Light and Vision

Abstract

THE old Greek philosophers who did so much thinking and so little experimenting had queer ideas about light and vision. Empedokles, who died about 420 B.C., considered it necessary to record the fact that darkness is not a real thing, but privation of light; and that the moon shines with reflected light, but he thought that the sun is the primary fire of the light of the sky reflected in a crystalline spheroid. Demo-critus, who died about 370 B.C., held that vision was to be explained by emanations or exceedingly thin husks or films which were continually being detached or thrown off from the surface of bodies, and that they penetrated into the sense-organs through fine passages or pores. We admit this in the case of taste and of smell. These ghost-like forms or images were called eidola (618″Xa), whence we have the word idol (a very different kind of image from those considered in optical books), and were supposed to be ever passing from the object to the moist and receptive surface of the eye straight into the mind. Aristotle, who died about 325 B.C., seems to have objected to some of the earlier theories. He scarcely alludes to light and vision in “De Physica,” but there is some reason to suppose that a treatise by him on optics has been lost. More than two centuries later Lucretius, the scientific poet, discussed the theory at great length in the fourth book of “De Natura Rerum.” He used the expression simulacra quasi membranae, resemblances like films, peeled off from the upper surface of things, flying hither and thither on one side and the other through the air. Simulacra was also used for gliosts, and he goes on to explain how they terrify us in sleep. He also attempted to explain the action of curved mirrors, of the distance of the image behind a mirror, and why the theory does not work in the dark.