of {223} light upon
the mother substance, and the vital response to W is the sensation of
white.

In the second stage, the mother substance is capable of giving off two
smaller cleavage products, Y and B. Y is split off by the long waves
of light, and B by the short waves, and the vital response to Y is the
sensation of yellow, that to B the sensation of blue. But suppose
that, chemically, Y + B = W: then, if Y and B are both split off at
the same time in the same cone, they immediately unite into W, and the
resulting sensation is white, and neither yellow nor blue.

[Illustration: Fig. 38.--The cleavage products, in the three stages of
the color sense. The "mother substance" is not represented in the
diagram, but only the cleavage products which, according to the
Ladd-Franklin theory, are the direct stimuli for the color sensations.
(Figure text: 1--white, 2--yellow blue, 3--red green blue)]

Similarly, in the third stage, the mother substance is capable of
giving off _three_ cleavage products, R, G and B; and there are three
corresponding vital responses, the sensations of red, green and blue.
But, chemically, R + G = Y; and therefore, if R and G are split off at
the same time, they unite chemically into Y and give the sensation of
yellow. If R, G and B are all split off at the same time, they unite
chemically as follows: R + G = Y, and Y + B = W; and therefore the
resulting sensation is that of white.

This theory of cleavage products is in good general agreement with
chemical principles, and it does justice to all the facts of color
vision, as detailed in the preceding pages. It should be added that
"for black, the theory supposes that, {224} in the interest of a
continuous field of view, objects which reflect no light at all upon
the retina have correlated with them a definite non-light
sensation--that of black." [Footnote: Quotation from Dr.
Ladd-Franklin.]

Adaptation

Sensory adaptation is a change that occurs in other senses also, but
it is so much more important in the sense of sight than elsewhere that
it may best be considered here. The stimulus continues, the sensation
ceases or diminishes--that is the most striking form of sensory
adaptation. Continued action of the same stimulus puts the sense into
such a condition that it responds differently from at first, and
usually more weakly. It is much like fatigue, but it often is more
positive and beneficial than fatigue.

The sense of smell is v