ABSTRACT
Theories of vision were advanced at a very early date. Amongst the ancients, an "emission" theory was generally favoured; according to this, vision was caused by something flowing out from the eye to the object viewed, so that the sensation of sight was comparable to that of touch. There were certain modifications of this theory; Empedocles (c. 450 B.C.) for example, tried to explain vision by recourse to the doctrine of "ocular beams", which supposed a dual emission of particles from the eye and from the object itself; these two streams on meeting excited the sensation of vision. In Plato three elements were deemed necessary. First a visual stream of light or divine fire was believed to be emitted from the eye itself; this combined with the light of the sun and these two meeting with an emanation from the object viewed completed the act of seeing. The doctrine of ocular beams and emission theories was opposed by Aristotle (384-322 B.C.) who maintained that light was due to a pressure, or an action transmitted from the object to the eye. Such a belief postulated the existence of some medium between the two by means of which this action could be transmitted; a void, even if it existed (and Aristotle denied this) would prove impenetrable to light. In some respects Aristotle's views on the nature of light approached more closely to the theories now current than did those generally accepted in the Middle Ages; Poggendorff, in his comprehensive study of the history of physics is of the opinion that Aristotle's conception of the nature of light bears so striking a resemblance to the theories current at the end of the nineteenth century that anyone unfamiliar with the latter could get a very presentable picture of them from the writings of Aristotle*. But in spite of the powerful influence which the great Stagyrite philosopher exerted upon ancient and mediaeval thought, the doctrine of ocular beams in one fonn or another persisted until nearly the close of the sixteenth century.
