The ability to focus clearly on nearby objects— accommodation—decreases with age. This is known as presbyopia. It explains why older people tend to hold books at arm's length. Bifocals are glasses incorporating two lenses for each eye. The lower section of each glass corrects for presbyopia, bringing nearby objects into focus.
American statesman Benjamin Franklin (1706- 1790) is normally credited with the invention of bifocals—although no one knows for sure who devised them, or when they did so. Franklin, and a few others, may have been wearing bifocals since the 1760s, but the first mention he makes of his "double spectacles" is in a letter dated August 21,1784. Franklin certainly did much to publicize bifocals and, as a man of science—and of poor eyesight—he certainly understood the principles behind their function.
Each "lens" of Franklin's bifocal glasses consisted of two distinct pieces of glass, but toward the end of the nineteenth century Louis de Wecker (1832-1906) devised a way to fuse the two lenses into one. The term "bifocals" was actually introduced by John Isaac Hawkins (1772-1854) in 1826 to distinguish them from glasses he introduced that incorporated three distinct lenses. Hawkins's "trifocals" overcame a problem faced by older people who wear bifocals: while distant and nearby objects are in focus, vision in the middle distance range suffers.
Today, glasses are available with a much wider range of focal lengths, not just two or three. These "varifocals," first seen in the late 1950s, offer a smooth transition from one focal length to another.
