gh not unlimited, is at least much larger than
seven. Between red and orange, for instance, there are quite a number
of distinguishable orange-reds and reddish oranges.

If now we ask what differences in the stimulus give rise to the three
kinds of difference in visual sensation that were spoken of
previously, we find that color-tone depends on the wave-length of the
light, brightness on the energy of the stimulus, i.e., on the
amplitude of the vibration, and saturation on the mixture of long and
short wave-lengths in a complex light-stimulus--the more mixture, the
less saturation.

These are the general correspondences between the light stimulus and
the visual sensation; but the whole relationship is much more complex.
Brightness depends, not only on the energy of the stimulus, but also
on wave-length. The {214} retina is tuned to waves of medium length,
corresponding to the yellow, which arouse much brighter sensation than
long or short waves of the same physical energy. Otherwise put, the
sensitivity of the retina is greatest for medium wavelengths, and
decreases gradually towards the ends of the spectrum, ceasing
altogether, as has been said, at wavelengths of 760 at the red end and
of 390 at the violet end.

Saturation, depending primarily on amount of mixture of different
wave-lengths, depends also on the particular wavelengths acting, and
also on their amplitude. So, the red and blue of the spectrum are more
saturated than the yellow and green; and very bright or very dim
light, however homogeneous, gives a less saturated sensation than a
stimulus of medium strength.

Color Mixing

Color-tone depends on the wave-length, as has been said, but this is
far from the whole truth; the whole truth, indeed, is one of the most
curious and significant facts about color vision. We have said that
each color-tone is the response to a particular wave-length. But any
color-tone can be got without its particular wave-length being present
at all; all that is necessary is that wave-lengths centering about
this particular one shall be present. A mixed light, consisting of two
wave-lengths, the one longer and the other shorter than the particular
wave which when acting alone gives a certain color-tone, will give
that same color-tone. For example, the orange color resulting from the
isolated action of a wave-length of 650 is given also by the combined
action of wave-lengths of 600 and 700, in amounts suitably
proportioned to