Test-Tube Vision

The human eye, says Biology Professor George Wald of Harvard, is like a camera, with a slow film for bright light and a fast film for dim light. At a Cleveland meeting of the Society of Biological Chemists, Dr. Wald told how he and two associates have duplicated in a test tube the action of the eye's fast film.

When the light is reasonably bright, the eye sees by means of millions of microscopic "cones" of the retina. As the light dims, the cones go out of action. Tiny "rods," which are much more sensitive, take over their duties. Only one quantum* of light is needed to make a rod tell the brain that it is seeing something.

Scientists have known for a long time that the photochemical action in the rods is connected in some way with a red substance, rhodopsin, which forms in the rods when the light gets dim. This is how eyes become "dark-adapted." Only when their rods are well fortified with rhodopsin can they respond to faint glimmers of light.

To find out how rhodopsin works, Biologist Wald extracted a protein called opsin from the eyes of freshly killed animals and mixed it with vitamin A and two enzymes (organic catalysts): alcohol oxidase (from horse livers) and cozymase (from yeast). When this mixture is placed in the dark, the enzymes convert the vitamin A to retinene, a yellow pigment. Then the retinene combines with the opsin to form bright red rhodopsin.

When the mixture is exposed to light, even very faint light, all the chemical processes go into reverse. The rhodopsin divides into retinene and opsin. The retinene reverts to vitamin A. This is just what happens when light shines into a dark-adapted eye: the rhodopsin in the rods is suddenly decomposed.

Dr. Wald is not quite sure how the destruction of rhodopsin by light produces nerve impulses that the brain interprets as vision. But since the impulses are electrical, he suspects that they may be started by some electrical consequence of the destruction of the rhodopsin. One possibility: that sulfhydryl (sulphur and hydrogen) groups, are exposed when the rhodopsin molecules disintegrate. These are lively chemicals with the necessary punch to start an electrical commotion.

Next project for Dr. Wald and his group will be to make their chemical mixture produce enough electricity to excite a nerve. If they accomplish this, they will have constructed something close to a completely man-made eye.

*The smallest possible quantity of each wave length of light.

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