roman Catholic Church, who wrote on philosophy, theology, and science. In a treatise of 1450, De beryllo (Concerning Beryl), Nicholas described that semi-precious, sometimes transparent stone, "to which a concave as well as a convex form is given; by looking through it you reach what was previously invisible." Even though beryllo was a Latin word used at the time for spectacles, the stone itself was used as a magnifying glass rather than in spectacles, for which cheaper and lighter concave glass lenses were by then available.
Nicholas, however, may have been aware of the recent development of concave lenses in Florence, through his Florentine friend Paolo Toscanelli (1397-1482). The need for literacy among the merchant and political elite of Florence probably led to the discovery of concave lenses by an anonymous inventor before 1450. Certainly by 1451 Florence had overtaken Venice as the manufacturing center for high-quality spectacles, including concave lenses for myopes, which are first mentioned in a letter of August 25 that year. On October 21, 1462, the Duke of Milan ordered three dozen spectacles from his resident ambassador at Florence, including a dozen with concave lenses for improving distance vision. Although the order was filled within days, suggesting a large-scale manufacture and a stock of lenses ready to be ground by several specialist shops, these were not prescription lenses in the modern sense. However, the Florentines understood that vision declines with age and made convex lenses in different strengths for five-year age groups from 30 years old, and concave in two strengths. Lenses bought by the Duke of Milan were ground for distant, near, and normal sight. The latter suggests that spectacles had become a fashion accessory at the Milanese court, perhaps to make courtiers look more intellectual, and perhaps also because the elderly Duke and his young Duchess both wore glasses with concave lenses for their myopia.
Venice challenged Florence in the large-scale production of quality spectacles, which also spread to Germany and other parts of Europe. However, Florence may have gained its advantage in optical research through the rediscovery in the early fifteenth century of the principles governing linear perspective, by the Florentine artists Filippo Brunelleschi (1377-1446) and Leon Battista Alberti (1407-1472). "Perspective" at this period meant mostly "optics," a branch of mathematics, and Brunelleschi and Alberti used geometry to create the illusion of space and distance on a flat or curved surface. This is also connected with map-making, the perspective projection of the curved surface of Earth onto a flat surface, practiced by Paolo Toscanelli, who was perhaps the source of Nicholas of Cusa's knowledge of Florentine optics.
The invention of concave and convex spectacles aided the revival of learning in the early Renaissance, adding years of eyesight to older readers. The increasing refinement of spectacles at the moment when printing was invented in 1450 may be no coincidence. The rapid spread of printing increased the supply of books and increased the number of readers, enlarging the market for spectacles, encouraging cost cutting and the production of specialized lenses. In turn the availability of spectacles allowed more readers to read small pages of small type, allowing printers to reduce the cost of their books by using less paper. The spread of printing accompanied the spread of spectacles, which raised standards of book and manuscript production by placing more emphasis on fine workmanship, accuracy, neatness, and detail, for example in miniature illustrations in manuscripts.
This carried over into other aspects of European society. Effectively, spectacles doubled the skilled craft workforce by doubling the working life of skilled craftsmen, especially those who did fine jobs—scribes and readers, instrument and toolmakers, metalworkers, and close weavers. The demand for precision-ground lenses encouraged improvements to the basic lathe, as instrument makers required precision parts. So conversely, spectacles pushed Europe towards the invention of precision instruments found nowhere else. Europeans further developed the crude instruments borrowed from other societies to create a range of gauges, micrometers, and many other tools linked to precision measurement and control. This process led to the establishment of a machine-tool industry, of machines to make other machines, and continues in the precisely fitted parts of modern articulated machines. Thus Europe could move to skilled replication, batch and then mass production of identical goods that did not rely on skills learned over long apprenticeship, but on finely made machines.
Spectacles magnified the authority of the eye and allowed people to challenge received authority on the basis that seeing was believing, after overcoming some initial resistance. George Bartisch in the first book on eye diseases, entitled Ophthalmodouleia (The service of the eye, 1583), could not imagine how an imperfect eye could see better through a lens. Contemporary theory held that the eye emitted a visual spirit, which created vision when reflected back from objects. Only after Galileo's (1564-1642) improved telescope of 1609 encouraged Johannes Kepler (1571-1630) to formulate modern optical theory in his book Dioptrice (Concerning measuring altitude, 1611) did the eye become accepted as an optical instrument that received light rays reflected from objects. Thereafter, the cosmic world seen through telescopes, and the microcosmic world seen through microscopes, would be used to develop entirely new systems of scientific knowledge in the seventeenth century.
GLYN PARRY
Derry, T.K., and Trevor I. Williams. A Short History of Technology. Oxford: Oxford University Press, 1960.
Dreyfus, John. Into Print: Selected Writings on Printing History, Typography and Book Production. London: The British Library, 1994.
Landes, David S. The Wealth and Poverty of Nations: Why Some Are So Rich and Some So Poor. New York: W.W. Norton, 1998.
Mumford, Lewis. Technics and Civilization. London, 1934.
Singer, C., E.J. Hall, A.R. Holmyard, and Trevor I. Williams. A History of Technology. 8 vols. Oxford: Oxford University Press, 1954-84, vol. 3 (1957).
Ilardi, Vincent. "Eyeglasses and Concave Lenses in Fifteenth-Century Florence and Milan: New Documents." Renaissance Quarterly 29, no. 4 (Winter 1976): 341-60.
Rosen, Edward. "The Invention of Eyeglasses." Journal of the History of Medicine 2, no. 1 (January 1956): 13-46 and no. 2 (April 1956): 183-218.
The development of sidepieces, or "temples," by 1730 allowed a further change to spectacles in the 1780s. The human eye as it ages adjusts its focus less rapidly when moving from close work to distance vision, and may need a combination of both concave and convex lenses to see distant and near objects properly. However, the eye can adjust to using such lenses, called bifocals, when they are held at sufficient distance from the eyes along the nose by the "temples." The first craftsman to perceive the need for bifocals and to find a solution to their construction was typical of Western civilization in that he was an inveterate scientific experimenter and tinkerer with machinery, always seeking ways to improve upon received knowledge. Realizing that a combination of convex and concave lenses in one spectacle frame would enable his aging eyes both to work close up with machinery and adjust to seeing at a distance, he worked out a way of grinding such lenses on a lathe and putting them into a frame. His name was Benjamin Franklin.
GLYN PARRY
The word "lens" owes its origin to the Latin word for lentils, the tiny beans that have from ancient times been an important ingredient in the cuisine of the Mediterranean region. The convex shape of lentils resulted in thier Latin name being coined for glass possessing the same shape.
Because of the way in which lenses refract light that strikes them, they are used to concentrate or disperse light. Light entering a lens can be altered in many different ways according, for example, to the composition, size, thickness, curvature and combination of the lens used. Many different kinds of lenses are manufactured for use in such devices as cameras, telescopes, microscopes and eyeglasses. Copying machines, image scanners, optical fiber transponders and cutting-edge semiconductor production equipment are other more recent devices in which the ability of lenses to diffuse or condense light is put to use.
The human body has evolved in a rough and ready response to environmental demands. The eye is no exception to this rule, for very few people have perfect vision. The many with imperfect vision fall into two general groups, the farsighted who have trouble focusing on near objects (presbyopes) and the nearsighted who have poor vision beyond a very short distance (myopes). Presbyopes require spectacles with convex lenses that curve outwards on both surfaces, myopes need lenses that are concave, or curve inwards. Attempts to correct human vision are as old as human society. The oldest known lens, of polished rock crystal, was found in the ruins of ancient Nineveh. The classical Roman writer Seneca is said to have read all the books in Rome by using a glass globe of water to enlarge the handwritten letters. However, spectacles in some way shaped to sit on the face, (as opposed to eyeglasses, which are held to the eye but are not on the face for long periods), seem to have been an invention of medieval Europe. Over the period from 1450 to 1700 they helped to change the way in which Europeans both perceived the world and operated within it.
The first spectacles contained convex lenses and were only effective for presbyopia or farsightedness, which normally occurs around the age of 40 as the crystalline lens of the eye hardens. They may have developed from "reading stones," segments of glass spheres used by presbyopic monks to read manuscripts by holding the glass against the letters. Convex spectacles seem to have evolved by chance, not through optical theory, although medieval Europe had acquired some scientific knowledge of optics from the Muslim world. The Muslim mathematician and natural philosopher Ibn al-Haitham (c.965-1039), called Alhazen by Europeans, wrote about the properties of lenses in a work translated from Arabic into Latin in 1266. In 1267 the English monk and scientist Roger Bacon (c. 1214-1294) wrote about his experiments in using convex lenses to correct vision, advocating their use to help old people.
Convex spectacles seem to have been invented around 1285. The first reference to spectacles dates from 1289, in a manuscript written about the Popozo family from Tuscany, Italy. The Dominican Friar Giordano da Rivalto of Pisa, in a sermon delivered in Florence in 1305 or 1306, claimed that spectacles had been invented nearly 20 years earlier, and that he had met the unnamed inventor. Historians differ over whether Florence or Pisa was the site of this invention, though it has been considered so important that for centuries patriotic historians of Italian cities have altered manuscripts and invented evidence to claim the prestige of the invention for their city. Venice became an early center of the mass production of lenses. The glass blowers of the Venetian suburb of Murano produced thicker and clearer glass, better for grinding high-quality lenses, than elsewhere in Europe, just as they had produced the best "reading stones." The Venetian crystalworkers' guild laid down regulations for producing "glass discs for the eyes" in 1301, and by c. 1320 a guild of spectacle-makers existed there.
These earliest spectacles had no sidepieces, or temples, the invention of which in 1725-1730 is attributed to Edward Scarlatt of London. Medieval spectacles were riveted at the center and had leather grips to hold on to the bridge of the nose. Some pictures show readers holding spectacles on the face by hand, and some frames were made of leather to reduce their weight. By the 1360s the early Renaissance writer Petrarch could refer to spectacles for the elderly as if they were commonplace in Florence, and in paintings of this period and the fifteenth century they are often included in portraits of saints and scholars to signify piety and learning. By the late fifteenth century their use had spread so far outside the elite that artists increasingly used them to signify folly or senility.
Making concave lenses was more difficult than convex ones, which evolved naturally from the magnification observed through convex reading stones. Any convex lens will magnify, and even inaccurate medieval lenses helped presbyopes. However, concave lenses have an important relationship with literacy, since they enable shortsighted myopes to read even small letters and to write more clearly themselves. The invention of concave lenses is sometimes attributed to Nicholas Krebs (1400?-1464), better known as Nicholas of Cusa, a Cardinal, senior politician, and diplomat of the Roman Catholic Church, who wrote on philosophy, theology, and science. In a treatise of 1450, De beryllo (Concerning Beryl), Nicholas described that semi-precious, sometimes transparent stone, "to which a concave as well as a convex form is given; by looking through it you reach what was previously invisible." Even though beryllo was a Latin word used at the time for spectacles, the stone itself was used as a magnifying glass rather than in spectacles, for which cheaper and lighter concave glass lenses were by then available.
Nicholas, however, may have been aware of the recent development of concave lenses in Florence, through his Florentine friend Paolo Toscanelli (1397-1482). The need for literacy among the merchant and political elite of Florence probably led to the discovery of concave lenses by an anonymous inventor before 1450. Certainly by 1451 Florence had overtaken Venice as the manufacturing center for high-quality spectacles, including concave lenses for myopes, which are first mentioned in a letter of August 25 that year. On October 21, 1462, the Duke of Milan ordered three dozen spectacles from his resident ambassador at Florence, including a dozen with concave lenses for improving distance vision. Although the order was filled within days, suggesting a large-scale manufacture and a stock of lenses ready to be ground by several specialist shops, these were not prescription lenses in the modern sense. However, the Florentines understood that vision declines with age and made convex lenses in different strengths for five-year age groups from 30 years old, and concave in two strengths. Lenses bought by the Duke of Milan were ground for distant, near, and normal sight. The latter suggests that spectacles had become a fashion accessory at the Milanese court, perhaps to make courtiers look more intellectual, and perhaps also because the elderly Duke and his young Duchess both wore glasses with concave lenses for their myopia.
Venice challenged Florence in the large-scale production of quality spectacles, which also spread to Germany and other parts of Europe. However, Florence may have gained its advantage in optical research through the rediscovery in the early fifteenth century of the principles governing linear perspective, by the Florentine artists Filippo Brunelleschi (1377-1446) and Leon Battista Alberti (1407-1472). "Perspective" at this period meant mostly "optics," a branch of mathematics, and Brunelleschi and Alberti used geometry to create the illusion of space and distance on a flat or curved surface. This is also connected with map-making, the perspective projection of the curved surface of Earth onto a flat surface, practiced by Paolo Toscanelli, who was perhaps the source of Nicholas of Cusa's knowledge of Florentine optics.
The invention of concave and convex spectacles aided the revival of learning in the early Renaissance, adding years of eyesight to older readers. The increasing refinement of spectacles at the moment when printing was invented in 1450 may be no coincidence. The rapid spread of printing increased the supply of books and increased the number of readers, enlarging the market for spectacles, encouraging cost cutting and the production of specialized lenses. In turn the availability of spectacles allowed more readers to read small pages of small type, allowing printers to reduce the cost of their books by using less paper. The spread of printing accompanied the spread of spectacles, which raised standards of book and manuscript production by placing more emphasis on fine workmanship, accuracy, neatness, and detail, for example in miniature illustrations in manuscripts.
This carried over into other aspects of European society. Effectively, spectacles doubled the skilled craft workforce by doubling the working life of skilled craftsmen, especially those who did fine jobs—scribes and readers, instrument and toolmakers, metalworkers, and close weavers. The demand for precision-ground lenses encouraged improvements to the basic lathe, as instrument makers required precision parts. So conversely, spectacles pushed Europe towards the invention of precision instruments found nowhere else. Europeans further developed the crude instruments borrowed from other societies to create a range of gauges, micrometers, and many other tools linked to precision measurement and control. This process led to the establishment of a machine-tool industry, of machines to make other machines, and continues in the precisely fitted parts of modern articulated machines. Thus Europe could move to skilled replication, batch and then mass production of identical goods that did not rely on skills learned over long apprenticeship, but on finely made machines.
Spectacles magnified the authority of the eye and allowed people to challenge received authority on the basis that seeing was believing, after overcoming some initial resistance. George Bartisch in the first book on eye diseases, entitled Ophthalmodouleia (The service of the eye, 1583), could not imagine how an imperfect eye could see better through a lens. Contemporary theory held that the eye emitted a visual spirit, which created vision when reflected back from objects. Only after Galileo's (1564-1642) improved telescope of 1609 encouraged Johannes Kepler (1571-1630) to formulate modern optical theory in his book Dioptrice (Concerning measuring altitude, 1611) did the eye become accepted as an optical instrument that received light rays reflected from objects. Thereafter, the cosmic world seen through telescopes, and the microcosmic world seen through microscopes, would be used to develop entirely new systems of scientific knowledge in the seventeenth century.
GLYN PARRY
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Derry, T.K., and Trevor I. Williams. A Short History of Technology. Oxford: Oxford University Press, 1960.
Dreyfus, John. Into Print: Selected Writings on Printing History, Typography and Book Production. London: The British Library, 1994.
Landes, David S. The Wealth and Poverty of Nations: Why Some Are So Rich and Some So Poor. New York: W.W. Norton, 1998.
Mumford, Lewis. Technics and Civilization. London, 1934.
Singer, C., E.J. Hall, A.R. Holmyard, and Trevor I. Williams. A History of Technology. 8 vols. Oxford: Oxford University Press, 1954-84, vol. 3 (1957).
Ilardi, Vincent. "Eyeglasses and Concave Lenses in Fifteenth-Century Florence and Milan: New Documents." Renaissance Quarterly 29, no. 4 (Winter 1976): 341-60.
Rosen, Edward. "The Invention of Eyeglasses." Journal of the History of Medicine 2, no. 1 (January 1956): 13-46 and no. 2 (April 1956): 183-218.
The development of sidepieces, or "temples," by 1730 allowed a further change to spectacles in the 1780s. The human eye as it ages adjusts its focus less rapidly when moving from close work to distance vision, and may need a combination of both concave and convex lenses to see distant and near objects properly. However, the eye can adjust to using such lenses, called bifocals, when they are held at sufficient distance from the eyes along the nose by the "temples." The first craftsman to perceive the need for bifocals and to find a solution to their construction was typical of Western civilization in that he was an inveterate scientific experimenter and tinkerer with machinery, always seeking ways to improve upon received knowledge. Realizing that a combination of convex and concave lenses in one spectacle frame would enable his aging eyes both to work close up with machinery and adjust to seeing at a distance, he worked out a way of grinding such lenses on a lathe and putting them into a frame. His name was Benjamin Franklin.
GLYN PARRY
The word "lens" owes its origin to the Latin word for lentils, the tiny beans that have from ancient times been an important ingredient in the cuisine of the Mediterranean region. The convex shape of lentils resulted in thier Latin name being coined for glass possessing the same shape.
Because of the way in which lenses refract light that strikes them, they are used to concentrate or disperse light. Light entering a lens can be altered in many different ways according, for example, to the composition, size, thickness, curvature and combination of the lens used. Many different kinds of lenses are manufactured for use in such devices as cameras, telescopes, microscopes and eyeglasses. Copying machines, image scanners, optical fiber transponders and cutting-edge semiconductor production equipment are other more recent devices in which the ability of lenses to diffuse or condense light is put to use.
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