Illusion - Is Seeing Really Believing?/Illusion - Is Seeing Really Believing (1998)(Marshall Media)[Mac-PC].iso/mac/ILLUSION/SROCK_TX.CXT/00044_Text_rel01t.txt

Text File | 1996-12-31 | 2KB | 64 lines

    The experiment Descartes
described depends on
understanding how  the lens
of an eyeΓÇöΓÇôor any convex
lensΓÇöΓÇôcreates an image.  The
lens, when it is shaped for
optimal focus, bends, or
refracts, all the rays of light
emanating from the same source
to focus them at a single point.
From any point on an object,
light is focused by the lens on a
single point on the retina.  In
this way, rays emanating from
the outline of an object are
focused on the retina so as to
form a corresponding outline
although, as we shall see, that
outline will be different from
the object's outline ΓÇöΓÇô distorted,
one might say ΓÇöΓÇô and will be
different from time to time as
well.

    Because we are concerned
with perceptual constancy in
this chapter, not the sharpness
of an image (which affects
acuity), we can disregard
variations in the shape of the
lens and the action of the
cornea, the eye's transparent
outer covering which
cooperates with the lens to
focus light, and assume that
only one ray of light emanates
from any point of an object. For
our purposes, then, the image
on the retina is equivalent to
that formed on a screen in a
camera obscura. A camera
obscura consists of a pinhole
opening, instead of the larger,
variable aperture of the eye or
an ordinary camera, and a
screen for direct viewing,
rather than the retina or the
chemically treated, light-
sensitive surface of film.
Because the pinhole is small,
for all intents and purposes we
can say that only one ray of
light passes through it from any
point in a scene. Thus, the
problem of focusing does not
arise. The shape of the screen
of the camera obscura is flat,
not spherical as it is in the eye.
However, this difference does
not affect the formation of an
image or the problem of
constancy under discussion.