LECTURE  VII

§ I. Relation of the Senses of Sight and Hearing. § II. Positive
and Neg.ative After-images. § III. Mechanical and Chemical
Senses. § IV. Phenomena of Contrast. § V. General Law
of Relativity.

§  I

IN the light of what we already know regarding the pro¬
perties of sight and hearing, we can see that the best way
to understand the relation between these two senses is to turn
to their two essential differences. The first is this: that,
whereas a subjectively simple tone-sensation can only be caused
by a simple objective movement of the air, in the case of sight
any vibration of ether, simple and complex alike, produces a
simple light-sensation. And the second consists in the fact
that the simple tonal qualities can only be varied in two
directions to produce higher and lower tones, whereas light-
sensations form two series, the chromatic and the achromatic,'
each consisting of a number of fundamental qualities and
intermediate tints, while the chromatic series, owing to the
subjective relationship of the terminal colours of the spectrum,
further constitutes a manifold of qualities which returns upon
itself. The system of simple tones, therefore, can be represented
by a straight line ; that of light-sensations requires a figure of
three dimensions for its expression. Besides these general
differences, there are further distinctions dependent on various
properties which attach to the sense of sight, either exclusively,
or to a greater extent than to any of the other senses.

§11
If we produce a tone {e.g., by striking a string or tuning-fork),
and  then suddenly arrest the vibrations of the sounding body.