Bifocals are eyeglasses that have two distinct optical powers in one lens. The top part of the lens corrects distance vision, while the bottom part of the lens corrects near vision. The bottom part of the lens can be one of several shapes, including a half-moon, round segment, or narrow rectangular area. Bifocals were first invented by Benjamin Franklin, who glued the top half of one lens to the bottom half of another. Modern bifocal lenses have evolved into more advanced types, including flat-top bifocals and no-line or progressive lenses. Flat-top bifocals have a large area devoted to distance vision, with a small insert or segment for near vision. No-line or progressive lenses have a smooth change in focus from the top of the lens (distance) to the bottom of the lens (reading distance) . Bifocals are commonly prescribed to people over the age of 40 who have presbyopia, which is the natural degradation of vision due to the aging process.