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Concept
Verre double foyer
Résumé
Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.
Benjamin Franklin is generally credited with the invention of bifocals. He decided to saw his lenses in half so he could read the lips of speakers of French at court, the only way he could understand them. Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between George Whatley and John Fenno, editor of the Gazette of the United States, suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought. Despite this, the College of Optometrists concluded:
Unless further evidence emerges all we can say for certain is that Franklin was one of the first people to wear split bifocals and this act of wearing them caused his name to be associated with the type from an early date. This no doubt contributed greatly to their popularisation. The evidence implies, however, that when he sought to order lenses of this type the London opticians were already familiar with them. Other members of Franklin's circle of British friends may have worn them even earlier, from the 1760s, but it is at best uncertain (and arguably improbable?) that split bifocal lenses had a famous gentleman inventor.
Since many inventions are developed independently by more than one person, it is possible that the invention of bifocals may have been such a case.
John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Benjamin Franklin.
In 1955, Irving Rips of Younger Optics created the first seamless or "invisible" bifocal, a precursor to progressive lenses. This followed Howard D. Beach's 1946 work in "blended lenses", O'Conner's "Ultex" lens in 1910, and Isaac Schnaitmann's single-piece bifocal lens in 1837.
Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper.
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