He invented bifocal spectacles. Writing in August 1784 to his friend George Whately, Franklin expressed great personal pleasure in the “invention of Double Spectacles, which, serving for distant objects as well as near ones, make my eyes as useful to me as ever they were.” Whately asked for more information. “The same convexity of glass through which a man sees clearest and best at the distance proper for reading is not the best for greater distances,” Franklin replied. “I therefore had formerly two pair of spectacles, which I shifted occasionally, as in traveling I sometimes read, and often wanted to regard the prospects. Finding this change troublesome, and not always sufficiently ready, I had the glasses cut and half of each kind associate in the same circle. By this means, as I wear my spectacles constantly, I have only to move my eyes up or down, as I wanted to see distinctly far or near, the proper glasses being always ready.” (Dorling Kindersley/UIG/Bridgeman Images)