His explanation does not
depend upon mere hypothesis or supposition, but is plainly based upon
anatomical investigation as well as on geometrical discussion.
[Sidenote: Determines the function of the retina.] He determines that
the retina is the seat of vision, and that impressions made by light
upon it are conveyed along the optic nerve to the brain. Though it might
not be convenient, at the time when Alhazen lived, to make such an
acknowledgment, no one could come to these conclusions, nor, indeed,
know anything about these facts, unless he had been engaged in the
forbidden practice of dissection. [Sidenote: Explains single vision.]
With felicity he explains that we see single when we use both eyes,
because of the formation of the visual images on symmetrical portions of
the two retinas. To the modern physiologist the mere mention of such
things is as significant as the occurrence of an arch in the interior of
the pyramid is to the architect. But Alhazen shows that our sense of
sight is by no means a trustworthy guide, and that there are illusions
arising from the course which the rays of light may take when they
suffer refraction or reflexion. It is in the discussion of one of these
physical problems that his scientific greatness truly shines forth.
[Sidenote: Traces the course of a ray of light through the air.] He is
perfectly aware that the atmosphere decreases in density with increase
of height; and from that consideration he shows that a ray of light,
entering it obliquely, follows a curvilinear path which is concave
toward the earth; and that, since the mind refers the position of an
object to the direction in which the ray of light from it enters the
eye, the result must be an illusion as respects the starry bodies; they
appear to us, to use the Arabic term, nearer to the _zenith_ than they
actually are, and not in their true place. [Sidenote: Astronomical
refraction.] We see them in the direction of the tangent to the curve of
refraction as it reaches the eye. Hence also he shows that we actually
see the stars, and the sun, and the moon before they have risen and
after they have set--a wonderful illusion. He shows that in its passage
through the air the curvature of a ray increases with the increasing
density, and that its path does not depend on vapours that chance to be
present, but on the variation of density in the medium. [Sidenote: The
horizontal sun and moon.] To this refraction he truly refers the
shor