Eyeglasses are corrective lenses mounted in frames that help those with vision problems see clearly. The lenses are shaped in order to bend light rays so that they will focus at the back of the eye, the retina. Some people who can see distant objects clearly but to whom near objects look blurry suffer from hyperopia or farsightedness. As explained by Franciscus Donders (1818-1889), a Dutch physiologist, the cause of farsightedness is that the eyeball is too shallow and that the image actually focuses beyond the eye. To correct hyperopia, convex corrective lenses are used to make the light rays converge or come together on the retina. Some people suffer from myopia or nearsightedness, in which the image is focused in front of the retina so that only near objects can be seen clearly. Concave lenses can be worn to diverge the light rays and permit light from...