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The Dancing Mouse by Robert M. Yerkes

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did not always occur; some individuals had to be trained to discriminate
gray No. 10 from gray No. 20. As soon as an individual had been so trained
that the ability to choose the lighter of these grays was perfect, it was
tested with No. 10 in combination with No. 15. If these in turn proved to
be discriminable, No. 10 could be used with No. 14, with No. 13, and so on
until either the limit of discrimination or that of the series had been

That it was not necessary to use other combinations than 10 with 20, and
10 with 15 is demonstrated by the results of Table 13. Mouse No. 420,
whose behavior was not essentially different from that of three other
individuals which were tested for gray discrimination, learned with
difficulty to choose gray No. 10 even when it was used with No. 20. Two
series of ten tests each were given to this mouse daily, and not until the
twentieth of these series (200 tests) did he succeed in making ten correct
choices in succession. Immediately after this series of correct choices,
tests with grays No. 10 and No. 15 were begun. In the case of this amount
of brightness difference twenty series failed to reveal discrimination.
The average number of right choices in the series is slightly in excess of
the mistakes, 5.8 as compared with 4.2.

From the experiments with gray papers we may conclude that under the
conditions of the tests the amount by which Nendel's gray No. 10 differs
in brightness from No. 20 is near the threshold of discrimination, or, in
other words, that the difference in the brightness of the adjacent grays
of Figure 16 is scarcely sufficient to enable the dancer to distinguish



The Delicacy of Brightness Discrimination

No. 420

AND 20 AND 15
NO. 10 NO. 2 NO. 10 NO. 15

1 Jan. 26 5 5 Feb. 6 8 2
2 27 8 2 6 5 5
3 28 6 4 7 9 1
4 28 2 8 7 7 3
5 29 1 9 8 5 5
6 29 6 4 8 6 4
7 30 9 1 9 5 5
8 30 7 3 9 6 4
9 31 6 4 10 8 2
10 31 4 6 10 3 7
11 Feb. 1 7 3 11 4 6
12 1 8 2 11 4 6
13 2 7 3 12 7 3
14 2 8 2 12 7 3
15 3 9 1 13 6 4
16 3 9 1 13 4 6
17 4 6 4 14 4 6
18 4 9 1 14 5 5
19 5 6 4 15 5 5
20 5 10 0 15 8 2

Averages 6.6 3.4 5.8 4.2

This result of the tests with gray papers surprised me very much at the
time of the experiments, for all my previous observation of the dancer had
led me to believe that it is very sensitive to light. It was only after a
long series of tests with transmitted light, in what is now to be
described as the Weber's law apparatus, that I was able to account for the
meager power of discrimination which the mice exhibited in the gray tests.
As it happened, the Weber's law experiment contributed quite as
importantly to the solution of our first problem as to that of the second,
for which it was especially planned.

For the Weber's law experiment a box similar to that used in the previous
brightness discrimination experiments (Figure 14) was so arranged that its
two electric-boxes could be illuminated independently by the light from
incandescent lamps directly above them. The arrangements of the light-box
and the lamps, as well as their relations to the other important parts of
the apparatus, are shown in Figure 17. The light-box consisted of two
compartments, of which one may be considered as the upward extension of
the left electric-box and the other of the right electric-box. The light-
box was pivoted at A and could be turned through an angle of 180 deg. by the
experimenter. Thus, by the turning of the light-box, the lamp which in the
case of one test illuminated the left electric-box could be brought into
such a position that in the case of the next test it illuminated the right
electric-box. The practical convenience of this will be appreciated when
the number of times that the brightnesses of the two boxes had to be
reversed is considered. The light-box was left open at the top for
ventilation and the prevention of any considerable increase in the
temperature of the experiment box. In one side of each of the compartments
of the light-box a slit (B, B of the figure) was cut out for an
incandescent lamp holder. A strip of leatherette, fitted closely into inch
grooves at the edges of the slit, prevented light from escaping through
these openings in the sides of the light-box. By moving the strips of
leatherette, one of which appears in the figure, C, the lamps could be
changed in position with reference to the bottom of the electric-box. A
scale, S, at the edge of each slit enabled the experimenter to determine
the distance of the lamp from the floor of the electric-box. The front of
the light-box was closed, instead of being open as it appears in the

[Illustration: Figure l7.--Weber's law apparatus for testing brightness
discrimination. Lower part, discrimination box similar to that of Figure
14. Upper part, rotatory light-box, pivoted at A, and divided into two
compartments by a partition P in the middle. L, L, incandescent lamps
movable in slits, B, B, in which a narrow strip of leatherette, C, serves
to prevent the escape of light. S, scale.]

This apparatus has the following advantages. First, the electric-boxes,
between which the mouse is expected to discriminate by means of their
difference in brightness, are illuminated from above and the light
therefore does not shine directly from the lamps into the eyes of the
animal, as it approaches the entrances to the boxes. Choice is required,
therefore, between illuminated spaces instead of between two directly
illuminated surfaces. Second, the amount of illumination of each electric-
box can be accurately measured by the use of a photometer. Third, since
the same kind of lamp is used in each box, and further, since the lamps
may be interchanged at any time, discrimination by qualitative instead of
quantitative difference in illumination is excluded. And finally, fourth,
the tests can be made expeditiously, conveniently, and under such simple
conditions that there should be no considerable error of measurement or of
observation within the range of brightness which must be used.

It was my purpose in the experiment with this apparatus to ascertain how
great the difference in the illumination of the two electric-boxes must be
in order that the mouse should be able to choose the brighter of them.
This I attempted to do by fixing an incandescent lamp of a certain known
illuminating power at such a position in one compartment of the light-box
that the electric-box below it was illuminated by what I call a standard
value, and by moving the incandescent lamp in the other compartment of the
light-box until the illumination of the electric-box below it was just
sufficiently less than that of the standard to enable the dancer to
distinguish them, and thereby to choose the brighter one. The light which
was changed from series to series I shall call the _variable_, in contrast
with the _standard_, which was unchanged.

The tests, which were made in a dark-room under uniform conditions, were
given in series of fifty each; usually only one such series was given per
day, but sometimes one was given in the morning and another in the
afternoon of the same day. To prevent choice by position the lights were
reversed in position irregularly, first one, then the other, illuminating
the right electric-box. For the fifty tests of each initial series the
order of the changes in position was as follows: standard (brighter light)
on the _l_ (left), _l, r_ (right), _r, l, l, r, r, l, r, l, r, l, l, r, r,
l, l, r, r, l, l, l, r, r, r, l, r, l, r, r, r, l, l, l, r, r, r, l, l, r,
l, r, l, r, l, r, l, r, l_. Twenty-five times in fifty the standard light
illuminated the right electric-box, and the same number of times it
illuminated the left electric-box. When a second series was given under
the same conditions of illumination, a different order of change was

In order to discover whether Weber's law holds in the case of the
brightness vision of the dancer it was necessary for me to determine the
just perceivable difference between the standard and the variable lights
for two or more standard values. I chose to work with three values, 5, 20,
and 80 hefners, and I was able to discover with a fair degree of accuracy
how much less than 5, 20, or 80 hefners, as the case might be, the
variable light had to be in order that it should be discriminable from the
other. For the work with the 5 hefner standard I used 2-candle-power
lamps,[1] for the 20, 4-candle-power, and for the 80, 16-candle-power.

[Footnote 1: I give merely the commercial markings of the lamps. They had
been photometered carefully by two observers by means of a Lummer-Brodhun
photometer and a Hefner amyl acetate lamp previous to their use in the
experiment. For the photometric measurements in connection with the
Weber's law tests I made use of the Hefner lamp with the hope of attaining
greater accuracy than had been possible with a standard paraffine candle,
in the case of measurements which I had made in connection with the
experiments on color vision that are reported in Chapters IX and X. The
Hefner unit is the amount of light produced by an amyl acetate lamp at a
flame height of 40 mm. (See Stine's "Photometrical Measurements.") A
paraffine candle at a flame height of 50 mm. is equal to 1.2 Hefner

For reasons which will soon appear, Weber's law tests were made with only
one dancer. This individual, No. 51, had been thoroughly trained in white-
black discrimination previous to the experiments in the apparatus which is
represented in Figure 17. Having given No. 51 more than two hundred
preliminary tests in the Weber's law apparatus with the electric-boxes
sufficiently different in brightness to enable her to discriminate
readily, I began my experiments by trying to ascertain how much less the
value of the illumination of one electric-box must be in order that it
should be discriminable from a value of 20 hefners in the other electric-
box. In recording the several series of tests and their results hereafter,
I shall state in Hefner units the value of the fixed or standard light and
the value of the variable light, the difference between the two in terms
of the former, and the average number of wrong choices in per cent.

With the lamps so placed that the difference in the illumination of the
two electric-boxes was .53 of the value of the standard, that is about one
half, No. 51 made twenty wrong choices in one hundred, or 20 per cent.
When the difference was reduced to .36 (one third) the number of errors
increased to 36 per cent, and with an intermediate difference of .48 there
were 26 per cent of errors (see Table 14).

Are these results indicative of discrimination, or are the errors in
choice too numerous to justify the statement that the dancer was able to
distinguish the boxes by their difference in brightness? Evidently this
question cannot be answered satisfactorily until we have decided what the
percentage of correct choices should be in order that it be accepted as
evidence of ability to discriminate, or, to put it in terms of errors,
what percentage of wrong choices is indicative of the point of just
perceivable difference in brightness. Theoretically, there should be as
many mistakes as right choices, 50 per cent of each, when the two
electric-boxes are equally illuminated (indiscriminable), but in practice
this does not prove to be the case because the dancer tends to return to
that electric-box through which in the previous test it passed safely,
whereas it does not tend in similar fashion to reenter the box in which it
has just received an electric shock. The result is that the percentage of
right choices, especially in the case of series which have the right box
in the same position two, three, or four times in succession, rises as
high as 60 or 70, even when the visual conditions are indiscriminable.
Abundant evidence in support of this statement is presented in Chapters
VII and IX, but at this point I may further call attention to the results
of an experiment in the Weber's law apparatus which was made especially to
test the matter. The results appear under the date of May 27 in Table 14.
In this experiment, despite the fact that both boxes were illuminated by
80 hefners, the mouse chose the standard (the illumination in which it was
not shocked) 59 times in 100. In other words the percentage of error was
41 instead of 50. It is evident, therefore, that as low a percentage of
errors as 40 is not necessarily indicative of discrimination. Anything
below 40 per cent is likely, however, to be the result of ability to
distinguish the brighter from the darker box. To be on the safe side we
may agree to consider 25 wrong choices per 100 as indicative of a just
perceivable difference in illumination. Fewer mistakes we shall consider
indicative of a difference in illumination which is readily perceivable,
and more as indicative of a difference which the mouse cannot detect. The
reader will bear in mind as he examines Table 14 that 25 per cent of wrong
choices indicates the point of just perceivable difference in brightness.


Brightness vision


May 13 100 20 9.4 .53 20
15 100 20 12.8 .36 36
16 100 20 10.8 .46 26
20 50 80 37.6 .53 6
21 50 80 51.3 .36 10
22 100 80 71.1 .11 35
24 100 80 60.0 .25 21
25 100 80 65.0 .19 25
27 100 80 80 0 41
28 50 5 2.5 .50 18
29 50 5 4.0 .20 14
29 100 5 4.5 .10 25
31 50 5 4.25 .15 20
June 1 50 5 4.85 .03 48
2 50 20 15.0 .25 16
3 50 20 17.4 .13 22
3 100 20 18.0 .10 22
4 100 80 72.0 .10 18
5 100 5 4.5 .10 12
7 100 5 4.67 .067 46
8 50 80 74.67 .067 56
9 50 20 18.67 .067 44

If we apply this rule to the results of the first tests, reported above,
it appears that a standard of 20 hefners was distinguished from a variable
of 9.4 hefners (.53 difference), for the percentage of errors was only 20.
But in the case of a difference of .36 in the illuminations lack of
discrimination is indicated by 36 per cent of errors. A difference of .46
gave a frequency of error so close to the required 25 (26 per cent) that I
accepted the result as a satisfactory determination of the just
perceivable difference for the 20 hefner standard and proceeded to
experiment with another standard value.

The results which were obtained in the case of this second standard, the
value of which was 80 hefners, are strikingly different from those for the
20 hefner standard. Naturally I began the tests with this new standard by
making the differences the same as those for which determinations had been
made in the case of the 20 standard. Much to my surprise only 6 per cent
of errors resulted when the difference in illumination was .53. I finally
discovered that about .19 difference (about one fifth) could be
discriminated with that degree of accuracy which is indicated by 25 per
cent of mistakes.

So far as I could judge from the results of determinations for the 20 and
the 80 hefner standards, Weber's law does not hold for the dancer. With
the former a difference of almost one half was necessary for
discrimination; with the latter a difference of about one fifth could be
perceived. But before presenting additional results I should explain the
construction of Table 14, and comment upon the number of experiments which
constitutes a set.

The table contains the condensed results of several weeks of difficult
experimentation. From left to right the columns give the date of the
initial series of a given set of experiments, the number of experiments in
the set, the value of the standard light in hefners, the value of the
variable light, the difference between the lights in terms of the standard
(the variable was always less than the standard), and the percentage of
errors or wrong choices. Very early in the investigation I discovered that
one hundred tests with any given values of the lights sufficed to reveal
whatever discriminating ability the mouse possessed at the time. In some
instances either the presence or the lack of discrimination was so clear,
as the result of 50 tests (first series), that the second series of 50 was
not given. Consequently in the table the number of tests for the various
values of the lights is sometimes 100, sometimes 50.

After finishing the experiments with the 80 standard on May 27 (see table)
I decided, in spite of the evidence against Weber's law, to make tests
with 5 as the standard, for it seemed not impossible that the lights were
too bright for the dancer to discriminate readily. I even suspected that I
might have been working outside of the brightness limits in which Weber's
law holds, if it holds at all. The tests soon showed that a difference of
one tenth made discrimination possible in the case of this standard. If
the reader will examine the data of the table, he will note that a
difference of .20 gave 14 per cent of mistakes; a difference of .03, 48
per cent. Evidently the former difference is above the threshold, the
latter below it. But what of the interpretation of the results in terms of
Weber's law? The difference instead of being one half or one fifth, as it
was in the cases of the 20 and 80 standards respectively, has now become
one tenth. Another surprise and another contradiction!

Had these three differences either increased or decreased regularly with
the value of the standard I should have suspected that they indicated a
principle or relationship which is different from but no less interesting
than that which Weber's law expresses. But instead of reading 5 standard,
difference one tenth; 20 standard, difference one fifth; 80 standard,
difference one half: or 5 standard, difference one half; 20 standard,
difference one fifth; 80 standard, difference one tenth: they read 5
standard, difference one tenth; 20 standard, difference one half; 80
standard, difference one fifth. What does this mean? I could think of no
other explanation than that of the influence of training. It seemed not
impossible, although not probable, that the mouse had been improving in
ability to discriminate day by day. It is true that this in itself would
be quite as interesting a fact as any which the experiment might reveal.

To test the value of my supposition, I made additional experiments with
the 20 standard, the results of which appear under the dates June 2 and 3
of the table. These results indicate quite definitely that the animal had
been, and still was, improving in her ability to discriminate. For instead
of requiring a difference of about one half in order that she might
distinguish the 20 standard from the variable light she was now able to
discriminate with only 22 per cent of errors when the difference was one

As it seemed most improbable that improvement by training should continue
much longer, I next gave additional tests with the 80 standard. Again a
difference of one tenth was sufficient for accurate discrimination (18 per
cent of errors). These series were followed immediately by further tests
with the 5 standard. As the results indicated greater ease of
discrimination with a difference of one tenth in the case of this standard
than in the case of either of the others I was at first uncertain whether
the results which I have tabulated under the dates June 3, 4, and 5 of the
table should be interpreted in terms of Weber's law.

Up to this point the experiments had definitely established two facts:
that the dancer's ability to discriminate by means of brightness
differences improves with training for a much longer period and to a far
greater extent than I had supposed it would; and that a difference of one
tenth is sufficient to enable the animal to distinguish two lights in the
case of the three standard values, 5, 20, and 80 hefners. The question
remains, is this satisfactory evidence that Weber's law holds with respect
to the brightness vision of the dancer, or do the results indicate rather,
that this difference is more readily detected in the case of 5 as a
standard (12 per cent error) than in the case of 20 as a standard (22 per
cent error)?

For the purpose of settling this point I made tests for each of the three
standards with a difference of only one fifteenth. In no instance did I
obtain the least evidence of ability to discriminate. These final tests,
in addition to establishing the fact that the limit of discrimination for
No. 51, after she had been subjected to about two thousand tests, lay
between one tenth and one fifteenth, proved to my satisfaction, when taken
in connection with the results already discussed, that Weber's law does
hold for the brightness vision of the dancer.

In concluding this discussion of the Weber's law experiment I wish to call
attention to the chief facts which have been revealed, and to make a
critical comment. In my opinion it is extremely important that the student
of animal behavior should note the fact that the dancer with which I
worked week after week in the Weber's law investigation gradually improved
in her ability to discriminate on the basis of brightness differences
until she was able to distinguish from one another two boxes whose
difference in illumination was less than one tenth[1] that of the brighter
box. At the beginning of the experiments a difference of one half did not
enable her to choose as certainly as did a difference of one tenth after
she had chosen several hundred times. Evidently we are prone to
underestimate the educability of our animal subjects.

[Footnote 1: Under the conditions of the experiment I was unable to
distinguish the electric-boxes when they differed by less than one

The reason that the experiments were carried out with only one mouse must
now be apparent. It was a matter of time. The reader must not suppose that
my study of this subject is completed. It is merely well begun, and I
report it here in its unfinished state for the sake of the value of the
method which I have worked out, rather than for the purpose of presenting
the definite results which I obtained with No. 51.

The critical comment which I wish to make for the benefit of those who are
working on similar problems is this. The phosphor bronze wires, on the
bottom of the electric-boxes, by means of which an electric shock could be
given to the mouse when it chose the wrong box, are needless sources of
variability in the illumination of the boxes. They reflect the light into
the eyes of the mouse too strongly, and unless they are kept perfectly
clean and bright, serious inequalities of illumination appear. To avoid
these undesirable conditions I propose hereafter to use a box within a
box, so that the wires shall be hidden from the view of the animal as it
attempts to discriminate.

A brief description of the behavior of the dancer in the brightness
discrimination experiments which have been described may very
appropriately form the closing section of this chapter. For the
experimenter, the incessant activity and inexhaustible energy of the
animal are a never-failing source of interest and surprise. When a dancer
is inactive in the experiment box, it is a good indication either of
indisposition or of too low a temperature in the room. In no animal with
which I am familiar is activity so much an end in itself as in this odd
species of mouse. With striking facility most of the mice learn to open
the wire swing doors from either side. They push them open with their
noses in the direction in which they were intended by the experimenter to
work, and with almost equal ease they pull them open with their teeth in
the direction in which they were not intended to work. In the rapidity
with which this trick is learned, there are very noticeable individual
differences. The pulling of these doors furnished an excellent opportunity
for the study of the imitative tendency.

When confronted with the two entrances of the electric-boxes, the dancer
manifested at first only the hesitation caused by being in a strange
place. It did not seem much afraid, and usually did not hesitate long
before entering one of the boxes. The first choice often determined the
majority of the choices of the preference series. If the mouse happened to
enter the left box, it kept on doing so until, having become so accustomed
to its surroundings that it could take time from its strenuous running
from _A_ by way of the left box to the alley and thence to _A_, to examine
things in _B_ a little, it observed the other entrance and in a seemingly
half-curious, half-venturesome way entered it. In the case of other
individuals, the cardboards themselves seemed to determine the choices
from the first.

The electric shock, as punishment for entering the wrong box, came as a
surprise. At times an individual would persistently attempt to enter, or
even enter and retreat from the wrong box repeatedly, in spite of the
shock. This may have been due in some instances to the effects of fright,
but in others it certainly was due to the strength of the tendency to
follow the course which had been taken most often previously. The next
effect of the shock was to cause the animal to hesitate before the
entrances to the boxes, to run from one to the other, poking its head into
each and peering about cautiously, touching the cardboards at the
entrances, apparently smelling of them, and in every way attempting to
determine which box could be entered safely. I have at times seen a mouse
run from one entrance to the other twenty times before making its choice;
now and then it would start to enter one and, when halfway in, draw back
as if it had been shocked. Possibly merely touching the wires with its
fore paws was responsible for this simulation of a reaction to the shock.
The gradual waning of this inhibition of the forward movement was one of
the most interesting features of the experiment. Could we but discover
what the psychical states and the physiological conditions of the animal
were during this period of choosing, comparative psychology and physiology
would advance by a bound.

If the conditions at the entrances of the two boxes were discriminable,
the mouse usually learned within one hundred experiences to choose the
right box without much hesitation. Three distinct methods of choice were
exhibited by different individuals, and to a certain extent by the same
individual from time to time. These methods, which I have designated
_choice by affirmation_, _choice by negation_, and _choice by comparison_,
are of peculiar interest to the psychologist and logician.

When an individual runs directly to the entrance of the right box, and,
after stopping for an instant to examine it, enters, the choice may be
described as recognition of the right box. I call it choice by affirmation
because the act of the animal is equivalent to the judgment--"this is it."
If instead it runs directly to the wrong box, and, after examining it,
turns to the other box and enters without pause for examination, its
behavior may be described as recognition of the wrong box. This I call
choice by negation because the act seems equivalent to the judgment--"this
is not it." Further, it seems to imply the judgment--"therefore the other
is it." In the light of this fact, this type of choice might appropriately
be called choice by exclusion. Finally, when the mouse runs first to one
box and then to the other, and repeats this anywhere from one to fifty
times, the choice may be described as comparison of the boxes; therefore,
I call it choice by comparison. Certain individuals choose first by
comparison, and later almost uniformly by affirmation and negation.
Whenever the conditions are difficult to discriminate, choice by
comparison occurs most frequently and persistently. If, however, the
conditions happen to be absolutely indiscriminable, as was true, for
example, in the case of the sound tests, in certain of the Weber's law
tests, and in the plain electric-box tests, the period of hesitation
rapidly increases during the first three or four series of tests, then the
mouse seems to lessen its efforts to discriminate and more and more tends
to rush into one of the boxes without hesitation or examination, and
apparently with the expectation of a shock, but with the intention of
getting it over as soon as possible. Now and then under such conditions
there is a marked tendency to enter the same box each time.
Indiscriminable conditions are likely to render the animals fearful of the
experiment; instead of going from _A_ to _A_ willingly, they fight against
making the trip. They refuse to pass from _A_ to _B_; and when in _B_,
they fight against being driven toward the entrances to the electric-

In marked contrast with this behavior on the part of the mouse under
conditions which do not permit it to choose correctly is that of the
animal which has learned what is expected of it. The latter, far from
holding back or fighting against the conditions which urge it forward, is
so eager to make the trip that it sometimes has to be forced to wait while
the experimenter records the results of the tests. There is evidence of
delight in the freedom of movement and in the variety of activity which
the experiment furnishes. The thoroughly trained dancer runs into _B_
almost as soon as it has been placed in _A_ by the experimenter; it
chooses the right entrance by one of the three methods described above,
immediately, or after whirling about a few times in _B_; it runs through
_E_ and back to _A_ as quickly as it can, and almost before the
experimenter has had time to record the result of the choice it is again
in _B_ ready for another choice.



Is the dancing mouse able to discriminate colors as we do? Does it possess
anything which may properly be called color vision? If so, what is the
nature of its ability in this sense field? Early in my study of the mice I
attempted to answer these and similar questions, for the fact that they
are completely deaf during the whole or the greater part of their lives
suggested to me the query, are they otherwise defective in sense
equipment? In the following account of my study of color vision, I shall
describe the evolution of my methods in addition to stating the results
which were obtained and the conclusions to which they led me. For in this
case the development of a method of research is quite as interesting as
the facts which the method in its various stages of evolution revealed.

Observation of the behavior of the dancer under natural conditions caused
me to suspect that it is either defective in color vision or possesses a
sense which is very different from human vision. I therefore devised the
following extremely simple method of testing the animal's ability to
distinguish one color from another. In opposite corners of a wooden box 26
cm. long, 23 cm. wide, and 11 cm. deep, two tin boxes 5 cm. in diameter
and 1.5 cm. deep were placed, as is shown in part I of Figure 18. One of
these boxes was covered on the outside with blue paper (_B_ of Figure 18),
and the other with orange[1] (_O_ of Figure 18). A small quantity of
"force" was placed in the orange box. As the purpose of the test was to
discover whether the animals could learn to go directly to the box which
contained the food, the experiments were made each morning before the mice
had been fed. The experimental procedure consisted in placing the
individual to be tested in the end of the large wooden box opposite the
color boxes, and then permitting it to run about exploring the box until
it found the food in the orange box. While it was busily engaged in eating
a piece of "force" which it had taken from the box and was holding in its
fore paws, squirrel fashion, the color boxes were quickly and without
disturbance shifted in the directions indicated by the arrows of Figure
18, I. Consequently, when the animal was ready for another piece of
"force," the food-box was in the corresponding corner of the opposite end
of the experiment box (position 2, 18, II). After the mouse had again
succeeded in finding it, the orange box was shifted in position as is
indicated by the arrows in Figure 18, II. Thus the tests were continued,
the boxes being shifted after each success on the part of the animal in
such a way that for no two successive tests was the position of the food-
box the same; it occupied successively the positions 1, 2, 3, and 4 of the
figure, and then returned to 1. Each series consisted of 20 tests.

[Footnote 1: These were the Milton Bradley blue and orange papers.]

[Illustration: FIGURE 18.--Food-box apparatus for color discrimination
experiments. _O_, orange food-box; _B_, blue food-box; 1, 2, 3, 4,
different positions of the food-boxes, _O_ and _B_; I, II, III, IV,
figures in which the arrows indicate the direction in which the food-boxes
were moved.]

[Illustration: FIGURE 19.--Food-box apparatus with movable partitions.
_O_, orange food-box; _B_, blue food-box; _X_, starting point for mouse;
_A_, point at which both food-boxes become visible to the mouse as it
approaches them; 1, 2, two different positions of the food-boxes; _T_,
_T_, movable partitions. (After Doctor Waugh.)]

An improvement on this method, which was suggested by Doctor Karl Waugh,
has been used by him in a study of the sense of vision in the common
mouse. It consisted in the introduction, at the middle of the experiment
box, of two wooden partitions which were pivoted on their mid-vertical
axes so that they could be placed in either of the positions indicated in
Figure 19. Let us suppose that a mouse to be tested for color vision in
this apparatus has been placed at _X_. In order to obtain food it must
pass through _A_ and choose either the orange or the blue box. If it
chooses the former, the test is recorded as correct; if it goes to the
blue box first, and then to the orange, it is counted an error. While the
animal is eating, the experimenter shifts the boxes to position 1 of
Figure 19, and at the same time moves the partitions so that they occupy
the position indicated by the dotted lines. The chief advantage of this
improvement in method is that the animal is forced to approach the color
boxes from a point midway between them, instead of following the sides of
the experiment box, as it is inclined to do, until it happens to come to
the food-box. This renders the test fairer, for presumably the animal has
an opportunity to see both boxes from _A_ and can make its choice at that
point of vantage.

Two males, A and B, of whose age I am ignorant, were each given seventeen
series of tests in the apparatus of Figure 18. A single series, consisting
of twenty choices, was given daily. Whether the animal chose correctly or
not, it was allowed to get food; that is, if it went first to the blue
box, thus furnishing the condition for a record of error, it was permitted
to pass on to the orange box and take a piece of "force." No attempt was
made to increase the animal's desire for food by starving it. Usually it
sought the food-box eagerly; when it would not do so, the series was
abandoned and work postponed. "Force" proved a very convenient form of
food in these tests. The mice are fond of it, and they quickly learned to
take a flake out of the box instead of trying to get into the box and sit
there eating, for when they attempted the latter they were promptly pushed
to one side by the experimenter and the box, as well as the food, was
removed to a new position.

The results of the tests appear in Table 15. No record of the choices in
the first two of the 17 series was kept. The totals therefore include 15
series, or 300 tests, with each individual. Neither the daily records nor
the totals of this table demonstrate choice on the basis of color
discrimination. Either the dancers were not able to tell one box from the
other, or they did not learn to go directly to the orange box. It might be
urged with reason that there is no sufficiently strong motive for the
avoidance of an incorrect choice. A mistake simply means a moment's delay
in finding food, and this is not so serious a matter as stopping to
discriminate. I am inclined, in the light of result of other experiments,
to believe that there is a great deal in this objection to the method.
Reward for a correct choice should be supplemented by some form of
punishment for a mistake. This conclusion was forced upon me by the
results of these preliminary experiments on color vision and by my
observation of the behavior of the animals in the apparatus. At the time
the above tests were made I believed that I had demonstrated the inability
of the dancer to distinguish orange from blue, but now, after two years'
additional work on the subject, I believe instead that the method was

The next step in the evolution of a method of testing the dancer's color
vision was the construction of the apparatus (Figures 14 and 15) which was
described in Chapter VII. In connection with this experiment box the basis
for a new motive was introduced, namely, the punishment of mistakes by an
electric shock. Colored cardboards, instead of the white, black, or grays
of the brightness tests, were placed in the electric-boxes.




1 Dec. 6 -- -- -- --
2 7 -- -- -- --
3 8 12 8 12 8
4 9 10 10 9 11
5 10 15 5 10 10
6 11 10 10 12 8
7 12 9 11 9 11
8 13 10 10 9 11
9 14 12 8 12 8
10 15 13 7 12 8
11 16 13 7 10 10
12 17 12 8 10 10
13 18 11 9 10 10
14 19 13 7 8 12
15 20 13 7 9 11
16 22 14 6 12 8
17 23 10 10 9 11

TOTALS 177 123 153 147

In preliminary tests, at the rate of four per day, the colored cardboards
were placed only at the entrances to the boxes, not inside, and as was
true also in the case of brightness tests under like conditions, no
evidence of discrimination was obtained from ten days' training. This
seemed to indicate that a considerable area of the colored surface should
be exposed to the mouse's view, if discrimination were to be made
reasonably easy.

This conclusion was supported by the results of other preliminary
experiments in which rectangular pieces of colored papers[1] 6 by 3 cm.,
were placed on the floor at the entrances to the electric-boxes, instead
of on the walls of the boxes. Mouse No. 2 was given five series of ten
tests each with a yellow card to indicate the right box and a red card at
the entrance to the wrong box. At first he chose the red almost uniformly,
and at no time during these fifty tests did he exhibit ability to choose
the right box by color discrimination. I present the results of these
series in Table 16, because they indicate a fact to which I shall have to
refer repeatedly later, namely, that the brightness values of different
portions of the spectrum are not the same for the dancer as for us.
Previous to this yellow-red training, No. 2, as a result of ten days of
white-black training (two tests per day), had partially learned to go to
the brighter of the two electric-boxes. It is possible therefore that the
choice of the box in the case of these color experiments was in reality
the choice of what appeared to the mouse to be the brighter box. If this
were not true, how are the results of Table 16 to be accounted for?

[Footnote 1: These were the only Hering papers used in my experiments.]



In Color Discrimination Box with 6 by 3 cm. Pieces of Hering
Papers at Entrances to Boxes

No. 2

1906 (Yellow) (Red)
1 Jan. 16 1 9
2 17 3 7
3 18 4 6
4 19 5 5
5 20 5 5

Without further mention of the many experiments which were necessary for
the perfecting of this method of testing color vision, I may at once
present the final results of the tests which were made with reflected
light. These tests were made with the discrimination apparatus in
essentially the same way as were the brightness discrimination tests of
Chapter VII.

In all of the color experiments, unless otherwise stated, a series of ten
tests each day was given, until satisfactory evidence of discrimination or
proof of the lack of the ability to discriminate had been obtained. The
difficulties of getting conclusive evidence in either direction will be
considered in connection with the results themselves. For all of these
tests with reflected light the Milton Bradley colored papers were used.
These colored papers were pasted on white cardboard carriers. I shall
designate, in the Bradley nomenclature, the papers used in each

With colored cardboards inside the electric-boxes as well as at their
entrances (see Figure 14 for position of cardboards) blue-orange tests
were given to Nos. 2 and 3 until they discriminated perfectly. The papers
were Bradley's blue tint No. 1 and orange. Number 2 was perfect in the
twelfth series (Table 17), No 3 in the fourteenth and again in the
sixteenth. They were then tested with a special brightness check series
which was intended by the experimenter to reveal any dependence upon a
possible brightness difference rather than upon the color difference of
the boxes.




1 Jan. 26 7 3 1 9
2 27 7 3 5 5
3 28 7 3 6 4
4 29 7 3 7 3
5 30 7 3 4 6
6 31 10 0 7 3
7 Feb. 1 9 1 7 3
8 2 8 2 6 4
9 3 9 1 9 1
10 5 7 3 5 5
11 6 8 2 5 5
12 7 10 0 5 5
Special brightness check series (see Table 18)
13 8 10 0 7 3
Special light blue-dark blue series
14 9 8 2 10 0
15 10 9 1 9 1
Special light blue-dark blue series
16 11 9 1 10 0
Special brightness
check series
17 12 10 0 9 1



Brightness check series Mouse No. 2, Series 13
Feb. 8, 1906


1 Light blue on right
Orange on left Right ____

2 Light blue on left
Orange on right Right ____

3 Light blue on right
Red substituted for orange Right ____

4 Light blur on left
Red substituted for orange Right ____

5 Dark blue on right
Orange on left Right ____

6 Dark blue on left
Orange on right Right ____

7 Dark blue on left
Orange on right Right ____

8 Dark blue on right
Red substituted for orange Right ____

9 Dark blue on left
Red substituted for orange Right ____

10 Dark blue on left
Red substituted for orange Right ____

Totals 10 0

The nature of this brightness check series, as well as the results which
No. 2 gave when tested by it, may be appreciated readily by reference to
Table 18. Tint No. 1 of the blue, which is considerably brighter, in my
judgment, than the Bradley blue, was replaced at intervals in this series
by the latter. For it was thought that in case the mouse were choosing the
blue of the series because it seemed brighter than the orange, this
substitution might mislead it into choosing the orange. These blues are
referred to in the table as light blue (tint No. 1) and dark blue
(standard blue). Again a change in the opposite direction was made by
substituting Bradley red for orange. As this was for the human eye the
substitution of a color whose brightness was considerably less than that
of the orange, it seemed possible that the mouse, if it had formed the
habit of choosing the box which seemed the darker, might by this change be
misled into choosing the red instead of the light blue. In a word, changes
in the conditions of the experiments were made in such a way that now one
color, now the other, appeared to be the brighter. But I did not attempt
to exclude brightness discrimination on the part of the mouse by
dependence upon the human judgment of brightness equality, for it is
manifestly unsafe to assume that two colors which are of the same
brightness for the human eye have a like relation for the eye of the
dancer or of any other animal. My tests of color vision have been
conducted without other reference to human standards of judgment or
comparisons than was necessary for the description of the experimental
conditions. In planning the experiments I assumed neither likeness nor
difference between the human retinal processes and those of the dancer. It
was my purpose to discover the nature of the mouse's visual discriminative

As is indicated in the tables, neither the substitution of dark blue for
light blue, nor the replacement of the orange by red or dark blue rendered
correct choice impossible, although certain of the combinations did render
choice extremely difficult. In other words, despite all of the changes
which were made in the brightness of the cardboards in connection with the
light blue-orange tests, the mice continued to make almost perfect
records. What are we to conclude from this? Either that the ability to
discriminate certain colors is possessed by the dancer, or that for some
reason the tests are unsatisfactory. If it be granted that the possibility
of brightness discrimination was excluded in the check series, the first
of these alternatives apparently is forced upon us. That such a
possibility was not excluded, later experiments make perfectly clear. The
fact was that not even in the check series was the brightness value of the
orange as great as that of the blue. Consequently the mice may have chosen
the brighter box each time while apparently choosing the blue.

Although conclusive proof of the truth of this statement is furnished only
by later experiments, the results of the light blue-orange series, as
given in Table 17, strongly suggest such a possibility. Mouse No. 3 had
not been experimented with previous to these color discrimination tests.
Her preference for the orange, which in the case of the first series was 9
to 1, consequently demands an explanation. If she had been trained
previously to choose the white instead of the black, as was true of No. 2,
it might be inferred that she went to the orange box because it appeared
brighter than the blue. As this explanation is not available, we are
driven back upon the results of the white-black preference tests in
Chapter VII, which proved that many dancers prefer the black to the white.
This may mean that they prefer the lower degree of brightness or
illumination, and if so it might be argued, in turn, that the orange was
chosen by No. 3 because it appeared darker than the blue. Since, as has
already been stated, the orange was far brighter for me than the blue,
this would also mean that the brightness values of different colors are
not the same for man and mouse.

Practically the same kind of color tests as those described for Nos. 2 and
3 were given to Nos. 1000 and 5. The results appear in Table 19. These
tests followed upon the formation of a habit to choose white instead of
black (that is, the greater brightness). From the first both No. 1000 and
No. 5 chose the light blue in preference to the orange or the red. It
therefore seems probable that the former was considerably brighter than
the latter. Number 1000, to be sure, was led into three erroneous choices
by the brightness check series (series 7), but, on the other hand, No. 5
was not at all disturbed in her choices by similar check tests. It seems
natural to conclude from these facts that both of these mice chose the
blue at first because of its relatively greater brightness, and that they
continued to do so for the same reason. In other words, their behavior
indicates that the brightness check tests were valueless because not
enough allowance had been made for the possible differences between the
vision of mouse and man.

No. 1000 No. 5
SERIES DATE Condition Right Wrong Right Wrong
(Light (Orange (Light (Orange
Blue or or Blue or
Dark Red) Dark Red)
Blue) Blue)
1 Jan. 25 Blue-red 8 2 10 0
2 26 Blue-red or
Light blue-orange 10 0 10 0
3 27 Light blue-orange 10 0 5 5
4 29 Light blue-orange 9 1 8 2
5 30 Light blue-orange 10 0 8 2
6 31 Light blue-orange 10 0 10 0
7 Feb. 1 Light blue-orange
or Dark blue-red 7 3 10 0

If only the final results of my experiments with the dancer and the
conclusions to which they lead were of interest, all of this description
of experiments which served merely to clear the ground and thus make
possible crucial tests might be omitted. It has seemed to me, however,
that the history of the investigation is valuable, and I am therefore
presenting the evolution of my methods step by step. To be sure, not every
detail of this process can be mentioned, and only a few of the individual
results can be stated, but my purpose will have been fulfilled if I
succeed in showing how one method of experimentation pointed the way to
another, and how one set of results made possible the interpretation of

As the results of my color vision experiments seemed to indicate that the
red end of the spectrum appears much darker to the dancer than to us,
tests were now arranged with colors from adjacent regions of the spectrum,
green and blue. The papers used were the Bradley green and tint No. 1 of
the blue. They were not noticeably different in brightness for the human
eye. Green marked the box to be chosen. Three of the individuals which had
previously been used in the light blue-orange series, and which therefore
had perfect habits of going to the light blue, were used for the green-
light blue tests. Of these individuals, No. 1000 became inactive on the
fifth day of the experiment, and the tests with him were discontinued.
Twenty series were given to each of the other mice, with the results which
appear in Table 20. To begin with, both No. 4 and No. 5 exhibited a
preference for the light blue, as a result of the previous light blue-
orange training. As this preference was gradually destroyed by the
electric shock which was received each time the light blue box was
entered, they seemed utterly at a loss to know which box to enter.
Occasionally a record of six, seven, or even eight right choices would be
made in a series, but in no case was this unquestionably due to color
discrimination; usually it could be explained in the light of the order of
the changes in the positions of the cardboards. For example, series 9, in
which No. 5 made a record of 8 right and 2 wrong, had green on the right
for the first three tests. The animal happened to choose correctly in the
first test, and continued to do so three times in succession simply
because there was no change in the position of the cardboards. I have
occasionally observed a record of seven right choices result when it was
perfectly evident to the observer that the mouse could not discriminate
visually. It was to avoid unsafe conclusions and unfair comparisons, as
the result of such misleading series, that three perfect series in
succession were required as evidence of a perfectly formed habit of



Date No. 1000 No. 4 No. 5

1 Feb.3 2 8 3 7 3 7

2 5 7 3 5 5 5 5

3 6 5 5 6 4 5 5

4 7 5 5 5 5 5 5

5 8 2 8 5 5 4 6

6 9 7 3 7 3

7 10 4 6 3 7

8 10 6 4 4 6

9 12 6 4 8 2

10 13 6 4 6 4

11 14 5 5 3 7

12 15 6 4 7 3

13 16 5 5 7 3

14 17 3 7 6 4

15 19 6 4 6 4

16 20 7 3 5 5

17 21 4 6 8 2

18 22 3 7 4 6

19 23 6 4 4 6

20 24 6 4 5 5

Twenty series, 200 tests for each of the individuals in the experiment,
yielded no evidence whatever of the dancer's ability to tell green from
blue. As it has already been proved that they readily learn to choose the
right box under discriminable conditions, it seems reasonable to conclude
either that they lack green-blue vision, or that they have it in a
relatively undeveloped state.

If it be objected that the number of training tests given was too small,
and that the dancer probably would exhibit discrimination if it were given
1000 instead of 200 tests in such an experiment, I must reply that the
behavior of the animal in the tests is even more satisfactory evidence of
its inability to choose than are the results of Table 20. Had there been
the least indication of improvement as the result of 200 tests, I should
have continued the experiment; as a matter of fact, the mice each day
hesitated more and more before choosing, and fought against being driven
toward the entrance to the experiment box. That they were helpless was so
evident that it would have been manifestly cruel to continue the

With Odor of All Cardboards the Same

A MAR. 7 8 2 5 5
B 7 3 7 2 8
1 14 3 7 6 4
2 15 4 6 4 6
3 16 5 5 5 5
4 19 4 6 4 6
5 20 5 5 6 4
6 21 4 6 8 2
7 22 8 2 4 6
8 23 4 6 6 4
9 24 6 4 4 6
10 25 4 6 6 4

Further color tests with reflected light were made with violet and red.
Two dancers, Nos. 998 and 7, neither of which had been in any experiment
previously, were subjected to the ten series of tests whose results are to
be found in Table 21. In this experiment the cardboards used had been
coated with shellac to obviate discrimination by means of odor. It is
therefore impossible to give a precise description of the color or
brightness by referring to the Bradley papers.[1] Both the violet and the
red were rendered darker, and apparently less saturated, by the coating.

[Footnote 1: The violet was darker than Bradley's shade No. 2, and the red
was lighter than Bradley's red.]

These violet-red tests were preceded by two series of preference tests
(_A_ and _B_), in which no shock was given and escape was possible through
either electric-box. Although the results of these preference tests as
they appear in Table 21 seem to indicate a preference for the red on the
part of No. 998, examination of the record sheets reveals the fact that
neither animal exhibited color preference, but that instead both chose by
position. Number 998 chose the box on the right 15 times in 20, and No. 7
chose the box on the left 15 times in 20.

Ten series of tests with the violet-red cardboards failed to furnish the
least indication of discrimination. The experiment was discontinued
because the mice had ceased to try to discriminate and dashed into one or
the other of the boxes on the chance of guessing correctly. When wrong
they whirled about, rushed out of the red box and into the violet
immediately. They had learned perfectly as much as they were able to learn
of what the experiment required of them. Although we are not justified in
concluding from this experiment that dancers cannot be taught to
distinguish violet from red, there certainly is good ground for the
statement that they do not readily discriminate between these colors.

The experiments on color vision which have been described and the records
which have been presented will suffice to give the reader an accurate
knowledge of the nature of the results, only a few of which could be
printed, and of the methods by which they were obtained.

In brief, these results show that the dancer, under the conditions of the
experiments, is not able to tell green from blue, or violet from red. The
evidence of discrimination furnished by the light blue-orange tests is not
satisfactory because the conditions of the experiment did not permit the
use of a sufficiently wide range of brightnesses. It is obvious,
therefore, that a method of experimentation should be devised in which the
experimenter can more fully control the brightness of the colors which he
is using. I shall now describe a method in which this was possible.



There are three well-known ways in which colors may be used as stimuli in
experiments on animals: by the use of colored papers (reflected light); by
the use of a prism (the spectrum which is obtained may be used as directly
transmitted or as reflected light); and by the use of light filters
(transmitted light). In the experiments on the color vision of the dancer
which have thus far been described only the first of these three methods
has been employed. Its advantages are that it enables the experimenter to
work in a sunlit room, with relatively simple, cheap, and easily
manipulated apparatus. Its chief disadvantages are that the brightness of
the light can neither be regulated nor measured with ease and accuracy.
The use of the second method, which in many respects is the most desirable
of the three, is impracticable for experiments which require as large an
illuminated region as do those with the mouse; I was therefore limited to
the employment of light filters in my further tests of color

The form of filter which is most conveniently handled is the colored
glass, but unfortunately few glasses which are monochromatic are
manufactured. Almost all of our so-called colored glasses transmit the
light of two or more regions of the spectrum. After making spectroscopic
examinations of all the colored glasses which were available, I decided
that only the ruby glass could be satisfactorily used in my experiments.
With this it was possible to get a pure red. Each of the other colors was
obtained by means of a filter, which consisted of a glass box filled with
a chemical solution which transmitted light of a certain wave length.

For the tests with transmitted light the apparatus of Figures 20 and 21
was constructed. It consisted of a reaction-box essentially the same as
that used in the brightness vision tests, except that holes were cut in
the ends of the electric-boxes, at the positions _G and R_ of Figure 20,
to permit the light to enter the boxes. Beyond the reaction-box was a long
light-box which was divided lengthwise into two compartments by a
partition in the middle. A slit in the cover of each of these compartments
carried an incandescent lamp _L_ (Figure 20). Between the two lamps, _L,
L_, and directly over the partition in the light-box was fastened a
millimeter scale, _S_, by means of which the experimenter could determine
the position of the lights with reference to the reaction-box. The light-
box was separated from the reaction-box by a space 6 cm. wide in which
moved a narrow wooden carrier for the filter boxes. This carrier, as shown
in Figure 20, could be moved readily from side to side through a distance
of 20 cm. The filter boxes, which are represented in place in Figures 20
and 21, consisted of three parallel-sided glass boxes 15 cm. long, 5 cm.
wide, and 15 cm. deep. Each box contained a substance which acted as a ray
filter. Tightly fitted glass covers prevented the entrance of dust and the
evaporation of the solutions in the boxes. Figures 20 and 21 represent the
two end boxes, _R, R_, as red light filters and the middle one, _G_, as a
green light filter. Three filters were used thus side by side in order
that the position of a given color with reference to the electric-boxes
might be changed readily. As the apparatus was arranged, all the
experimenter had to do when he wished to change from green-left, red-right
to green-right, red-left was to push the carrier towards the right until
the green filter covered the hole on the right at the end of the electric-
box. When this had been accomplished the red filter at the left end of the
carrier covered the hole on the left at the end of the electric-box. Thus
quickly, noiselessly, easily, and without introducing any other change in
conditions than that of the interchange of lights, the experimenter was
able to shift the positions of his colored lights at will.

[Illustration: FIGURE 20.--Color discrimination apparatus. _A,_ nest-box;
_B,_ entrance chamber; _R, R,_ red filters; _G,_ green filter; _L, L,_
incandescent lamps in light-box; _S,_ millimeter scale on light-box; _I,_
door between _A_ and _B; O, O,_ doors between alleys and _A_.]

[Illustration: FIGURE 21--Ground plan of color discrimination apparatus.
_E, E_, exits from electric-boxes. _LB_, light-box; _R, G, R_, filter
boxes on carrier; _L_, left electric-box; _R_, right electric-box; _IC_
induction apparatus; _C_, electric cell; _K_, key; _S_, millimeter scale.]

In the tests which are now to be reported, three portions of the spectrum
were used: the red end, the blue-violet end, and a middle region, chiefly
green. The red light was obtained by the use of a filter which was made by
placing two plates of ruby glass in one of the glass boxes, filling the
box with filtered water and then sealing it to prevent evaporation. The
blue-violet was obtained by the use of a filter box which contained a 5
per cent solution of copper ammonium sulphate. The green, which, however,
was not monochromatic, was obtained by the use of a filter box which
contained a saturated solution of nickel nitrate. These three sets of
filters were examined spectroscopically both before the experiments had
been made and after their completion.[1] The red filters, of which I had
two for shifting the lights, transmitted only red light. The blue-violet
filters, two also, at first appeared to transmit only portions of the blue
and violet of the spectrum, but my later examination revealed a trace of
green. It is important to note, however, that the red and the blue-violet
filters were mutually exclusive in the portions of the spectrum which they
transmitted. Of all the filters used the green finally proved the least
satisfactory. I detected some yellow and blue in addition to green in my
first examination, and later I discovered a trace of red. Apparently the
transmitting power of the solutions changed slightly during the course of
the experiments. On this account certain solutions are undesirable for
experiments on color vision, for one must be certain of the constancy of
the condition of stimulation. It is to be understood, of course, that each
of the three filters transmitted, so far as the eye is concerned, only the
color named. I consider the red filter perfectly satisfactory, the blue-
violet very good, and the green poor. Henceforth, in testing color vision
in animals, I shall make use of colored glasses as filters, if it is in
any way possible to obtain or have manufactured blue, green, and yellow
glasses which are as satisfactory as the ruby.

[Footnote 1: A Janssen-Hoffman spectroscope was used.]

The apparatus needs no further description, as its other important
features were identical with those of the reflected light experiment box.
The use of artificial light for the illumination of the electric-boxes
made it necessary to conduct all of the following tests in a dark-room.
The method of experimentation was practically the same as that already
described. A mouse which had been placed in _A_ by the experimenter was
permitted to enter _B_ and thence to return to _A_ by entering one of the
electric-boxes, the red or blue or green one, as the case might be.
Mistakes in choice were punished by an electric shock. One further point
in the method demands description and discussion before the results of the
tests are considered, namely, the manner of regulating and measuring the
brightness of the lights.

Regulating brightness with this apparatus was easy enough; measuring it
accurately was extremely difficult. The experimenter was able to control
the brightness of each of the two colored lights which he was using by
changing the position or the power of the incandescent lamps in the light-
box. The position of a lamp could be changed easily between tests simply
by moving it along toward or away from the electric-box in the slit which
served as a lamp carrier. As the distance from the entrances of the
electric-boxes to the further end of the light-box was 120 cm., a
considerable range or variation in brightness was possible without change
of lamps. Ordinarily it was not necessary to change the power of the
lamps, by replacing one of a given candle power by a higher or lower,
during a series of tests. Both the candle power of the lamps and their
distance from the filters were recorded in the case of each test, but for
the convenience of the reader I have reduced these measurements to candle
meters[1] and report them thus in the descriptions of the experiments.

[Footnote 1: The illuminating power of a standard candle at a distance of
one meter.]

But measuring the actual brightness of the red light or the green light
which was used for a particular series of tests, and the variations in
their brightnesses, was not so simple a matter as might appear from the
statements which have just been made. The influence of the light filters
themselves upon the brightness must be taken into account. The two red
filters were alike in their influence upon the light which entered them,
for they were precisely alike in construction, and the same was true of
the two blue-violet filters. The same kind of ruby glass was placed in
each of the former, and a portion of the same solution of copper ammonium
sulphate was put into each of the filter boxes for the latter. But it is
difficult to say what relation the diminution in brightness caused by a
red filter bore to that caused by a blue-violet or a green filter. My only
means of comparison was my eye, and as subjective measurement was
unsatisfactory for the purposes of the experiment, no attempt was made to
equalize the amounts of brightness reduction caused by the several
filters. So far as the value of the tests themselves, as indications of
the condition of color vision in the dancer is concerned, I have no
apology for this lack of measurement, but I do regret my inability to give
that accurate objective statement of brightness values which would enable
another experimenter with ease and certainty to repeat my tests. The
nearest approach that it is possible for me to make to such an objective
measurement is a statement of the composition and thickness of the filters
and of the candle-meter value of the light when it entered the filter. The
distance from this point to the entrance to the electric-box was 20 cm.

To sum up and state clearly the method of defining the brightness of the
light in the following experiments: the candle-meter value of each light
by which an electric-box was illuminated, as determined by the use of a
Lummer-Brodhun photometer and measurements of the distance of the source
of light from the filter, is given in connection with each of the
experiments. This brightness value less the diminution caused by the
passage of the light through a filter, which has been defined as to
composition and thickness of the layer of solution, gives that degree of
brightness by which the electric-box was illuminated.

Tests of the dancer's ability to discriminate green and blue[1] in the
transmitted light apparatus were made with four animals. An incandescent
lamp marked 16-candle-power was set in each of the light-boxes. These
lamps were then so placed that the green and the blue seemed to be of
equal brightness to three persons who were asked to compare them
carefully. Their candle-meter values in the positions selected were
respectively 18 and 64, as appears from the statement of conditions at the
top of Table 22.

[Footnote 1: Hereafter the light transmitted by the blue-violet filter
will be referred to for convenience as blue.]



Brightnesses Equal for Human Eye

Green 18 candle meters Blue 64 candle meters

A and B[1] April 2 10 10 12 8
1 3 6 4 5 5
2 4 5 5 6 4
3 5 5 5 5 5
4 6 5 5 5 5
5 7 7 3 5 5
6 8 7 3 3 7
7 9 7 3 5 5
8 10 3 7 7 3
9 11 5 5 4 6
10 12 5 5 6 4
[Footnote: A single preference series of twenty tests.]

Numbers 10 and 11 exhibited no preference for either of these colors in
the series of 20 tests which preceded the training tests, and neither of
them gave evidence of ability to discriminate as the result of ten series
of training tests. In this case, again, the behavior of the animals was as
strongly against the inference that they can tell green from blue as are
the records of choices which appear in the table. Granted, that they are
unable to discriminate green from blue when these colors are of about the
same brightness for the human eye, what results when they differ markedly
in brightness? Table 23 furnishes a definite answer to this question.
Numbers 5 and 12 were given eight series of green-blue tests with each
light at 18 candle meters. Little, if any, evidence of discrimination
appeared. Then, on the supposition that the difference was not great
enough for easy discrimination, the blue light was reduced almost to 0,
the green being left at 18. The tests (series 9) immediately indicated
discrimination. For series 10 the green was made 64 candle meters, the
blue 18, and again there was discrimination. These results were so
conclusively indicative of the lack of color vision and the presence of
brightness vision, that there appeared to be no need of continuing the
experiment further.

Accepting provisionally the conclusion that the dancers cannot tell green
from blue except by brightness differences, we may proceed to inquire
whether they can discriminate other colors. Are green and red

Green-red discrimination now was tested by a method which it was hoped
might from the first prevent dependence upon brightness. The light in the
light-box on the left was so placed that it had a value of 18 candle
meters, that in the light-box on the right so that it had a value of 1800
candle meters. Neither light was moved during the first four series of the
green-red tests which were given to Nos. 151 and 152.



Brightnesses Different for Human Eye

Green 18 candle meters Blue 18 candle meters

No. 5 No. 12

1 April 10 6 4 5 5
2 11 5 5 7 3
3 12 6 4 7 3
4 13 4 6 7 3
5 14 7 3 5 5
6 15 4 6 6 4
7 16 6 4 8 2
8 17 5 5 4 6

As it was now evident that the intensity difference was not sufficient to
render discrimination easy, the blue was reduced to 0 and the green left
at 18.

9 17 7 3 8 2

Now the brightnesses were made, green 64, blue 18, just the reverse of
those of series of Table 22.

10 17 8 2 8 2

Each of these series consisted of 20 tests instead of 10. As a result of
the arrangement of the lights just mentioned, the green appeared to me
very much brighter than the red when it was on the right and very much
darker when it was on the left. If this were true for the mouse also, it
is difficult to see how it could successfully depend upon brightness for
guidance in its choices. Such dependence would cause it to choose now the
green, now the red.

The first four series of green-red tests so clearly demonstrated
discrimination, of some sort, that it was at once necessary to alter the
conditions of the experiment. The only criticism of the above method of
excluding brightness discrimination, of which I could think, was that the
red at no time had been brighter than the green. In other words, that
despite a value of 1800 candle meters for the red and only 18 candle
meters for the green, the latter still appeared the brighter to the mouse.
To meet this objection, I made the extreme brightness values 1 and 1800
candle meters in some of the later series, of which the results appear in
Table 24. From day to day different degrees of brightness were used, as is
indicated in the second column of the table. Instead of having first one
color and then the other the brighter, after the fourth series I changed
the position of the lights each time the position of the filters was
changed; hence, the table states a certain brightness value for each color
instead of for each electric-box.

Series 5 to 14 so clearly indicated discrimination, that it seemed
necessary to devise some other means than that of changing the
brightnesses of the colored lights themselves to test the assumption that
the animals were choosing the brighter light. I therefore removed the
light filters so that the colors which had been present as conditions of
discrimination were lacking, and arranged the apparatus so that first one
box, then the other, was illuminated the more brightly. The purpose of
this was to discover whether as the result of their green-red training the
mice had acquired the habit of choosing uniformly either the lighter or
the darker box. One series was given under the conditions of illumination
specified in Table 24 with the result that the brighter box was chosen
eight times in ten by No. 151 and every time by No. 152. Since neither of
these individuals had previously been trained by white-black tests to go
to the white, and since, furthermore, the dancers usually manifest a
slight preference for the lower instead of the higher illumination, this
result may be interpreted as indicative of dependence upon brightness in
the previous color tests. It looks very much indeed as if the green had
been chosen, not because of its greenness, but on account of its
relatively greater brightness.

This test of brightness preference was followed by two series, 16 and 17,
under conditions similar to those of the first four series of the table.
For series 16 the value of the light in the left box was 1 candle meter,
that of the light in the right box 1800 candle meters. Discrimination was
perfect. For series 17 the value for the left remained at 1 candle meter,
but that of the right box was decreased to 0. In this series No. 152 was
entirely at a loss to know which box to choose. Of course this was an
entirely new set of conditions for choice, namely, a colored box, the
green or the red as the case might be, beside a dark box, the one which
was not illuminated. If the mice really had been choosing correctly
because of a habit of avoiding the red or of seeking the green, this
method should bring out the fact, for the red box, since with it the
disagreeable electric shock had always been associated, should be a box to
be avoided. For No. 151 this seemed to be the case.

Series 23 to 27 of Table 24 were given as final and crucial tests of the
relation of brightness discrimination to color discrimination. As it is
not possible to express in a simple formula the conditions of the tests, a
sample series which indicates the brightness of the colors in each of the
twenty tests of a series, and in addition the results given by No. 151 in
the first of these final series, is reproduced in Table 25. For an animal
which had presumably learned perfectly to choose green in preference to
red, the record of 8 mistakes in 20 choices as a result of changes in
relative brightness is rather bad, and it renders doubtful the existence
of color discrimination in any of these experiments. No. 152 showed no
ability whatever to choose the green in the first of the series (series 23
of Table 24) of which that of Table 25 is a sample. His record, 10
mistakes in 20 choices, was even poorer than that of No. 151. That both of
these mice learned to choose fairly accurately in these final tests is
shown by the results of series 24, 25, 26, and 27. I must admit, however,
that these records indicate little ability on the part of the animals to
discriminate colors.



Brightnesses Extremely Different for Human Eye
Intensities are given in candle meters (c.m.)

NO. 151 NO. 152

1 April 26 18 c.m. on left
1800 c.m. on right 11 9 7 13
2 27 Same 16 4 16 4
3 28 Same 20 0 17 3
4 29 Same 19 1 19 1
5 30 Green 18 c.m.
Red 18 c.m. 9 1 10 0
6 30 Green 64 c.m.
Red 18 c.m. 9 1 8 2
7 May 1 Green 6 c.m.
Red 1500 c.m. 7 3 9 1
8 1 Green 4 c.m.
Red 1500 c.m. 8 2 7 3
9 2 Both varied from
4 to 1500 c.m. 18 2 18 2
10 3 Green 2 c.m.
Red 1800 c.m. 6 4 7 3
11 3 Same 10 0 10 0
12 4 Same 7 3 8 2
13 4 Same 8 2 6 4
14 5 Green 1 c.m.
Red 1800 c.m. 19 1 19 1

Filters were now removed. An illumination of 15 c.m. was established on
one side and an illumination of 0 on the other side, in order to ascertain
whether the mice would choose the brighter box. This was done to test the
assumption that the green in the previous tests had always appeared
brighter to the mice than did the red, and that in consequence they had
chosen the brighter box instead of the green box.


No. 151 No. 152


15 May 5 Brighter 15 c.m. 8[1] 2[2] 10[1] 0[2]
Darker 0 c.m.
16 5 1 c.m. on left
1800 c.m. on right 10 0 10 0
17 5 1 c.m. on left
0 c.m. on right 9 1 4 6
18 5 Green 18 c.m.
Red 18 c.m. 19 1 17 3
19 9 Same 9 1 9 1
20 9 Same 10 0 10 0
21 10 Same 10 0 10 0
22 11 Same 10 0 10 0
23 June 1 Both varied from
1 to 1800 c.m. 12 8 10 10
24 2 Same 18 2 14 6
25 June 3 Both varied from
2 to 1800 c.m. 19 1 17 3
26 4 Same 17 3 17 3
27 5 Same 18 2 18 2

[Footnote 1: Brighter.]
[Footnote 2: Darker.]

These long-continued and varied tests with Nos. 151 and 152 revealed three
facts: that the mice depend chiefly upon brightness differences in visual
discrimination; that they probably have something which corresponds to our
red-green vision, although their color experience may be totally unlike
ours; and that the red end of the spectrum seems much darker to them than
to us, or, in other words, that the least refrangible rays are of lower
stimulating value for them than for us.



June 1, 1906 No. 151

1 Green on left Green 4, Red 448 Right --
2 Green on right Green 448, Red 4 Right --
3 Green on right Green 4, Red 448 Right --
4 Green on left Green 448, Red 4 Right --
5 Green on left Green 3, Red 1800 -- Wrong
6 Green on right Green 1800, Red 3 -- Wrong
7 Green on right Green 3, Red 1800 -- Wrong
8 Green on left Green 1800, Red 3 Right --
9 Green on right Green 5, Red 34 Right --
10 Green on left Green 34, Red 5 Right --
11 Green on right Green 6, Red 74 Right --
12 Green on left Green 74, Red 6 Right --
13 Green on left Green 4, Red 448 -- Wrong
14 Green on right Green 448, Red 4 Right --
15 Green on right Green 4, Red 448 -- Wrong
16 Green on left Green 448, Red 4 Right --
17 Green on right Green 3, Red 1800 -- Wrong
18 Green on left Green 1800, Red 3 -- Wrong
19 Green on right Green 1800, Red 3 -- Wrong
20 Green on left Green 3, Red 1800 Right --

Totals 12 8

So many of the results of my color experiments have indicated the all-
important role of brightness vision that I have hesitated to interpret any
of them as indicative of true color discrimination. But after I had made
all the variations in brightness by which it seemed reasonable to suppose
that the mouse would be influenced under ordinary conditions, and after I
had introduced all the check tests which seemed worth while, there still
remained so large a proportion of correct choices that I was forced to
admit the influence of the quality as well as of the intensity of the
visual stimulus.

The first of the facts mentioned above, that brightness discrimination is
more important in the life of the mouse than color discrimination, is
attested by almost all of the experiments whose results have been
reported. The second fact, namely, that the dancer possesses something
which for the present we may call red-green vision, also has been proved
in a fairly satisfactory manner by both the reflected and the transmitted
light experiments. I wish now to present, in Table 26, results which
strikingly prove the truth of the statement that red appears darker to the
dancer than to us.

The brightness conditions which appeared to make the discrimination
between green and red most difficult were, so far as my experiments permit
the measurement thereof, green from 1 to 4 candle meters with red from
1200 to 1600. Under these conditions the red appeared extremely bright,
the green very dark, to the human subject.

According to the description of conditions in Table 26, Nos. 2 and 5 were
required to distinguish green from red with the former about 3 candle
meters in brightness and the latter about 1800 candle meters. In the
eighth series of 20 tests, each of these animals made a perfect record. As
it seemed possible that they had learned to go to the darker of the two
boxes instead of to the green box, I arranged the following check test.
The filters were removed, the illumination of one electric-box was made 74
candle meters, that of the other 3, and the changes of the lighter box
from left to right were made at irregular intervals. In February, No. 2
had been trained to go to the black in black-white tests, and at the same
time No. 5 had been trained to go to the white in white-black tests. The
results of these brightness check tests, as they appear in the table,
series 8 _a_, are indeed striking. Number 2 chose the darker box each
time; No. 5 chose it eight times out of ten. Were it not for the fact that
memory tests four weeks after his black-white training had proved that No.
2 had entirely lost the influence of his previous experience (he chose
white nine times out of ten in the memory series), it might reasonably be
urged that this individual chose the darker box because of his experience
in the black-white experiment. And what can be said in explanation of the
choices of No. 5? I can think of no more reasonable way of accounting for
this most unexpected result of the brightness tests than the assumption
that both of these animals had learned to discriminate by brightness
difference instead of by color.



Brightnesses Different for Human Eye

No. 2 No. 5


1 May 7 Green
Red 1800 c.m. 10 10 12 8
2 8 Same 12 8 11 9
3 9 Same 15 5 14 6
4 10 Same 18 2 12 8
5 11 Same 18 2 14 6
6 12 Same 19 1 16 4
7 13 Same 19 1 18 2
8 14 Same 20 0 20 0

Brightness tests without colors were now given to determine whether the
mice had been choosing the brighter or the darker instead of the green.


NO. 2 NO. 5


8a 14 Brighter 74 c.m. 0[1] 10[2] 2[1] 8[2]
Darker 3 c.m.
9 15 3 c.m. on left
1800 c.m. on right 8 12 16 4
10 16 4 c.m. on left
36 c.m. on right 5 5 7 3
11 16 Green 4 c.m.
Red 36 c.m. 9 1 8 2
12 17 11 c.m. on left
1800 c.m. on right 7 3 6 4
13 17 Green 11 c.m.
Red 1800 c.m. 9 1 8 2
14 18 Mixed values
3 to 1800 c.m. 7 3 8 2
15 19 Same 7 3 7 3
16 20 Same 7 3 7 3
17 21 Same 7 3 9 1
18 22 Same 9 1 8 2
19 23 Same 7 3 9 1
20 24 Same 10 0 8 2
21 25 Same 10 0 9 1
22 26 Same 9 1 10 0

[Footnote 1: Brighter]
[Footnote 2: Darker]

Immediately after the brightness series, the influence of making first one
color, then the other, the brighter was studied. Throughout series 9 the
brightness value of the left box remained 3 candle meters, that of the
right side 1800 candle meters. Number 2 was so badly confused by this
change that his mistakes in this series numbered 12; No. 5 made only 4
incorrect choices. Then series after series was given under widely
differing conditions of illumination. The expression "mixed values," which
occurs in Table 26 in connection with series 14 to 22 inclusive, means
that the brightnesses of the green and the red boxes were changed from
test to test in much the way indicated by the sample series of Table 25.
In view of the results of these 22 series, 320 tests for each of two mice,
it is evident that the dancer is able to discriminate visually by some
other factor than brightness. What this factor is I am not prepared to
say. It may be something akin to our color experience, it may be distance
effect. No other possibilities occur to me.

Table 26 shows that discrimination was relatively easy for Nos. 2 and 5
with green at 3 candle meters and red at 1800. That their discrimination
was made on the basis of the greater brightness of the red, instead of on
the basis of color, is indicated by the results of the brightness check
series 8a. Increase in the brightness of the green rendered discrimination
difficult for a time, but it soon improved, and by no changes in the
relative brightness of the two colors was it possible to prevent correct

In addition to giving point to the statement that red appears darker to
the dancer than to us, the above experiment shows that the animals depend
upon brightness when they can, and that their ability to discriminate
color differences is extremely poor, so poor indeed that it is doubtful
whether their records are better than those of a totally color blind
person would be under similar conditions. Surely in view of such results
it is unsafe to claim that the dancer possesses color vision similar to

Perfectly trained as they were, by their prolonged green-red tests, to
choose the green, or what in mouse experience corresponds to our green,
Nos. 2 and 5 offered an excellent opportunity for further tests of blue-
green discrimination. For in view of their previous training there should
be no question of preference for the blue or of a tendency to depend upon
brightness in the series whose results constitute Table 27.


NO. 2 NO. 5


1 June 1 Blue 74 c.m.
Green 36 c.m. 3 7 3 7
2 2 Same 5 5 4 6
3 3 Same 5 5 6 4
4 4 Same 6 4 3 7
5 5 Same 6 4 5 5
6 6 Blue 21 c.m.
Green 21 c.m. 6 4 7 3
7 7 Same 2 8 3 7
8 8 Same 5 5 4 6
9 9 Same 3 7 6 4
10 10 Same 2 8 4 6
11 12 Same 6 4 3 7
12 13 Blue 36 c.m.
Green 21 c.m. 3 7 4 6
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
(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

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.

Again I must call attention to the fact that the behavior of the mice in
these experiments is even more significant of their lack of discriminating
ability than are the numerical results of the tables. After almost every
series of tests, whether or not it came out numerically in favor of
discrimination, I was forced to add the comment, "No satisfactory evidence
of discrimination."

We have now examined the results of green-red, green-blue, and blue-green
tests. One other important combination of the colors which were used in
these experiments is possible, namely, blue-red. This is the most
important of all the combinations in view of the results already
described, for these colors represent the extremes of the visible
spectrum, and might therefore be discriminable, even though those which
are nearer together in the spectral series were not.


No. 2 No. 205

1 July 31 1800 c.m. on left
24 c.m. on right 5 5 6 4
2 Aug. 1 21 c.m. on left
1800 c.m. on right 6 4 6 4
3 2 1800 c.m. on left
21 c.m. on right 8 2 6 4
4 3 19 c.m. on left
1800 c.m. on right 9 1 6 4
5 4 1800 c.m. on left
7 c.m. on right 7 3 5 5
6 5 6 c.m. on left
1800 c.m. on right 10 0 7 3
7 6 18 c.m. on left
74 c.m. on right 10 0 9 1
8 7 1800 c.m. on left
7 c.m. on right 8 2 8 2
9 8 7 c.m. on left
1800 c.m. on right 7 3 8 2
10 9 Mixed values
6 to 1800 c.m. 8 2 9 1
11 10 Blue 3 c.m.
Red 1800 c.m. 7 3 6 4

Brightness tests were now made, without the use of colors.

11a 10 4 6 5 5

12 10 Blue 3 c.m.
Red 8 c.m. 4 6 6 4
13 11 Blue 3 c.m.
Red 7200 c.m. 8 2 5 5
14 13 Mixed values
3 to 7200 c.m. 7 3 7 3
15 13 Same 7 3 9 1
16 14 Blue 3 to 6 c.m.
Red 112 to 3650 c.m. 10 0 10 0

Series were now given to test the assumption that red appears dark to the

17 14 Darkness on one side
Red 3 c.m. 5 5 7 3
18 14 Blue 3 to 3650 c.m.
Red 3 to 3650 c.m. 10 0 10 0
19 15 Darkness on one side
Red 3 c.m. 5 5 4 6
20 15 Blue 3 to 3650 c.m.
Red 3 to 3650 c.m. 10 0 9 1
21 16 Darkess on one side
Red 72 c.m. 5 5 7 3
22 16 Darkness on one side
Red 1800 c.m. 6 4 10 0

As is shown by the results in Table 28, no combination of brightnesses
rendered correct choice impossible in the case of the blue-red tests which
are now to be described. Choice was extremely difficult at times, even
more so perhaps than the table would lead one to suppose, and it is quite
possible that color played no part in the discrimination. But that
brightness difference in the colors was not responsible for whatever
success these mice attained in selecting the right box is proved by the
brightness-without-color series which follows series II of the table.
Neither No. 2 nor No. 205 showed preference for the lighter or the darker
box. At the end of the sixteenth blue-red series, I was convinced that one
of two conclusions must be drawn from the experiment: either the dancers
possess a kind of blue-red vision, or red is of such a value for them that
no brightness of visible green or blue precisely matches it.

The latter possibility was further tested by an experiment whose results
appear in series 17 to 22 inclusive, of Table 28. The conditions of series
17 were a brightness value of 0 in one box (darkness) and in the other red
of a brightness of 3 candle meters. Despite the fact that they had been
perfectly trained in _blue-red tests_ to avoid the red, neither of the
mice seemed able to discriminate the red from the darkness and to avoid
it. This was followed by a series in which the brightness of both the blue
and the red was varied between 3 and 3650 candle meters, with the striking
result that neither mouse made any mistakes. In series 19 red was used
with darkness as in series 17, and again there was a total lack of
discrimination. Series 20 was a repetition of series 18, with practically
the same result. I then attempted to find out, by increasing the
brightness of the red, how great must be its value in order that the
dancers should distinguish it readily from darkness. For the tests of
series 21 it was made 72 candle meters, but discrimination did not clearly
appear. At 1800 candle meters, as is shown in series 22, the red was
sufficiently different in appearance from total darkness to enable No. 205
to discriminate perfectly between the two electric-boxes. For No. 2
discrimination was more difficult, but there was no doubt about his
ability. It would appear from these tests that the dancers had not learned
to avoid red. Therefore we are still confronted with the question, can
they see colors?


With the Electric-boxes Precisely Alike Visually

No. 151 No. 152

1 Sept. 29 6 4 4 6
2 30 5 5 6 6

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