THE SENSE OF SIGHT: BRIGHTNESS VISION
The sense of sight in the dancer has received little
attention hitherto. In the literature there are
a few casual statements to the effect that it is of
importance. Zoth, for example (31 p. 149), remarks
that it seems to be keenly developed; and other writers,
on the basis of their observation of the animal’s
behavior, hazard similar statements. The descriptions
of the behavior of blinded mice, as given by Cyon,
Alexander and Kreidl, and Kishi (p.47), apparently
indicate that the sense is of some value; they do
not, however, furnish definite information concerning
its nature and its role in the daily life of the animal.
The experimental study of this subject which is now
to be described was undertaken, after careful and
long-continued observation of the general behavior
of the dancer, in order that our knowledge of the nature
and value of the sense of sight in this representative
of the Mammalia might be increased in scope and definiteness.
The results of this study naturally fall into three
groups: (1) those which concern brightness vision,
(2) those which concern color vision, and (3) those
which indicate the role of sight in the life of the
dancer.
Too frequently investigators, in their work on vision
in animals, have assumed that brightness vision and
color vision are inseparable; or, if not making this
assumption, they have failed to realize that the same
wave-length probably has markedly different effects
upon the retinal elements of the eyes of unlike organisms.
In a study of the sense of sight it is extremely important
to discover whether difference in the quality, as
well as in the intensity, of a visual stimulus influences
the organism; in other words, whether color sensitiveness,
as well as brightness sensitiveness, is present.
If the dancer perceives only brightness or luminosity,
and not color, it is evident that its visual world
is strikingly different from that of the normal human
being. The experiments now to be described were
planned to show what the facts really are.
[Illustration: Figure 14.—Discrimination
box. W, electric-box with white cardboards;
B, electric-box with black cardboards. Drawn by
Mr. C.H. Toll.]
As a means of testing the ability of the dancer to
distinguish differences in brightness, the experiment
box represented by Figures 14 and 15 was devised.
Figure 14 is the box as seen from the position of the
experimenter during the tests. Figure 15 is its
ground plan. This box, which was made of wood,
was 98 cm. long, 38 cm. wide, and 17 cm. deep, as
measured on the outside. The plan of construction
and its significance in connection with these experiments
on vision will be clear from the following account
of the experimental procedure. A mouse whose brightness
vision was to be tested was placed in the nest-box,
A (Figure 15). Thence by pushing open the swinging
