George Wald (1906-1997), American biochemist. Wald studied rhodopsin (visual purple), the light- sensitive pigment in the rod cells of the eye's retina. Rods are nerve cells involved in night vision. In 1956, Wald showed that rhodopsin cons- ists of the protein opsin and a particular form of retinal, a derivative of vitamin A (retinol). He found that light causes the retinal to change form and separate from the opsin, resulting in a nerve signal passing to the brain. The retinal is converted into vitamin A and then slowly back to rhodopsin. Wald linked vitamin A deficiency to night blindness and retinopathy. He shared the 1967 Nobel Prize for physiology or medicine.