Figure 14.1. Cross section of the human eyeball, our organ of vision. The
muscles of the iris regulate the size of the pupillary opening. The interior of
the eyeball is filled with a transparent gelatinous fluid called vitreous humor.
Light coming from the center of the visual field (what we are
looking directly at) is focused by the lens onto the center of the retina,
a region called the fovea (Latin for “pit”). In the fovea the density of
photoreceptor cells is highest, so visual acuity (ability to see fine de-
tail) is best for the region of space where we are directly looking.
The human retina contains two major kinds of photoreceptor
cells: rods and cones. Rod cells are rod shaped, very numerous, dis-
tributed throughout most of the retina, and sensitive to even very
small amounts of light. Cone cells are cone shaped, mostly located at
the fovea, and respond to higher intensity rather than very dim light.
In the human eye there are three types of cone cells, with each type