Another experiment will further illustrate this method of study. An
apparatus is so contrived that a colored disk can be made darker or
brighter by the operator, and a measure of the change be recorded.
(See illustration on opposite page, rear group.) The persons operated
on do not know what change is made, or whether any will be made or
not. They first look at the disk for ten seconds, taking good note of
its color. Next, the operator changes the shade (or not) as he sees
fit. Then for another ten seconds the subject judges the shade of
color, but this time performs meanwhile a sum in addition as the
operator calls to him simple numbers.
The experiment is to determine how the appearance of the color
changes, by reason of dividing the attention between observing the
disk and performing the addition. Do the colors of a rival's bonnet
really grow more glaring the harder they are looked at? To explain
this is to touch on a social as well as an esthetic problem.
Diversion of attention changes the appearance of distances as well as
of colors. A large frame covered with black cloth stands vertical. Two
tiny white disks are held in place on the cloth by invisible threads
manipulated behind the frame by the operator. When the disks are set a
given distance apart they rest close upon the smooth black ground. The
eye sees but two white spots in a free field, and may judge the
distance between them without complication. This is done for ten
seconds, as with the color disks. Then the spots are covered, and
their distance apart slightly changed (or not) by the operator. Again
they are shown, and now judged for ten seconds while adding figures.
The mental process of addition changes the judgment of the distance.
You will say it is a familiar experience that the road seems longer or
shorter as the mind is busy or not. But it is not a familiar thing to
determine the law of such lengthening and shortening for definite
distances, and under precise mental condition, as in the above
experiment.