Leonardo da Vinci was, in 1508, the first to suggest the use of contact lenses to correct defective vision. In Germany, nearly four centuries later, the first contact lens manufactured was a glass shell that covered the entire surface of the eye. Modern contact lenses appeared in the 1940s. Today, many Americans prefer them over eyeglasses for reasons of convenience or cosmetics, although spectacles offer far more protection for the eye.

 The inconvenience of conventional eyeglasses has led to the development of plastic corrective lenses that can be worn under the eyelids, directly over the eyeball. Such contact lenses minimize the danger of breakage that is always present with ordinary glasses, because, like the eye, contact lenses are protected from injury by the shape of the skull.

 Present-day contact lenses cover only the cornea of the eye; a special molding process permits precision fitting to the curvature of the cornea to minimize irritation. So-called soft lenses, now in common use, are made from a soft plastic material that molds itself to the shape of the cornea. Extended-wear contact lenses should be used only following careful consultation with an eye doctor.

 Research has been done with implanted lenses that reshape the cornea to correct focal defects. Another approach is the direct reshaping of the cornea through a surgical procedure called radial keratotomy. Although this operation is coming into increasing use, it can present problems and it has been criticized by a number of physicians.