Far more thoroughgoing tests than those of Rawitz or Panse were made by

Cyon (9 p. 218), who holds the unique position of being the only person on

record who has observed the adult dancer give definite reactions to

sounds. To a König Galton whistle so adjusted that it gave a tone of about

7000 complete vibrations per second, which is said to be about the pitch

of the voice of the dancer, some of the animals tested by Cyon responded

unmistakably, others not at all. In one group of four mice, two not only

reacted markedly to the sound of the whistle but apparently listened

intently, for as soon as the whistle was blown they ran to the side of the

cage and pressed their noses against the walls as if attempting to

approach the source of the stimulus. The remaining two mice gave not the

slightest indication that the sound acted as a stimulus. By the repetition

of this sound from eight to twelve times Cyon states that he was able to

arouse the mice from sleep. When thus disturbed, the female came out of

the nest box before the male. Similarly when the mice were disturbed by

the whistle in the midst of their dancing, the female was first to retreat

into the nest box. There is thus, according to Cyon, some indication of

sex, as well as individual, differences in sensitiveness to the sound of

the whistle. Cyon's statement that in order to evoke a response the

whistle must be held above the head of the dancer suggests at once the

possibility that currents of air or odors instead of sounds may have been

responsible for the reactions which he observed. The work of this

investigator justifies caution in the acceptance of his statements.

Neither the conditions under which the auditory tests were made nor the

condition of the animals is described with sufficient accuracy to make

possible the comparison of Cyon's work with that of other investigators.

As will appear later, it is of the utmost importance that the influence of

other stimuli than sound be avoided during the tests and that the age of

the mouse be known. The conclusion reached by Cyon is that some dancers

are able to hear sounds of about the pitch of their own cries.

The fact, emphasized by Cyon, that the mice respond to tones of about the

pitch of their own voice is of peculiar interest in its relation to the

additional statements made by the same author to the effect that the

female dancer is more sensitive to sounds than the male, and that the

males either do not possess a voice or are much less sensitive to

disagreeable stimuli than the females. In the case of the dancers which he

first studied (9 p. 218), Cyon observed that certain strong stimuli evoked

pain cries; but later in his investigation he noticed that four

individuals, all of which were males, never responded thus to disagreeable

stimulation (11 p.431). He asks, therefore, does this mean that the males

lack a voice or that they are less sensitive than the females? The fact

that he did not succeed in getting a definite answer to this simple

question is indicative of the character of Cyon's work. My dancers have

provided me with ample evidence concerning the matter. Both the males and

the females, among the dancers which I have studied, possess a voice.

The females, especially during periods of sexual excitement, are much more

likely to squeak than the males. At such times they give their shrill cry

whenever they are touched by another mouse or by the human hand. A slight

pinching of the tail will frequently cause the female to squeak, but the

male seldom responds to the same stimulus by crying out. The most

satisfactory way to demonstrate the existence of a voice in the male is to

subject him to the stimulating effect of an induced current, so weak that

it is barely appreciable to the human hand. To this unexpected stimulus

even the male usually responds by a sudden squeak. There can be no doubt,

then, of the possession of a voice by both males and females. The males

may be either less sensitive or less given to vocal expression, but they

are quite able to squeak when favorable conditions are presented. The

possession of a voice by an animal is presumptive evidence in favor of a

sense of hearing, but it would scarcely be safe to say that the mice must

be able to hear their own voices. Cyon, however, thinks that some dancers

can. What further evidence is to be had?

Although they obtained no visible motor reactions to such noises as the

clapping of the hands, the snapping of the fingers, or to the tones of

tuning forks of different pitches and the shrill tones of the Galton

whistle, Alexander and Kreidl (1 p. 547) are not convinced of the total

deafness of the dancer, for, as they remark, common mice which undoubtedly

hear do not invariably respond visibly to sounds. Furthermore, the

anatomical conditions revealed by their investigation of the ear of the

dancer are not such as to render sensitiveness to sounds impossible. They

recognize also that the existence of the ability to produce sounds is an

indication of hearing. They have no confidence in the results reported by

Cyon, for they feel that he did not take adequate precautions to guard

against the action of other than auditory stimuli.

Zoth (31 p. 170) has pointed out with reason and force that testing the

sensitiveness of the mice is especially difficult because of their

restlessness. They are almost constantly executing quick, jerky movements,

starting, stopping, or changing the direction of movement, and it is

therefore extremely difficult to tell with even a fair degree of certainty

whether a given movement which occurs simultaneously with a sound is a

response to the sound or merely coincident with it. With great care in the

exclusion of the influence of extraneous stimuli, Zoth tried a large

number of experiments to test the hearing of both young and adult dancers.

Not once did he observe an indubitable auditory reaction. As he says, "I

have performed numerous experiments with the Galton whistle, with a

squeaking glass stopper, with caps and cartridges, without being able to

come to any certain conclusion. With reference to the Galton whistle and

particularly to the tone which was said to have been heard extremely well

by Cyon's mice, I believe I am rather safe in asserting that my mice,

young (12-13 days) as well as old, do not react to the König Galton

whistle (7210 Vs.). They could not be awakened out of sleep by repetitions

of the sound, nor enticed out of their nests, and their dancing could not

be interrupted" (31 p. 170). Zoth's experiments appear to be the most

careful and critical of those thus far considered.

Last to be mentioned, but in many respects of greatest interest and value,

is the work of Kishi (21 p. 482) on the problem of hearing. To this acute

observer belongs the credit of calling attention emphatically to the ear

movements which are exhibited by the dancer. Frequently, as he remarks,

the ears move as if the animal were listening or trying to determine the

direction whence comes a sound, yet usually the mouse gives no other sign

of hearing. That the absence of ordinary reactions to sounds is due to

deafness, Kishi, like Panse, is led to doubt because his anatomical

studies have not revealed any defects in the organs of hearing which would

seem to indicate the lack of this sense.

This historical survey of the problem of hearing has brought out a few

important facts. No one of the several investigators of the subject, with

the exception of Cyon, is certain that the dancer can hear, and no one of

them, with the exception of Rawitz, is certain that it cannot hear! Cyon

almost certainly observed two kinds of dancing mice. Those of his dancers

which exhibited exceptional ability to climb in the vertical direction and

which also gave good evidence of hearing certain sounds may have been

hybrids resulting from the crossing of the dancer with a common mouse, or

they may have been exceptional specimens of the true dancer variety. A

third possibility is suggested by Rawitz's belief in the ability of the

young dancer to hear. Cyon's positive results may have been obtained with

immature individuals. I am strongly inclined to believe that Cyon did

observe two types of dancer, and to accept his statement that some of the

mice could hear, whereas others could not. It is evident, in the light of

our examination of the experimental results thus far obtained by other

investigators, that neither the total lack of sensitiveness to sounds in

the adult nor the presence of such sensitiveness in the young dancer has

been satisfactorily proved.

I shall now report in detail the results of my own study of the sense of

hearing in the dancer. As the behavior of the young differs greatly from

that of the adult, by which is meant the sexually mature animal, I shall

present first the results of my experiments with adults and later, in

contrast, the results obtained with mice from one to twenty-eight days

old.

My preliminary tests were made with noises. While carefully guarding

against the interference of visual, tactual, temperature, and olfactory

stimuli, I produced noises of varying degrees of loudness by clapping the

hands together suddenly, by shouting, whistling, exploding pistol caps,

striking steel bars, ringing an electric bell, and causing another mouse

to squeak. To these sounds a common mouse usually responds either by

starting violently, or by trembling and remaining perfectly quiet for a

few seconds, as if frightened. The adult dancers which I have tested, and

I have repeated the experiment scores of times during the last three years

with more than a hundred different individuals, have never given

unmistakable evidence of hearing. Either they are totally deaf or there is

a most surprising lack of motor reactions.

Precisely the same results were obtained in tests made with the Galton

whistle throughout its range of pitches, and with Appuun whistles which,

according to their markings, ranged from 2000 Vs. (C_4) to 48,000 (G_9),

but which undoubtedly did not correspond at all exactly to this range, and

with a series of König tuning forks which gave tones varying in pitch from

1024 to 16,382 complete vibrations.

I am willing to trust these experimental results the more fully because

during all the time I have had adult dancers under observation I have

never once seen a reaction which could with any fair degree of certainty

be referred to an auditory stimulus. Never once, although I have tried

repeatedly, have I succeeded in arousing a dancer from sleep by producing

noises or tones, nor have I ever been able to observe any influence of

sounds on the dance movements. All of Cyon's signs have failed with my

mice. Occasionally what looked like a response to some sound appeared, but

critical observation invariably proved it to be due to some other cause

than the auditory stimulus. A sound produced above the animal is very

likely to bring about a motor reaction, as Cyon claims; but I have always

found it to be the result of the currents of air or odors, which usually

influence the animal when the experimenter is holding any object above it.

I do not wish to maintain that Cyon's conclusions are false; I merely

emphasize the necessity for care in the exclusion of other stimuli. The

mice are extremely sensitive to changes in temperature, such, for example,

as are produced by the breath of the experimenter, and one must constantly

guard against the misinterpretation of behavior.

In a single experiment with mice over a month old, I observed what might

possibly indicate sensitiveness to sound. While holding a mouse, thirty-

five days old, in my hand I pursed my lips and made a very shrill sound by

drawing in air; the mouse seemed to start perceptibly according to the

indications given by my sense of touch. I repeated the stimulus several

times and each time I could see and feel the animal start slightly. With

two other individuals which I tested the reaction was less certain, and

with several others I failed to get any indication of response. This would

seem to prove that the three individuals which responded happened to be

sensitive to that particular tone at the age of five weeks. The test is

unsatisfactory because the vibrations from my own body may have brought

about the reaction instead of the air vibrations produced by my lips, and

I therefore merely mention it in the enumeration of the various

experimental tests which I have made.

If we should conclude from all the negative evidence that is available, or

that could be obtained, that the dancer is totally deaf, it might fairly

be objected that the conclusion is unsafe, since an animal does not

necessarily respond to stimuli by a visible change in the position or

relations of its body. Death feigning may fairly be considered a response

to a stimulus or stimulus complex, yet there may be no sign of movement.

The green frog when observed in the laboratory usually gives no indication

whatever, by movements that are readily observable, that it hears sounds

which occur about it, but I have been able to show by means of indirect

methods of study that it is stimulated by these same sounds.[1] Its rate

of respiration is changed by the sounds, and although a sound does not

bring about a bodily movement, it does very noticeably influence movements

in response to other stimuli which occur simultaneously with the sound. I

discovered that under certain rather simple experimental conditions the

green frog would regularly respond to a touch on the back by drawing its

hind leg up toward the body. Under the same conditions the sound of an

electric bell caused no visible movement of the leg, but if at the instant

the back was touched the bell was rung, the leg movement was much greater

than that brought about by the touch alone. This suggests at once the

desirability of studying the sense of hearing in the dancer by some

indirect method. The animal may be stimulated, and yet it may not give any

visible sign of the influence of the auditory stimulus.

[Footnote 1: "The Sense of Hearing in Frogs." _Journal of Comparative

Neurology and Psychology_, Vol. XV, p. 288, 1905.]

Were not the dancing so extremely variable in rapidity and duration, it

might be used as an index of the influence of auditory stimuli. Cyon's

statements would indicate that sounds interfere with the dancing, but as I

obtained no evidence of this, I worked instead with the following indirect

method, which may be called the method of auditory choice.

The apparatus which was used is described in detail in Chapter VII.

Figures 14 and 15 will greatly aid the reader in understanding its

essential features. Two small wooden boxes, identical in form and as

closely similar as possible in general appearance, were placed in a larger

box in such positions that a mouse was forced to enter and pass through

one of them in order to get to the nest-box. On the bottom of each of

these small boxes was a series of wires through which an electric current

could be made to pass at the will of the experimenter. The boxes could

readily be interchanged in position. At one side of the large wooden box

and beyond the range of vision of the mouse was an electric bell which

could be caused to ring whenever the mouse approached the entrance to one

of the small boxes. The point of the experiment was to determine whether

the dancer could learn to avoid the box-which-rang when it was approached.

The method of conducting the tests was as follows. Each day at a certain

hour the mouse was placed in that part of the large box whence it could

escape to the nest-box only by passing through one of the small boxes. If

it approached the wrong box (whether it happened to be the one on the

right or the one on the left depended upon the experimenter's decision),

the bell began to ring as a warning against entering; if it approached the

other box, all was silent. As motives for the choice of the box-which-did-

not-ring both reward and punishment were employed. The reward consisted of

freedom to return to the nest-box _via_ the passage which led from the

box-which-did-not-ring; the punishment, which consisted of a disagreeable

electric shock, was given whenever the mouse entered the wrong box, that

is, the one which had the sound as a warning. Entering the wrong box

resulted in a disagreeable stimulus and in the necessity of returning to

the large box, for the exit to the nest-box by way of the passage from

this box was closed. My assumption, on the basis of extended study of the

ability of the dancer to profit by experience, was that if it could hear

the sound of the bell it would soon learn to avoid the box-that-rang and

enter instead the one which had no sound associated with it.

Systematic tests were made with No. 4 from the 3d to the 12th of February,

inclusive, 1906. Each day the mouse was permitted to find his way to the

nest-box through one of the small boxes ten times in succession. Usually

the experimenter rang the bell alternately for the box on the left and the

box on the right. The time required for such a series of experiments

varied, according to the rapidity with which the mouse made his choice,

from ten to thirty minutes. If in these experiments the animal approached

and entered the right, or soundless box, directly, the choice indicated

nothing so far as ability to hear is concerned; if it entered the wrong,

or sounding box, despite the ringing of the bell, it indicated either the

lack of the influence of experience or inability to hear the sound; but if

it regularly avoided the box-which-sounded it thus gave evidence of

ability to hear the sound of the bell. The purpose of the test was to

determine, not whether the mouse could learn, but whether it could hear.

For ten successive days this experiment was carried on with No. 4 without

the least indication of increasing ability to avoid the wrong box by the

association of the sound of the bell with the disagreeable electric shock

and failure to escape to the nest-box. In fact, the experiment was

discontinued because it became evident that an impossible task had been

set for the mouse. Day by day as the tests were in progress I noticed that

the animal became increasingly afraid of the entrances to the small boxes;

it seemed absolutely helpless in the face of the situation. Partly because

of the definiteness of the negative results obtained with No. 4 and partly

because of the cruelty of subjecting an animal to disagreeable conditions

which it is unable to avoid, the experiment was not repeated with other

individuals. I have never conducted an experiment which gave me as much

discomfort as this; it was like being set to whip a deaf child because it

did not learn to respond to stimuli which it could not feel.

By a very similar method No. 18 was tested for his sensitiveness to the

noise and jar from the induction apparatus which was used in connection

with many of my experiments on vision and the modifiability of behavior.

In this experiment the wrong box was indicated by the buzzing sound of the

apparatus and the slight vibrations which resulted from it. Although No.

18 was tested, as was No. 4, for ten successive days, ten trials each day,

it gave no evidence of ability to avoid the box-which-buzzed.

Since both direct and indirect methods of testing the hearing of the

dancer have uniformly given negative results, in the case of mice more

than five weeks old, I feel justified in concluding that they are totally

deaf and not merely irresponsive to sounds.

Rawitz's statements, and the fact that what may have been auditory

reactions were obtained with a few individuals of five weeks of age,

suggest that the mice may be able to hear at certain periods of life. To

discover whether this is true I have tested the young of twenty different

litters from the first day to the twenty-eighth, either daily or at

intervals of two or three days. In these tests König forks, steel bars,

and a Galton whistle were used. The results obtained are curiously

interesting.

During the first two weeks of life none of the mice which I tested gave

any visible motor response to the various sounds used. During the third

week certain of the individuals responded vigorously to sudden high tones

and loud noises. After the third week I have seen only doubtful signs of

hearing. I shall now describe in detail the method of experimentation, the

condition of the animals, and the nature of the auditory reactions.

Between the twelfth and the eighteenth day the auditory canal becomes open

to the exterior. The time is very variable in different litters, for their

rate of growth depends upon the amount of nourishment which the mother is

able to supply. Without exception, in my experience, the opening to the

ear appears before the eyes are open. Consequently visual stimuli usually

are not disturbing factors in the auditory tests with mice less than

sixteen days old. There is also a sudden and marked change in the behavior

of the mice during the third week. Whereas, for the first fourteen or

eighteen days they are rather quiet and deliberate in their movements when

removed from the nest, some time in the third week their behavior suddenly

changes and they act as if frightened when taken up by the experimenter.

They jump out of his hand, squeak, and sometimes show fight. This is so

pronounced that it has attracted my attention many times and I have

studied it carefully to determine, if possible, whether it is due to some

profound change in the nervous system which thus suddenly increases the

sensitiveness of the animal or to the development of the sexual organs. I

am inclined to think that it is a nervous phenomenon which is intimately

connected with the sexual condition. Within a day or two after it appears

the mice usually begin to show auditory reactions and continue to do so

for three to five days.

I shall now describe the results obtained with a few typical litters. A

litter born of Nos. 151 and 152 gave uniformly negative results in all

auditory tests up to the fourteenth day. On that day the ears were open,

and the following observations were recorded. The five individuals of the

litter, four females and one male, were taken from the nest one at a time

at 7 A.M. and placed on a piece of paper in the bright sunlight. The

warmth of the sun soon quieted them so that auditory tests could be made

to advantage. As soon as an individual had become perfectly still, the

Galton whistle was held at a distance of about four inches from its head

in such a position that it could not be seen nor the currents of air

caused by it felt, and suddenly blown. Each of the five mice responded to

the first few repetitions of this stimulus by movements of the ears,

twitchings of the body, and jerky movements of the legs. The most violent

reactions resulted when the individual was lying on its back with its legs

extended free in the air. Under such circumstances the four legs were

often drawn together suddenly when the whistle was sounded. Similar

responses were obtained with the lip sound already mentioned. Two other

observers saw these experiments, and they agreed that there can be no

doubt that the mice responded to the sound. The sounds which were

effective lay between 5000 and 10,000 complete vibrations.

On the fifteenth day the eyes were just beginning to open. Three of the

mice responded definitely to the sounds, but the other two slightly, if at

all. On the sixteenth day they were all too persistently active for

satisfactory auditory tests, and on the seventeenth, although they were

tested repeatedly under what appeared to be favorable conditions, no signs

of sensitiveness were noted. Although I continued to test this litter, at

intervals of three or four days, for two weeks longer, I did not once

observe a response to sound.

This was the first litter with which I obtained perfectly definite, clear-

cut responses to sounds. That the reactive ability had not been present

earlier than the fourteenth day I am confident, for I had conducted the

tests in precisely the same manner daily up to the time of the appearance

of the reactions. To argue that the mice heard before the fourteenth day,

but were unable to react because the proper motor mechanism had not

developed sufficiently would be short-sighted, for if the response

depended upon the development of such a mechanism, it is not likely that

it would disappear so quickly. I am therefore satisfied that these

reactions indicate hearing.

With another litter the following results were obtained. On the thirteenth

day each of the eight members of the litter responded definitely and

uniformly to the Galton whistle, set at 5 (probably about 8000 complete

vibrations), and to a König steel bar of a vibration rate of 4096 Vs. The

largest individuals, for almost always there are noticeable differences in

size among the members of a litter, appeared to be most sensitive to

sounds.

On the fifteenth day and again on the seventeenth unmistakable responses

to sound were observed; on the eighteenth the responses were indefinite,

and on the nineteenth none were obtained. I continued the tests up to the

twenty-eighth day without further indications of hearing.

Certain individuals in this litter reacted so vigorously to the loud sound

produced by striking the steel bar a sharp blow and also to the Galton

whistle, during a period of five days, that I have no hesitation in saying

that they evidently heard during that period of their lives. Other members

of the litter seemed to be less sensitive; their reactions were sometimes

so indefinite as to leave the experimenter in doubt about the presence of

hearing.

A third litter, which developed very slowly because of lack of sufficient

food, first showed unmistakable reactions to sound on the twenty-first

day. On this day only two of the five individuals reacted. The reactions

were much more obvious on the twenty-second day, but thereafter they

became indefinite.

Still another litter, which consisted of one female and four males, began

to exhibit the quick, jerky movements, already mentioned, on the

fourteenth day. On the morning of the fifteenth day three members of the

litter definitely reacted to the tone of the steel bar, and also to the

hammer blow when the bar was held tightly in the hand of the experimenter.

My observations were verified by another experimenter. Two individuals

which appeared to be very sensitive were selected for special tests. Their

reactions were obvious on the sixteenth, seventeenth, and eighteenth days;

on the nineteenth day they were indefinite, and on the twentieth none

could be detected. Some individuals of this litter certainly had the

ability to hear for at least five days.

A sixth litter of four females and two males first gave indications of the

change in behavior which by this time I had come to interpret as a sign of

the approach of the period of auditory sensitiveness, on the seventeenth

day. I had tested them almost every day previous to this time without

obtaining evidence of hearing. The tests with the steel bar and the Galton

whistle were continued each day until the end of the fourth week without

positive results. To all appearances the individuals of this litter were

unable to hear at any time during the first month of life.

Practically the same results were obtained with another litter of four

females. The change in their behavior was obvious on the eighteenth day,

but at no time during the first month did they give any satisfactory

indications of hearing.

In the accompanying table, I have presented in condensed form the results

of my auditory tests in the case of twelve litters of young dancers.

TABLE 5

PERIOD OF AUDITORY REACTION IN YOUNG DANCERS

PARENTS No. in Change in Ears Auditory Reactions

Litter Behavior Open Appear Disappear

152+151 5 13th day 14th day 14th day 16th day

152+15l 8 (?) 13th day 13th day 17th day

152+151 5 13th day 13th day 13th day 17th day

152+151 4 10th day 12th day 13th day 15th day

410+415 5 14th day 15th day 15th day 19th day

410+415 6 13th day 14th day 14th day 18th day

420+425 2 12th day 14th day 14th day 17th day

210+215 5 17th day 13th day 17th day 19th day

210+215 6 11th day 14th day No reactions

220+225 6 16th day 14th day No reactions

220+225 6 17th day 13th day No reactions

212+211 4 15th day 14th day No reactions

Certain of the litters tested responded definitely to sounds, others gave

no sign of hearing at any time during the first four weeks of life. Of the

twelve litters for which the results of auditory tests are presented in

Table 5, eight evidently passed through an auditory period. It is

important to note that all except one of these were the offspring of Nos.

151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and

425. In fact every one of the litters in this line of descent which I have

tested, and they now number fifteen, has given indications of auditory

sensitiveness. And, on the other hand, only in a single instance have the

litters born of Nos. 210 and 215, or of their descendants, given evidence

of ability to hear.

These two distinct lines of descent may be referred to hereafter as the

400 and the 200 lines. I have observed several important differences

between the individuals of these groups in addition to the one already

mentioned. The 200 mice were sometimes gray and white instead of black and

white; they climbed much more readily and danced less vigorously than

those of the 400 group. These facts are particularly interesting in

connection with Cyon's descriptions of the two types of dancer which he

observed.

In criticism of my conclusion that the young dancers are able to hear

certain sounds for a few days early in life, and then become deaf, it has

been suggested that they cease to react because they rapidly become

accustomed to the sounds. That this is not the case, is evident from the

fact that the reactions often increase in definiteness during the first

two or three days and then suddenly disappear entirely. But even if this

were not true, it would seem extremely improbable that the mouse should

become accustomed to a sudden and startlingly loud sound with so few

repetitions as occurred in these tests. On any one day the sounds were not

made more than five to ten times. Moreover, under the same external

condition, the common mouse reacts unmistakably to these sounds day after

day when they are first produced, although with repetition of the stimulus

at short intervals, the reactions soon become indefinite or disappear.

The chief results of my study of hearing in the dancer may be summed up in

a very few words. The young dancer, in some instances, hears sounds for a

few days during the third week of life. The adult is totally deaf. Shortly

before the period of auditory sensitiveness, the young dancer becomes

extremely excitable and pugnacious.

CHAPTER VII

THE SENSE OF SIGHT: BRIGHTNESS VISION

The sense of sight in the dancer has received little attention hitherto.

In the literature there are a few casual statements to the effect that it

is of importance. Zoth, for example (31 p. 149), remarks that it seems to

be keenly developed; and other writers, on the basis of their observation

of the animal's behavior, hazard similar statements. The descriptions of

the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and

Kishi (p.47), apparently indicate that the sense is of some value; they do

not, however, furnish definite information concerning its nature and its

role in the daily life of the animal.

The experimental study of this subject which is now to be described was

undertaken, after careful and long-continued observation of the general

behavior of the dancer, in order that our knowledge of the nature and

value of the sense of sight in this representative of the Mammalia might

be increased in scope and definiteness. The results of this study

naturally fall into three groups: (1) those which concern brightness

vision, (2) those which concern color vision, and (3) those which indicate

the role of sight in the life of the dancer.

Too frequently investigators, in their work on vision in animals, have

assumed that brightness vision and color vision are inseparable; or, if

not making this assumption, they have failed to realize that the same

wave-length probably has markedly different effects upon the retinal

elements of the eyes of unlike organisms. In a study of the sense of sight

it is extremely important to discover whether difference in the quality,

as well as in the intensity, of a visual stimulus influences the organism;

in other words, whether color sensitiveness, as well as brightness

sensitiveness, is present. If the dancer perceives only brightness or

luminosity, and not color, it is evident that its visual world is

strikingly different from that of the normal human being. The experiments

now to be described were planned to show what the facts really are.

[Illustration: Figure 14.--Discrimination box. _W_, electric-box with

white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H.

Toll.]

As a means of testing the ability of the dancer to distinguish differences

in brightness, the experiment box represented by Figures 14 and 15 was

devised. Figure 14 is the box as seen from the position of the

experimenter during the tests. Figure 15 is its ground plan. This box,

which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as

measured on the outside. The plan of construction and its significance in

connection with these experiments on vision will be clear from the

following account of the experimental procedure. A mouse whose brightness

vision was to be tested was placed in the nest-box, A (Figure 15). Thence

by pushing open the swinging door at _I_, it could pass into the entrance

chamber, _B_. Having entered _B_ it could return to _A_ only by passing

through one of the electric-boxes, marked _W_, and following the alley to

_O_, where by pushing open the swing door it could enter the nest-box. The

door at _I_ swung inward, toward _B_, only; those at _O_, right and left,

swung outward, toward _A_, only. It was therefore impossible for the mouse

to follow any other course than _A-I-B-L-W-E-O_ or _A-I-B-R-W-E-O_. The

doors at _I_ and _O_ were pieces of wire netting of 1/2 cm. mesh, hinged

at the top so that a mouse could readily open them, in one direction, by

pushing with its nose at any point along the bottom. On the floor of each

of the electric-boxes, _W_, was an oak board 1 cm. in thickness, which

carried electric wires by means of which the mouse could be shocked in _W_

when the tests demanded it. The interrupted circuit constituted by the

wires in the two electric-boxes, in connection with the induction

apparatus, _IC_, the dry battery, _C_, and the hand key, _K_, was made by

taking two pieces of No. 20 American standard gauge copper wire and

winding them around the oak board which was to be placed on the floor of

each electric-box. The wires, which ran parallel with one another, 1/2 cm.

apart, fitted into shallow grooves in the edges of the board, and thus, as

well as by being drawn taut, they were held firmly in position. The coils

of the two pieces of wire alternated, forming an interrupted circuit

which, when the key _K_ was closed, was completed if the feet of a mouse

rested on points of both pieces of wire. Since copper wire stretches

easily and becomes loose on the wooden base, it is better to use phosphor

bronze wire of about the same size, if the surface covered by the

interrupted circuit is more than three or four inches in width. The

phosphor bronze wire is more difficult to wind satisfactorily, for it is

harder to bend than the copper wire, and it has the further disadvantage

of being more brittle. But when once placed properly, it forms a far more

lasting and satisfactory interrupted circuit for such experiments as those

to be described than does copper wire. In the case of the electric-boxes

under consideration, the oak boards which carried the interrupted circuits

were separate, and the two circuits were joined by the union of the wires

between the boxes. The free ends of the two pieces of wire which

constituted the interrupted circuit were connected with the secondary coil

of a Porter inductorium whose primary coil was in circuit with a No. 6

Columbia dry battery. In the light of preliminary experiments, made in

preparation for the tests of vision, the strength of the induced current

received by the mouse was so regulated, by changing the position of the

secondary coil with reference to the primary, that it was disagreeable but

not injurious to the animal. What part the disagreeable shock played in

the test of brightness vision will now be explained.

[Illustration: FIGURE 15.--Ground plan of discrimination box. _A_, nest-

box; _B_, entrance chamber; _W,W_, electric-boxes; _L_, doorway of left

electric-box; _R_, doorway of right electric-box; _E_, exit from electric-

box to alley; _I_, swinging door between _A_ and _B_; _O_, swinging door

between alley and _A_; _IC_, induction apparatus; _C_, electric cell; _K_,

key in circuit.]

An opportunity for visual discrimination by brightness difference was

provided by placing dead black cardboard at the entrance and on the inside

of one of the electric-boxes, as shown in Figure 14, _B_, and white

cardboard similarly in the other box. These cardboards were movable and

could be changed from one box to the other at the will of the

experimenter. The test consisted in requiring the mouse to choose a

certain brightness, for example, the white cardboard side, in order to

return to the nest-box without receiving an electric shock. The question

which the experimenter asked in connection with this test really is, Can a

dancer learn to go to the white box and thus avoid discomfort? If we

assume its ability to profit by experience within the limits of the number

of experiences which it was given, such a modification of behavior would

indicate discrimination of brightness. Can the dancer distinguish white

from black; light gray from dark gray; two grays which are almost of the

same brightness? The results which make up the remainder of this and the

following chapter furnish a definite answer to these questions.

To return to the experimental procedure, the mouse which is being tested

is placed by the experimenter in the nest-box, where frequently in the

early tests food and a comfortable nest were attractions. If it does not

of its own accord, as a result of its abundant random activity, pass

through _I_ into _B_ within a few seconds, it is directed to the doorway

and urged through. A choice is now demanded of the animal; to return to

the nest-box it must enter either the white electric-box or the black one.

Should it choose the white box, it is permitted to return directly to _A_

by way of the doorway _E_, the alley, and the swinging door at _O_, and it

thus gets the satisfaction of unobstructed activity, freedom to whirl, to

feed, and to retreat for a time to the nest. Should it choose to attempt

to enter the black box, as it touches the wires of the interrupted circuit

it receives a shock as a result of the closing of the key in the circuit

by the experimenter, and further, if it continues its forward course

instead of retreating from the "stinging" black box, its passage through

_E_ is blocked by a barrier of glass temporarily placed there by the

experimenter, and the only way of escape to the nest-box is an indirect

route by way of _B_ and the white box. Ordinarily the shock was given only

when the mouse entered the wrong box, not when it retreated from it; it

was never given when the right box was chosen. The box to be chosen,

whether it was white, gray, or black, will be called the right box. The

electric shock served as a means of forcing the animal to use its

discriminating ability. But the question of motives in the tests is not so

simple as might appear from this statement.

The reader will wonder why the mouse should have any tendency to enter

_B_, and why after so doing, it should trouble to go further, knowing, as

it does from previous experiences, that entering one of the electric-boxes

may result in discomfort. The fact is, a dancer has no very constant

tendency to go from _A_ to _B_ at the beginning of the tests, but after it

has become accustomed to the box and has learned what the situation

demands, it shows eagerness to make the trip from _A_ to _B_, and thence

by way of either the right or the left route to _A_. That the mouse should

be willing to enter either of the electric-boxes, after it has experienced

the shock, is even more surprising than its eagerness to run from _A_ to

_B_. When first tested for brightness discrimination in this apparatus, a

dancer usually hesitated at the entrance to the electric-boxes, and this

hesitation increased rapidly unless it were able to discriminate the boxes

by their difference in brightness and thus to choose the right one. During

the period of increasing hesitancy in making the choice, the experimenter,

by carefully moving from _I_ toward the entrances to the electric-boxes a

piece of cardboard which extended all the way across _B_, greatly

increased the mouse's desire to enter one of the boxes by depriving it of

dancing space in _B_. If an individual which did not know which entrance

to choose were permitted to run about in _B_, it would often do so for

minutes at a time without approaching the entrance to the boxes; but the

same individual, when confined to a dancing space 4 or 5 cm. wide in front

of the entrances, would enter one of the electric-boxes almost

immediately. This facilitation of choice by decrease in the amount of

space for whirling was not to any considerable extent the result of fear,

for all the dancers experimented with were tame, and instead of forcing

them to rush into one of the boxes blindly and without attempt at

discrimination, the narrowing of the space simply increased their efforts

to discriminate. The common mouse when subjected to similar experimental

conditions is likely to be frightened by being forced to approach the

entrances to the boxes, and fails to choose; it rushes into one box

directly, and in consequence it is as often wrong as right. The dancer

always chooses, but its eagerness to choose is markedly increased by the

restriction of its movements to a narrow space in front of the entrances

between which it is required to discriminate. It is evident that the

animal is uncomfortable in a space which is too narrow for it to whirl in

freely. It must have room to dance. This furnished a sufficiently strong

motive for the entering of the electric-boxes. It must avoid disagreeable

and unfavorable stimuli. This is a basis for attempts to choose, by visual

discrimination, the electric-box in which the shock is not given. It may

safely be said that the success of the majority of the experiments of this

book depended upon three facts: (1) the dancer's tendency to avoid

disagreeable external conditions, (2) its escape-from-confinement-

impelling need of space in which to dance freely, and (3) its abundant and

incessant activity.

Of these three conditions of success in the experiments, the second and

third made possible the advantageous use of the first. For the avoidance

of a disagreeable stimulus could be made use of effectively in the tests

just because the mice are so restless and so active. In fact their

eagerness to do things is so great that the experimenter, instead of

having to wait for them to perform the desired act, often is forced to

make them wait while he completes his observation and record. In this

respect they are unlike most other animals.

My experiments with the dancer differ from those which have been made by

most students of mammalian behavior in one important respect. I have used

punishment instead of reward as the chief motive for the proper

performance of the required act. Usually in experiments with mammals

hunger has been the motive depended upon. The animals have been required

to follow a certain devious path, to escape from a box by working a

button, a bolt, a lever, or to gain entrance to a box by the use of teeth,

claws, hands, or body weight and thus obtain food as a reward. There are

two very serious objections to the use of the desire for food as a motive

in animal behavior experiments--objections which in my opinion render it

almost worthless in the case of many mammals. These are the discomfort of

the animal and the impossibility of keeping the motive even fairly

constant. However prevalent the experience of starvation may be in the

life of an animal, it is not pleasant to think of subjecting it to extreme

hunger in the laboratory for the sake of finding out what it can do to

obtain food. Satisfactory results can be obtained in an experiment whose

success depends chiefly upon hunger only when the animal is so hungry that

it constantly does its best to obtain food, and when the desire for food

is equally strong and equally effective as a spur to action in the

repetitions of the experiment day after day. It is easy enough to get

almost any mammal into a condition of utter hunger, but it is practically

impossible to have the desire for food of the same strength day after day.

In short, the desire for food is unsatisfactory as a motive in animal

behavior work, first, because a condition of utter hunger, as has been

demonstrated with certain mammals, is unfavorable for the performance of

complex acts, second, because it is impossible to control the strength of

the motive, and finally, because it is an inhumane method of

experimentation.

In general, the method of punishment is more satisfactory than the method

of reward, because it can be controlled to a greater extent. The

experimenter cannot force his subject to desire food; he can, however,

force it to discriminate between conditions to the best of its knowledge

and ability by giving it a disagreeable stimulus every time it makes a

mistake. In other words, the conditions upon which the avoidance of a

disagreeable factor in the environment depends are far simpler and much

more constant than those upon which the seeking of an agreeable factor

depends. Situations which are potentially beneficial to the animal attract

it in varying degrees according to its internal condition; situations

which are potentially disagreeable or injurious repel it with a constancy

which is remarkable. The favorable stimulus solicits a positive response;

the unfavorable stimulus demands a negative response.

Finally, in connection with the discussion of motives, it is an important

fact that forms of reward are far harder to find than forms of punishment.

Many animals feed only at long intervals, are inactive, do not try to

escape from confinement, cannot be induced to seek a particular spot, in a

word, do not react positively to any of the situations or conditions which

are employed usually in behavior experiments. It is, however, almost

always possible to find some disagreeable stimulus which such an animal

will attempt to avoid.

As it happens, the dancer is an animal which does not stand the lack of

food well enough to make hunger a possible motive. I was driven to make

use of the avoiding reaction, and it has proved so satisfactory that I am

now using it widely in connection with experiments on other animals. The

use of the induction shock, upon which I depended almost wholly in the

discrimination experiments with the dancer, requires care; but I am

confident that no reasonable objection to the conduct of the experiments

could be made on the ground of cruelty, for the strength of the current

was carefully regulated and the shocks were given only for an instant at

intervals. The best proof of the humaneness of the method is the fact that

the animals continued in perfect health during months of experimentation.

The brightness discrimination tests demanded, in addition to motives for

choice, adequate precautions against discrimination by other than visual

factors, and, for that matter, by other visual factors than that of

brightness. The mouse might choose, for example, not the white or the

black box, but the box which was to the right or to the left, in

accordance with its experience in the previous test. This would be

discrimination by position. As a matter of fact, the animals have a strong

tendency at first to go uniformly either to the right or to the left

entrance. This tendency will be exhibited in the results of the tests.

Again, discrimination might depend upon the odors of the cardboards or

upon slight differences in their shape, texture, or position. Before

conclusive evidence of brightness discrimination could be obtained, all of

these and other possibilities of discrimination had to be eliminated by

check tests. I shall describe the various precautions taken in the

experiments to guard against errors in interpretation, in order to show

the lengths to which an experimenter may be driven in his search for

safely interpretable results.

To exclude choice by position, the cardboards were moved from one

electric-box to the other. When the change was made regularly, so that

white was alternately on the right and the left, the mouse soon learned to

go alternately to the right box and the left without stopping to notice

the visual factor. This was prevented by changing the position of the

cardboards irregularly.

Discrimination by the odor, texture, shape, and position of the cardboards

was excluded by the use of different kinds of cardboards, by changing the

form and position of them in check tests, and by coating them with

shellac.

The brightness vision tests described in this chapter were made in a room

which is lighted from the south only, with the experiment box directed

away from the windows. The light from the windows shone upon the

cardboards at the entrances to the electric-boxes, not into the eyes of

the mouse as it approached them. Each mouse used in the experiments was

given a series of ten tests in succession daily. The experiment was

conducted as follows. A dancer was placed in _A_, where it usually ran

about restlessly until it happened to find its way into _B_. Having

discovered that the swing door at _I_ could be pushed open, the animal

seemed to take satisfaction in passing through into _B_ as soon as it had

been placed in or had returned to _A_. In _B_, choice of two entrances,

one of which was brighter than the other, was forced by the animal's need

of space for free movement. If the right box happened to be chosen, the

mouse returned to _A_ and was ready for another test; if it entered the

wrong box, the electric shock was given, and it was compelled to retreat

from the box and enter the other one instead. In the early tests with an

individual, a series sometimes covered from twenty to thirty minutes; in

later tests, provided the condition of discrimination was favorable, it

did not occupy more than ten minutes.

To exhibit the methods of keeping the records of these experiments and

certain features of the results, two sample record sheets are reproduced

below. The first of these sheets, Table 6, represents the results given by

No. 5, a female,[1] in her first series of white-black tests. Table 7

presents the results of the eleventh series of tests given to the same

individual.

[Footnote 1: It is to be remembered that the even numbers always designate

males; the odd numbers, females.]

In the descriptions of the various visual experiments of this and the

following chapters, the first word of the couplet which describes the

condition of the experiment, for example, white-black, always designates

the visual condition which the animal was to choose, the second that which

it was to avoid on penalty of an electric shock. In the case of Tables 6

and 7, for example, white cardboard was placed in one box, black in the

other, and the animal was required to enter the white box. In the tables

the first column at the left gives the number of the test, the second the

positions of the cardboards, and the third and fourth the result of the

choice. The first test of Table 6 was made with the white cardboard on the

box which stood at the left of the mouse as it approached from _A_, and,

consequently, with the black cardboard on the right. As is indicated by

the record in the "wrong" column, the mouse chose the black instead of the

white. The result of this first series was choice of the white box four

times as compared with choice of the black box six times. On the eleventh

day, that is, after No. 5 had been given 100 tests in this brightness

vision experiment, she made no mistakes in a series of ten trials (Table

7).

TABLE 6

BRIGHTNESS DISCRIMINATION

White-Black, Series 1

Experimented on No. 5 January 15, 1906

POSITION OF

TEST CARDBOARDS RIGHT WRONG

1 White left -- Wrong

2 White right -- Wrong

3 White left -- Wrong

4 White right -- Wrong

5 White left Right --

6 White right Right --

7 White left -- Wrong

8 White right Right --

9 White left -- Wrong

10 White right Right --

Totals 4 6

Before tests, such as have been described, can be presented as conclusive

proof of discrimination, it must be shown that the mouse has no preference

for the particular brightness which the arrangement of the test requires

it to select. That any preference which the mouse to be tested might have

for white, rather than black, or for a light gray rather than a dark gray,

might be discovered, what may be called preference test series were given

before the discrimination tests were begun. These series, two of which

were given usually, consisted of ten tests each, with the white

alternately on the left and on the right. The mouse was permitted to enter

either the white or the black box, as it chose, and to pass through to the

nest-box without receiving a shock and without having its way blocked by

the glass plate. The conditions of these preference tests may be referred

to hereafter briefly as "No shock, open passages." The preference tests,

which of course would be valueless as such unless they preceded the

training tests, were given as preliminary experiments, in order that the

experimenter might know how to plan his discrimination tests, and how to

interpret his results.

TABLE 7

BRIGHTNESS DISCRIMINATION

White-Black, Series II

Experimented on No. 5 February 2, 1906

POSITION

TEST OF CARDBOARDS RIGHT WRONG

1 White left Right --

2 White left Right --

3 White right Right --

4 White right Right --

5 White right Right --

6 White left Right --

7 White left Right --

8 White left Right --

9 White right Right --

10 White right Right --

Totals 10 0

The results given in the white-black preference tests by ten males and ten

females are presented in Table 8. Three facts which bear upon the

brightness discrimination tests appear from this table: (1) black is

preferred by both males and females, (2) this preference is more marked in

the first series of tests than in the second, and (3) it is slightly

stronger for the first series in the case of females than in the case of

males.

TABLE 8

WHITE-BLACK PREFERENCE TESTS

MALES FIRST SERIES SECOND SERIES

WHITE BLACK WHITE BLACK

No. 10 3 7 3 7

18 5 5 5 5

20 2 8 4 6

152 4 6 6 4

210 4 6 4 6

214 6 4 3 7

220 5 5 3 7

230 4 6 2 8

410 4 6 5 5

420 4 6 9 1

Averages 4.1 5.9 4.4 5.6

FEMALES FIRST SERIES SECOND SERIES

WHITE BLACK WHITE BLACK

No. 11 5 5 4 6

151 6 4 5 5

215 2 8 2 8

213 2 8 5 5

225 4 6 2 8

227 4 6 6 4

235 6 4 4 6

415 2 8 4 6

425 5 5 8 2

229 2 8 5 5

Averages 3.8 6.2 4.5 5.5

That the dancers should prefer to enter the dark rather than the light box

is not surprising in view of the fact that the nests in which they were

kept were ordinarily rather dark. But whatever the basis of the

preference, it is clear that it must be taken account of in the visual

discrimination experiments, for an individual which strongly preferred

black might choose correctly, to all appearances, in its first black-white

series. Such a result would demonstrate preference, and therefore one kind

of discrimination, but not the formation of a habit of choice by

discrimination. The preference for black is less marked in the second

series of tests because the mouse as it becomes more accustomed to the

experiment box tends more and more to be influenced by other conditions

than those of brightness. The record sheets for both series almost

invariably indicate a strong tendency to continue to go to the left or the

right entrance according to the way by which the animal escaped the first

time. This cannot properly be described as visual choice, for the mouse

apparently followed the previous course without regard to the conditions

of illumination. We have here an expression of the tendency to the

repetition of an act. It is only after an animal acquires considerable

familiarity with a situation that it begins to vary its behavior in

accordance with relatively unimportant factors in the situation. It is

this fact, illustrations of which may be seen in human life, as well as

throughout the realm of animal behavior, that renders it imperative that

an animal be thoroughly acquainted with the apparatus for experimentation

and with the experimenter before regular experiments are begun. Any animal

will do things under most experimental conditions, but to discover the

nature and scope of its ability it is necessary to make it thoroughly at

home in the experimental situation. As the dancer began to feel at home in

the visual discrimination apparatus it began to exercise its

discriminating ability, the first form of which was choice according to

position.

Since there appears to be a slight preference on the part of most dancers'

for the black box in comparison with the white box, white-black training

tests were given to fifty mice, and black-white to only four. The tests

with each individual were continued until it had chosen correctly in all

of the tests of three successive series (thirty tests). As the

reproduction of all the record sheets of these experiments would fill

hundreds of pages and would provide most readers with little more

information than is obtainable from a simple statement of the number of

right and wrong choices, only the brightness discrimination records of

Tables 6 and 7 are given in full.

As a basis for the comparison of the results of the white-black tests with

those of the black-white tests, two representative sets of data for each

of these conditions of brightness discrimination are presented (Tables 9

and 10). In these tables only the number of right and wrong choices for

each series of ten tests appears.

Tables 9 and 10 indicate--if we grant that the precautionary tests to be

described later exclude the possibility of other forms of discrimination--

that the dancer is able to tell white from black; that it is somewhat

easier, as the preference tests might lead us to expect, for it to learn

to go to the black than to the white, and that the male forms the habit of

choosing on the basis of brightness discrimination more quickly than the

female.

TABLE 9

WHITE-BLACK TESTS

No. 210 No. 215

AGE, 28 DAYS AGE, 28 DAYS

SERIES DATE RIGHT WRONG RIGHT WRONG

(WHITE) (BLACK) (WHITE) (BLACK)

A June 22 4 6 2 8

B 23 4 6 2 8

1 24 4 6 3 7

2 25 6 4 5 5

3 26 7 3 7 3

4 27 5 5 8 2

5 28 7 3 9 1

6 29 8 2 8 2

7 30 9 1 9 1

8 July 1 10 0 10 0

9 2 10 0 9 1

10 3 10 0 10 0

11 4 -- -- 10 0

12 5 -- -- 10 0

TABLE 10

WHITE-BLACK TESTS

No. 14 No. 13

AGE, 32 DAYS AGE, 32 DAYS

SERIES DATE RIGHT WRONG RIGHT WRONG

(WHITE) (BLACK) (WHITE) (BLACK)

1 May 13[1] 5 5 7 3

2 14 8 2 6 4

3 15 7 3 9 1

4 16 9 1 9 1

5 17 10 0 10 0

6 18 10 0 9 1

7 19 10 0 10 0

8 20 -- -- 10 0

9 21 -- -- 10 0

[Footnote 1: No preference tests were given.]

It is now necessary to justify the interpretation of these results as

evidence of brightness discrimination by proving that all other conditions

for choice except brightness difference may be excluded without

interfering with the animal's ability to select the right box. We shall

consider in order the possibility of discrimination by position, by odor,

and by texture and form of the cardboards.

The tendency which the dancer has in common with many, if not all, animals

to perform the same movement or follow the same path under uniform

conditions is an important source of error in many habit-formation

experiments. This tendency is evident even from casual observation of the

behavior of the dancer. The ease with which the habit of choosing the box

on the left or the box on the right is formed in comparison with that of

choosing the white box or the black box is strikingly shown by the

following experiment. Five mice were given one series of ten trials each

in the discrimination box of Figure 14 without the presence of cardboards

or of other means of visual discrimination. The electric shock was given

whenever the box on the left was entered. Thus without other guidance than

that of direction, for the boxes themselves were interchanged in position,

and, as was proved by additional tests, the animals were utterly unable to

tell one from the other, the mouse was required to choose the box on its

right. Only one of the five animals went to the box on the left after once

experiencing the electric shock. The results of the series are given in

Table 11.

TABLE 11

CHOICE BY POSITION

Choices of Choices of

Box on Right Box on Left

First mouse 9 1

Second mouse 8 2

Third mouse 9 1

Fourth mouse 9 1

Fifth mouse 9 1

This conclusively proves that the habit of turning in a certain direction

or of choosing by position can be formed more readily than a habit which

depends upon visual discrimination. A rough comparison justifies the

statement that it takes from six to ten times as long for the dancer to

learn to choose the white box as it does to learn to choose the box on the

right. Since this is true, it is exceedingly important that the

possibility of choice by position or direction of movement be excluded in

the case of tests of brightness discrimination. To indicate how this was

effectively accomplished in the experiments, the changes in the position

of the cardboards made in the case of a standard set of white-black series

are shown in Table 12. The number of the series, beginning at the top of

the table with the two lettered preference series, is given in the first

column at the left, the number of the tests at the top of the table, and

the position of the white cardboard, left or right, is indicated below by

the letters l (left) and r (right).

TABLE 12

POSITION OF WHITE CARDBOARDS FOR A SET OF 150 TESTS

SERIES 1 2 3 4 5 6 7 8 9 10

Preference

A l r l r l r l r l r

B r l r l r l r l r l

1 r l r l r l r l r l

2 l l r r l r l l r r

3 r r l r l l r l r l

4 l l l r r r l r r l

5 r l r l r l r l r l

6 l l r l r r l r l r

7 r l l l r r r l r l

8 r r l l r l r l r l

9 r r r l l l r l r l

10 l l l l r r r r l r

11 r l r r r l l l r l

12 r l r l r r l l r l

13 r l r l l l r r r l

14 l l l l r r r r l r

15 r l r r r l l l r l

It is to be noted that in the case of each series of ten tests the white

cardboard was on the left five times and on the right five times. Thus the

establishment of a tendency in favor of one side was avoided. The

irregularity of the changes in position rendered it impossible for the

mouse to depend upon position in its choice. It is an interesting fact

that the dancer quickly learns to choose correctly by position if the

cardboards are alternately on the left box and on the right.

The prevalent, although ill-founded, impression that mice have an

exceedingly keen sense of smell might lead a critic of these experiments

to claim that discrimination in all probability was olfactory rather than

visual. As precautions against this possibility the cardboards were

renewed frequently, so that no odor from the body of the mouse itself

should serve as a guiding condition, different kinds of cardboard were

used from time to time, and, as a final test, the cardboards were coated

with shellac so that whatever characteristic odor they may have had for

the dancer was disguised if not totally destroyed. Despite all these

precautions the discrimination of the boxes continued. A still more

conclusive proof that we have to do with brightness discrimination, and

that alone, in these experiments is furnished by the results of white-

black tests made with an apparatus which was so arranged that light was

transmitted into the two electric-boxes through a ground glass plate in

the end of each box. No cardboards were used. The illumination of each box

was controlled by changes in the position of the sources of light. Under

these conditions, so far as could be ascertained by critical examination

of the results, in addition to careful observation of the behavior of the

animals as they made their choices, there was no other guiding factor than

brightness difference. Nevertheless the mice discriminated the white from

the black perfectly. This renders unnecessary any discussion of the

possibility of discrimination by the texture or form of the cardboards.

Since a variety of precautionary tests failed to reveal the presence, in

these experiments, of any condition other than brightness difference by

which the mice were enabled to choose correctly, and since evidence of

ability to discriminate brightness differences was obtained by the use of

both reflected light (cardboards) and transmitted light (lamps behind

ground glass), it is necessary to conclude that the dancer possesses

brightness vision.

CHAPTER VIII

THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)

Since the ability of the dancer to perceive brightness has been

demonstrated by the experiments of the previous chapter, the next step in

this investigation of the nature of vision is a study of the delicacy of

brightness discrimination, and of the relation of the just perceivable

difference to brightness value. Expressed in another way, the problems of

this portion of the investigation are to determine how slight a difference

in brightness enables the dancer to discriminate one light from another,

and what is the relation between the absolute brightnesses of two lights

and that amount of difference which is just sufficient to render the

lights distinguishable. It has been discovered in the case of the human

being that a stimulus must be increased by a certain definite fraction of

its own value if it is to seem different. For brightness, within certain

intensity limits, this increase must be about one one-hundredth; a

brightness of 100 units, for example, is just perceivably different from

one of 101 units. The formulation of this relation between the amount of a

stimulus and the amount of change which is necessary that a difference be

noted is known as Weber's law. Does this law, in any form, hold for the

brightness vision of the dancing mouse?

Two methods were used in the study of these problems. For the first

problem, that of the delicacy of brightness discrimination, I first used

light which was reflected from gray papers, according to the method of

Chapter VII. For the second, the Weber's law test, transmitted light was

used, in an apparatus which will be described later. Either of these

methods might have been used for the solution of both problems. Which of

them is the more satisfactory is definitely decided by the results which

make up the material of this chapter, Under natural conditions the dancer

probably sees objects which reflect light more frequently than it does

those which transmit it; it would seem fairer, therefore, to require it to

discriminate surfaces which differ in brightness. This the use of gray

papers does. But, on the other hand, gray papers are open to the

objections that they may not be entirely colorless (neutral), and that

their brightness values cannot be changed readily by the experimenter. As

will be made clear in the subsequent description of the experiments with

transmitted light, neither of these objections can be raised in connection

with the second method of experimentation.

To determine the delicacy of discrimination with reflected light it is

necessary to have a series of neutral grays (colorless) whose adjacent

members differ from one another in brightness by less than the threshold

of discrimination of the animal to be tested. A series which promised to

fulfill these conditions was that of Richard Nendel of Berlin. It consists

of fifty papers, beginning with pure white, numbered 1, and passing by

almost imperceptible steps of decrease in brightness through the grays to

black, which is numbered 50. For the present we may assume that these

papers are so nearly neutral that whatever discrimination occurs is due to

brightness. The differences between successive papers of the series are

perceptible to man. The question is, can they, under favorable conditions

of illumination, be perceived by the dancer?

On the basis of the fact that the dancer can discriminate between white

and black, two grays which differed from one another in brightness by a

considerable amount were chosen from the Nendel series; these were numbers

10 and 20. It seemed certain, from the results of previous experiments,

that the discrimination of these papers by brightness difference would be

possible, and that therefore the use of papers between these two extremes

would suffice to demonstrate the delicacy of discrimination. In Figure 16

we have a fairly accurate representation of the relative brightness of the

Nendel papers Nos. 10, 15, and 20.

[Illustration: FIGURE 16. Three of Nendel's gray papers: Nos. 10, 15, and

20. To exhibit differences in brightness.]

Pieces of the gray papers were pasted upon cardboard carriers so that they

might be placed in the discrimination box as were the white and black

cardboards in the tests of brightness vision previously described. Mice

which had been trained to choose the white box by series of white-black

tests were now tested with light gray (No. 10) and dark gray (No. 20), my

assumption being that they would immediately choose the brighter of the

two if they were able to detect the difference. As a matter of fact this

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

reached.

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

them.

TABLE 13

GRAY DISCRIMINATION

The Delicacy of Brightness Discrimination

No. 420

GRAYS NOS. 10 GRAYS NOS. 20

AND 20 AND 15

SERIES DATE DATE

NO. 10 NO. 2 NO. 10 NO. 15

(RIGHT) (WRONG) (RIGHT) (WRONG)

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° 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

figure.

[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

followed.

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

units.]

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 reënter 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.

TABLE 14

RESULTS OF WEBER'S LAW EXPERIMENTS

Brightness vision

DATE NUMBER STANDARD VARIABLE DIFFERENCE % OF ERRORS

OF TESTS LIGHT LIGHT

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

tenth.

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

twentieth.]

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-

boxes.

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.

CHAPTER IX

THE SENSE OF SIGHT: COLOR VISION

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

defective.

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.

TABLE 15

ORANGE-BLUE TESTS, WITH FOOD-BOX

MOUSE A MOUSE B

SERIES DATE

1904 RIGHT WRONG RIGHT WRONG

(ORANGE) (BLUE) (ORANGE) (BLUE)

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.]

TABLE 16

YELLOW-RED TESTS

In Color Discrimination Box with 6 by 3 cm. Pieces of Hering

Papers at Entrances to Boxes

No. 2

SERIES DATE RIGHT WRONG

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

experiment.

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.

TABLE 17

LIGHT BLUE-ORANGE TESTS IN COLOR DISCRIMINATION BOX

SERIES DATE NO. 2 NO. 3

1906 RIGHT WRONG RIGHT WRONG

(LIGHT (ORANGE) (LIGHT (ORANGE)

BLUE) BLUE)

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

TABLE 18

LIGHT BLUE-ORANGE

Brightness check series Mouse No. 2, Series 13

Feb. 8, 1906

TEST CONDITION RIGHT WRONG

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

ability.

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.

TABLE 19

LIGHT BLUE-ORANGE AND DARK BLUE-RED TESTS

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

others.

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

discrimination.

TABLE 20

GREEN-LIGHT BLUE TESTS

Date No. 1000 No. 4 No. 5

SERIES 1906 RIGHT WRONG RIGHT WRONG RIGHT WRONG

(GREEN) (BLUE) (GREEN) (BLUE) (GREEN) (BLUE)

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

experiment.

TABLE 21

VIOLET-RED TESTS

With Odor of All Cardboards the Same

SERIES DATE NO. 7 NO. 998

RIGHT WRONG RIGHT WRONG

(VIOLET) (RED) (VIOLET) (RED)

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.

CHAPTER X

THE SENSE OF SIGHT: COLOR VISION (_Continued_)

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

discrimination.

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.