ish light of most artificial illuminants),
and by degrees the color sensation bleaches out so that the light
appears nearly white.

Dark adaptation is equivalent to sensitizing the retina for faint
light. Photographic plates can be made of more or less sensitiveness
for use with different illuminations; but the retina automatically
alters its sensitivity to fit the illumination to which it is exposed.

{226}

Rod and Cone Vision

You will notice, in the dark room, that while you see light and shade
and the forms of objects, you do not see colors. The same is true out
of doors at night. In other words, the kind of vision that we have
when the eye is dark-adapted is totally color-blind. Another
significant fact is that the fovea is of little use in very dim light.
These facts are taken to mean that dim-light vision, or _twilight
vision_ as it is sometimes called, is _rod vision_ and not cone
vision; or, in other words, that the rods and not the cones have the
great sensitiveness to faint light in the dark-adapted eye. The cones
perhaps become somewhat dark-adapted, but the rods far outstrip them
in this direction. The fovea has no rods and hence is of little use in
very faint light. The rods have no differential responsiveness to
different wave-lengths, remaining still in the "first stage" in the
development of color vision, and consequently no colors are seen in
faint light.

Rod vision differs then from cone vision in having only one response
to every wave-length, and in adapting itself to much fainter light. No
doubt, also, it is the rods that give to peripheral vision its great
sensitivity to moving objects.

After-Images

After-images, which might better be called after-sensations, occur in
other senses than sight, but nowhere else with such definiteness. The
main fact here is that the response outlasts the stimulus. This is
true of a muscle, and it is true of a sense organ. It takes a little
time to get the muscle, or the sense organ, started, and, once it is
in action, it takes a little time for it to stop. If you direct your
eyes towards the lamp, holding your hand or a book in front of them as
a screen, remove the screen for an {227} instant and then replace it,
you will continue for a short time to see the light after the external
stimulus has been cut off. This "positive after-image" is like the
main sensation, only weaker. There is also a "negative after-image",
best got by looking steadily at