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.



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 deg. by the experimenter. Thus, by the turning of the light-box, the lamp which in the case of one test illuminated the left electric-box could be brought into such a position that in the case of the next test it illuminated the right electric-box. The practical convenience of this will be appreciated when the number of times that the brightnesses of the two boxes had to be reversed is considered. The light-box was left open at the top for ventilation and the prevention of any considerable increase in the temperature of the experiment box. In one side of each of the compartments of the light-box a slit (B, B of the figure) was cut out for an incandescent lamp holder. A strip of leatherette, fitted closely into inch grooves at the edges of the slit, prevented light from escaping through these openings in the sides of the light-box. By moving the strips of leatherette, one of which appears in the figure, C, the lamps could be changed in position with reference to the bottom of the electric-box. A scale, S, at the edge of each slit enabled the experimenter to determine the distance of the lamp from the floor of the electric-box. The front of the light-box was closed, instead of being open as it appears in the figure.



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 reenter the box in which it has just received an electric shock. The result is that the percentage of right choices, especially in the case of series which have the right box in the same position two, three, or four times in succession, rises as high as 60 or 70, even when the visual conditions are indiscriminable. Abundant evidence in support of this statement is presented in Chapters VII and IX, but at this point I may further call attention to the results of an experiment in the Weber's law apparatus which was made especially to test the matter. The results appear under the date of May 27 in Table 14. In this experiment, despite the fact that both boxes were illuminated by 80 hefners, the mouse chose the standard (the illumination in which it was not shocked) 59 times in 100. In other words the percentage of error was 41 instead of 50. It is evident, therefore, that as low a percentage of errors as 40 is not necessarily indicative of discrimination. Anything below 40 per cent is likely, however, to be the result of ability to distinguish the brighter from the darker box. To be on the safe side we may agree to consider 25 wrong choices per 100 as indicative of a just perceivable difference in illumination. Fewer mistakes we shall consider indicative of a difference in illumination which is readily perceivable, and more as indicative of a difference which the mouse cannot detect. The reader will bear in mind as he examines Table 14 that 25 per cent of wrong choices indicates the point of just perceivable difference in brightness.



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.]



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.

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



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

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

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

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

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

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

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

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

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



TABLE 22

GREEN-BLUE TESTS

Brightnesses Equal for Human Eye

Green 18 candle meters Blue 64 candle meters

SERIES DATE NO. 10 NO. 11 1906 RIGHT WRONG RIGHT WRONG (GREEN) (BLUE) (GREEN) (BLUE) A and B[1] April 2 10 10 12 8 1 3 6 4 5 5 2 4 5 5 6 4 3 5 5 5 5 5 4 6 5 5 5 5 5 7 7 3 5 5 6 8 7 3 3 7 7 9 7 3 5 5 8 10 3 7 7 3 9 11 5 5 4 6 10 12 5 5 6 4 [Footnote: A single preference series of twenty tests.]