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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.
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