Since a variety of precautionary tests failed to reveal
the presence, in these experiments, of any condition
other than brightness difference by which the mice
were enabled to choose correctly, and since evidence
of ability to discriminate brightness differences
was obtained by the use of both reflected light (cardboards)
and transmitted light (lamps behind ground glass),
it is necessary to conclude that the dancer possesses
brightness vision.
THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)
Since the ability of the dancer to perceive brightness
has been demonstrated by the experiments of the previous
chapter, the next step in this investigation of the
nature of vision is a study of the delicacy of brightness
discrimination, and of the relation of the just perceivable
difference to brightness value. Expressed in another
way, the problems of this portion of the investigation
are to determine how slight a difference in brightness
enables the dancer to discriminate one light from another,
and what is the relation between the absolute brightnesses
of two lights and that amount of difference which
is just sufficient to render the lights distinguishable.
It has been discovered in the case of the human being
that a stimulus must be increased by a certain definite
fraction of its own value if it is to seem different.
For brightness, within certain intensity limits, this
increase must be about one one-hundredth; a brightness
of 100 units, for example, is just perceivably different
from one of 101 units. The formulation of this
relation between the amount of a stimulus and the
amount of change which is necessary that a difference
be noted is known as Weber’s law. Does
this law, in any form, hold for the brightness vision
of the dancing mouse?
Two methods were used in the study of these problems.
For the first problem, that of the delicacy of brightness
discrimination, I first used light which was reflected
from gray papers, according to the method of Chapter
VII. For the second, the Weber’s law test,
transmitted light was used, in an apparatus which
will be described later. Either of these methods
might have been used for the solution of both problems.
Which of them is the more satisfactory is definitely
decided by the results which make up the material
of this chapter, Under natural conditions the dancer
probably sees objects which reflect light more frequently
than it does those which transmit it; it would seem
fairer, therefore, to require it to discriminate surfaces
which differ in brightness. This the use of gray
papers does. But, on the other hand, gray papers
are open to the objections that they may not be entirely
colorless (neutral), and that their brightness values
cannot be changed readily by the experimenter.
As will be made clear in the subsequent description
of the experiments with transmitted light, neither
of these objections can be raised in connection with
the second method of experimentation.
