Here is a lantern throwing a steady light through a large tube.
(See illustration below, the right hand group.) By transparent
slides of colored glass or gelatine, the light may be made of any
color. At the end of the tube is a box, like a camera. The operator
covers his head with a cloth, and observes the color of the light as
it shines from the tube through, or on, a tiny hole in the dark
box. The size of the hole can be varied by moving slides, worked by
micrometer screws so fine that they measure the dimensions of the
hole to the four-hundredth of an inch.
[Illustration: STUDYING THE EFFECTS OF SOUND AND OF ATTENTION ON COLORS.]
The first step is to discover the "threshold" of each separate color.
That means the smallest-sized hole through which each color can be
distinguished. This varies for different colors. But now comes the
interesting point. The size of the hole, for any given _color seen_,
varies according to the nature of any _sound heard_ at the same time.
For instance, in order to distinguish a given red, the hole must be
larger or smaller, in proportion as the pitch of a musical tone is
lower or higher, fainter or stronger.
[Illustration: STUDYING THE EFFECTS OF COLORS ON JUDGMENTS OF TIME.]
The above experiment is one in a system of investigations, intended to
discover the laws by which the simplest sensations modify each other
under the simplest conditions. These are laws as fixed as the laws of
gravity, and, once determined, we may move on to study the combination
of these elements into the higher thought processes.