Today in 1785, Benjamin Franklin publicly announced the invention of the bifocal lens. Bifocals are eyeglasses that are made with two different optical powers rather than just one. Usually this second lens is near the bottom of the larger lens, allowing wearers to simply look down through it to read. Currently in development are lenses filled with liquid crystal, which will allow users to switch their entire lens between different optical powers on the fly.
Image Credit: Pindaddy