One reason for accepting outward-moving influences was the belief in the evil eye, whereby some people could allegedly harm others by looking at them with envy or other negative emotions. Nevertheless, some atomists admitted that influences could move both ways, not just into the eyes, but also outwards from the looker. 6 Intromission alone made vision into a passive process, and ignored the active role of attention. The intromission theory, ‘sending in’ – proposed by atomists such as Democritus and Lucretius and the Arab scholar Alhazen – suggested that vision was as a result of various kinds of substances travelling into the eye, with nothing coming out of the eye. He also clearly stated the principles of mirror reflection, recognizing the equality of what we now call the angles of incidence and reflection, and he explained virtual images in terms of the movement of visual rays outwards from the eyes. 4 Euclid recognized that light played a part in vision, but he said very little about the way it was related to the visual rays projecting outwards from the eyes. Euclid suggested that as a result of an active process of looking and finding there is a change in what is seen, even though the light entering the eye remains the same. This theory argues that vision is not merely passive. 3 We look at things, and can decide where to direct our attention. The extramission theory, which literally means ‘sending out’ - proposed by the Pythagorean School, Euclid and Ptolemy - suggested that a visual current was projected outwards from the eye, with sight proceeding from the eyes to the object seen.
1 For over two thousand years there were four main theories: the intromission theory, the extramission theory, theories combining intromission and extramission, and theories about the medium through which vision took place. This debate continued in the Arab world and in Europe in the Middle Ages. In the ancient world there was a long-running discussion about the nature of vision. Interconnections Between the Observer and the Observed in Quantum Physics.Extramission Theories in Science and Popular Belief.
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