a black-and-white or colored figure
for as long as fifteen or twenty seconds, and then directing the eyes
upon a medium gray background. After a moment a sensation develops in
which black takes the place of white and white of black, while for
each color in the original sensation the complementary color now
appears.

[Illustration: Fig. 39.--The visual response outlasts the stimulus.
The progress of time is supposed to be from left to right in the
diagram. After the stimulus ceases, the sensation persists for a time,
at first as a positive after-image, and then as a negative
after-image, a sort of back swing. (Figure text: stimulus, sensory
response)]

This phenomenon of the negative after-image is the same as that of
color adaptation. Exposing the retina for some time to light of a
certain color adapts the retina to that color, bleaches that color
sensation, and, as it were, subtracts that color (or some of it) from
the gray at which the eyes are then directed; and gray (or white)
minus a color gives the complementary color.

Contrast

Contrast is still another effect that occurs in other senses, but most
strikingly in vision. There is considerable in common between the
negative after-image and contrast; indeed, {228} the negative
after-image effect is also called "successive contrast". After looking
at a bright surface, one of medium brightness appears dark, while this
same medium brightness would seem bright after looking at a dark
surface. This is evidently adaptation again, and is exactly parallel
to what was found in regard to the temperature sense. After looking at
any color steadily, the complementary color appears more saturated
than usual; in fact, this is the way to secure the maximum of
saturation in color sensation. These are examples of "successive
contrast".

"Simultaneous contrast" is something new, not covered by adaptation,
but gives the same effects as successive contrast. If you take two
pieces of the same gray paper, and place one on a black background and
the other on white, you will find the piece on the black ground to
look much brighter than the piece on the white ground. Spots of gray
on colored backgrounds are tinged with the complementary colors. The
contrast effect is most marked at the margin adjoining the background,
and grows less away from this margin. Any two adjacent surfaces
produce contrast effects in each other, though we usually do not
notice them any more than we usua