problem now is to identify the elements. Notice
that we are not seeking for the physical elements of light, nor for
the primary pigments of the painter's art, but for the elementary
_sensations_. Our knowledge of physics and painting, indeed, is likely
to lead us astray. Sensations are our responses to the physical
stimulus, and the psychological question is, what fundamental
responses we make to this class of stimuli.

Suppose, without knowing anything of pigments or of the physics of
light, we got together a collection of bits of color of every shade
and tint, in order to see what we could discover about visual
sensations. Leaving aside the question of elements for the moment, we
might first try to _classify_ the bits of color. We could sort out a
pile of reds, a pile of blues, a pile of browns, a pile of grays,
etc., but the piles would shade off one into another. The salient fact
about colors is the gradual transition from one to another. We can
arrange them in _series_ better than we can classify them. They can be
serially arranged in three different ways, according to brightness or
intensity, according to color-tone, and according to saturation.

The _intensity series runs from light to dark_. We can arrange such a
series composed entirely of reds or blues or any other one color; or
we can arrange the whole collection of bits of color into a single
light-dark series. It is not always easy to decide whether a given
shade of one color is lighter or darker than a given shade of a
different color; but in a rough way, at least, every bit of whatever
color would have its place in the single intensity series. An
intensity series can, of course, be arranged in any other sense as
well as in sight.

The _color-tone series_ is best arranged from a collection consisting
entirely of full or saturated colors. Start the {207} series with any
color and put next to this the color that most resembles it in
color-tone, i.e., in specific color quality; and so continue, adding
always the color that most resembles the one preceding. If we started
with red, the next in order might be either a yellowish red or a
bluish red. If we took the yellowish red and placed it beside the red,
then the next in order would be a still more yellowish red, and the
series would run on to yellow and then to greenish yellow, green,
bluish green, blue, violet, purple, purplish red, and so back to red.
The color-tone series returns upon itself. It is a c