Chapter XIII.

Of Our Sight, and by what Means We See.

Democritus and Epicurus suppose that sight is caused by the insertion of little images into the visive organ, and by the reception of certain rays which return to the eye after meeting the object. Empedocles supposes that images are mixed with the rays of the eye; these he styles the rays of images. Hipparchus, that the visual rays extend from both the eyes to the superficies of bodies, and give to the sight the apprehension of those same bodies, after the same manner in which the hand touching the extremity of bodies gives the sense of feeling. Plato, that the sight is the splendor of united rays; there is a light which reaches some distance from the eyes into a cognate air, and there is likewise a light shed from bodies, which meets and joins with the fiery visual light in the intermediate air (which is liquid and mutable); and the union of these rays gives the sense of seeing. This is Plato's corradiancy, or splendor of united rays.