ating the
disks. The ordinary blue and yellow pigments give green when mixed,
because each of the two pigments contains green. The blue and yellow
neutralize each other, leaving green.

_Visual After-Images._ The stimulation of the retina has interesting
after effects. We shall mention here only the one known as _negative
after-images_. If one will place on the table a sheet of white paper,
and on this white paper lay a small piece of colored paper, and if he
will then gaze steadily at the colored paper for a half-minute, it will
be found that if the colored paper is removed one sees its complementary
color. If the head is not moved, this complementary color has the same
size and shape as the original colored piece of paper. The negative
after-image can be projected on a background at different distances, its
size depending on the distance of the background. The after-image will
be found to mix with an objective color in accordance with the
principles of color-mixture mentioned above.

After-image phenomena have some practical consequences. If one has been
looking at a certain color for some time, a half-minute or more, then
looks at some other color, the after-image of the first color mixes with
the second color.

_Adaptation._ The fact last mentioned leads us to the subject of
adaptation. If the eyes are stimulated by the same kind of light for
some time, the eyes become adapted to that light. If the light is
yellow, at first objects seem yellow, but after a time they look as if
they were illuminated with white light, losing the yellow aspect. But if
one then goes out into white light, everything looks bluish. The
negative after-image of the yellow being cast upon everything makes the
surroundings look blue, for the after-image of yellow is blue. All the
other colors act in a similar way, as do also black and white. If one
has been for some time in a dark room and then goes out to a lighter
place, it seems unusually light. And if one goes from the light to a
dark room, it seems unusually dark.

=Hearing or Audition.= Just as the eye is an organ sensitive to certain
frequencies of ether vibration, so the ear is an organ sensitive to
certain air vibrations. The reader should familiarize himself with the
physiology of the ear by reference to physiologies. The drum-skin, the
three little bones of the middle ear, and the cochlea of the inner ear
are all merely mechanical means of making possible the stimulation of
the