satisfactory theory of vision. Hero and Ptolemy both beli~ved that vision was produced by the emission of light from the eyes, but their theory did not provide a reasonable explanation of perspective, the effect whereby the apparent size of an object depends upon its distance from the observer. As we know today, and as Ibn al-Haytham understood in the eleventh century, vision results from light being reflected into the eye from the object observed, an idea that explains perspective. He correctly regarded the eye as an intercepting screen, comparable to those we use today to show movies or slides. When his revolutionary ideas on perspective passed into Europe during the Renaissance, they influenced the development not only of science but also of art. The use of improved knowledge of perspective to give a feeling of depth and movement became strikingly visible in the works of Italy's new school of painters, the Perspectivi, around 1500.

Furthermore, Ibn al-Haytham appreciated that an explanation of vision must take into account not only such physical factors as light, screens, lenses and so on, but also anatomical and psychological factors, and he realized that the eye must function in a manner consistent with the laws of optics.
Ibn al-Haytham proved that the perception of an image occurs not in the eyes but in the brain and that the location of an image is largely determined by psychological factors. Like Newton, Ibn alHaytham considered the problem of why a visual image produced within the brain is perceived as if it were located at some distance from the viewer, is the actual position of the object which produced it. Even today, most people do not find this surprising, although it is quite remarkable that images of the objects we see do not appear to be inside the head, here they actually exist, since they are simply electro~hemical versions of the scene inside the brain.

Ibn al-Haytham was aware of an even more subtle aspect of vision, namely, that when we see an object the brain automatically performs a memory retrieval procedure to see if it recognizes that object. The signals ultimately produced within the brain by light entering the eye cannot tell us that what we see is, for example, a loafofbread. Almost instantly, the brain scans its memory and compares the new information it has received through the eyes with data it has stored over the years. Ibn al-Haytham called this function of the brain "the distinguishing faculty" and realized that it is intimately tied to the entire process of seeing.

  That someone in the eleventh century realized that such complex questions existed is in itself noteworthy, but Ibn al-Haytham did not merely raise them, he attempted to provide answers. Explanations of these phenomena required him to construct a psychological theory of vision at a time when psychology was not recognized as a field of study. These ideas were quite different from the notions held by the Greeks and even by other contemporary Arab scientists.

The manner in which Ibn al-Haytham presented his theories in his Book of Optics is extremely interesting to the historian of science. He was both a mathematician and an experimenter, which allowed him to present his arguments with a power unmatched by previous scientists who rarely had experimental evidence to back up their assertions. Here lies another parallel between Newton and Ibn al-Haytham: they were both mathematicians and experimenters who made significant contributions


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