13 14 Same 5 5
14 15 Blue 62 c.m.
Green 21 c.m. 4 6
15 16 Same 5 5
16 17 Same 5 5
17 18 Same 6 4
Now, as a final test, blue and green glasses were
placed over the electric-boxes, the brightness of
the two was equalized for the human eye, and the tests
of series 18 and 19 were given to No. 2:—
NO.
2
SERIES DATE BRIGHTNESS VALUES
RIGHT WRONG
(Blue) (Green)
18 18 Blue 62 c.m.
Green 21 c.m 4 6
19 19 Same 6 4
20 20 Blue 21 c.m.
Green 88 c.m. 2 8
The green was now made much the brighter.
21 21 Blue 21 c.m.
Green 18 c.m. 7 3
22 23 Same 8 2
To begin with, the blue and the green were made quite
bright for the human subject, blue 74 candle meters,
green 36. Later the brightness of both was first
decreased, then increased, in order to ascertain whether
discrimination was conditioned by the absolute strength
of illumination. No evidence of discrimination
was obtained with any of the several conditions of
illumination in seventeen series of ten tests each.
On the supposition that the animals were blinded by
the brightness of the light which had been used in
some of the tests, similar tests were made with weaker
light. The results were the same. I am therefore
convinced that the animals did justice to their visual
ability in these experiments.
Finally, it seemed possible that looking directly
at the source of light might be an unfavorable condition
for color discrimination, and that a chamber flooded
with colored light from above and from one end would
prove more satisfactory. To test this conjecture
two thicknesses of blue glass were placed over one
electric-box, two plates of green glass over the other;
the incandescent lamps were then fixed in such positions
that the blue and the green within the two boxes appeared
to the experimenter, as he viewed them from the position
at which the mouse made its choice, of the same brightness.
Mouse No. 2 was given two series of tests, series
18 and 19, under these conditions, with the result
that he showed absolutely no ability to tell the blue
box from the green box. The opportunity was now
taken to determine how quickly No. 2 would avail himself
of any possibility of discriminating by means of brightness.
With the blue at 21 candle meters, the green was increased
to about 1800. Immediately discrimination appeared,
and in the second series (22 of Table 27) there were
only two mistakes.
The results of the blue-green experiments with light
transmitted from in front of the animal and from above
it are in entire agreement with those of the experiments
in which reflected light was used. Since the range
of intensities of illumination was sufficiently great
to exclude the possibility of blinding and of under
illumination, it is necessary to conclude that the
dancer does not possess blue-green vision.
