Latent Period, Duration, and Clearness. — In

each of the following four experiments record (1) the approximate latent time,* (2) duration, and (3) relative clearness.

a, Time, — Project the after-image of the black square first after a fixation of five seconds and then after a fixation of ten seconds,t allowing due time for recovery between the two.

6. Brightness. — Place the black and the gray squares edge to edge upon the white sheet ; fixate the adjoining edges and project the double image upon a white background.

c. Color. — Place the red, the green, the yellow, and the blue squares in a cluster upon the white sheet ; mark

* The latent time of an after-image is the time which elapses between the withdrawal of the stimulus and the beginning of the appearance of the after-image.

fThis time should be adapted to the individual needs, the requirement being that a relatively short exposure-time shall be eompared with a longer one.

10 VISUAL AFTER-IMAGES

a point in the center of the cluster; fixate the central point and project the four-colored af ter-image upon a white baekground.

d. Area. — Cut a pièce of white paper about eight millimeters square and place it and the white square five millimeters apart upon the large black square ; fixate a middle point and project the double image upon a white baekground.*

Long stimulation, large brightness-difference between the object and its baekground, and a moderately large area are conducive to a short latent time, long duration, and cleamess of the af ter-image, t Thèse laws are, however, subject to numerous qualifications.

Thèse experiments merely show how it is possible to work out laws of the relation between stimuli and after-images.^ The rule is to study one f actor at a time and keep ail other conditions constant. The factor which is studied may be varied, as in the above separating of time, brightness, and area under controUable conditions, and the effect of such variations may be measured.

The certainty that the image shall appear dépends mainly upon the exposure-time, the area, and the brightness of the stimulus, but also upon such subjective conditions as are mentioned in the foUowing paragraph.

* A millimeter scale is printed on the last page of this book. Fold the margin of that page so as to bring the scale marking to the edge of the page.

f The variations with color are perhaps essentially due to the différence in the brightness of the color. For this reason blue is the most effective color in this group on a white baekground.

J See Franz, "Af ter- images," The Psychological Review Mono* graph Supplément, Vol. III, No. 12.

VISUAL AFTER-IMAGES 11

If any one of thèse is inadéquate the image f ails to appear. As a rule, the after-image may be produced if the object is presented under such conditions as to be clearly perceptible. Thus, an electric spark, which endures only for an infinitésimal part of a second, may produce an after-image ; in the présent experiments the observer may not hâve notioed the after-image of the white seen through the pinhole, but objects of that size may produce af ter-images under favorable conditions; and one may not be able to get a distinct image of a white plate upon a white tablecloth because the brightness-difference is too small.

The duration of the after-image varies within wide limits. . In extrême cases the impression has been known to persist for days and months, as with Newton, who looked at the sun with his right eye and then saw the sun continually before him for many days. The ordinary duration is, however, limited to a few seconds, and the image passes away gradually. In addition to such objective conditions as exposure-time, area, and brightness which we hâve noted, there are subjective factors, such as practice, expectation, attention, etc., which condition the duration.

11. Indirect Vision. — Fixate a point about seven centimeters away f rom the center of the back square ; it then stimulâtes the indirect field of the retina. Project the after-image. Compare the effectiveness of this image with the foregoing images from the direct field.

The best after-images are obtained from the center of the retina. Clearness and duration gradually de-

12 VISUAL AFTER-IMAGES

crease toward the periphery, and it is very difficult to get any image f rom the extrême periphery of the retina. In this respect the af ter-image behaves like the primary image in perception.

12. Periodicity. — Fixate the black square nntil it begins to blur and then project the af ter-image in the most favorable way, and observe that it recurs again and again. Record the number of appearances.

Under favorable circumstances the image may appear twenty or thirty times. This periodic récurrence is a fundamental law. Sometimes the usual séquence of positive and négative phases may be observed in each period.

The cause of after-images lies chiefly in the fact that, as soon as a light stimulus ceases to act upon a given portion of the retina, a reaction of the chemically antagonistic sort follows. The play of the after-image in ail its transitions through brightness and color follows the course of this reaction in the retinal éléments.*

* A good account of the principal physiological théories of color-vision is found in Calkins, "Introduction to Psychology," pp. 464-79.

CHAPTEE II

VISUAL CONTKAST

For One,*

EvERY sensation is différent from what it would hâve been if it had been experienced together with, or in séquence to, some other sensation. One of the best illustrations of this "law of relativity" is to be found in contrast, which we shall now study in the sensé of sight.

Thèse experiments should be performed in good diffused daylight. TJnless otherwise directed, the object must invariably be viewed ^hrough the tissue-paper. The foregoing exercise has taught the importance of avoiding fatigue for color and brightness. Make prompt judgments and avoid unnecessary exposure of the eyes to the figures.

1. Brightness-contrast. — Lay the black and the white squares about three centimeters apart upon the background ; place a gray bar upon each of them and cover the whole with the tissue-paper. Compare the

*Take ail the small squares from the envelope. Cut two bars from the small gray square and two from the pale green square each 5 millimeters wide. Use the page at the end of the Dook containing the millimeter scale for a background, and cover the colors laid upon it with the facing sheet of tissue-paper.

14 VISUAL CONTRAST

brightness of the two gray bars and record which is the brighter.

The two gray bars are exaetly alike in brightness; but, by contrast with their backgrounds, one beeomes a light gray and the other a dark gray.

There are two kinds of contrast, successive and simultaneous. Successive contrast is in many respects synonymous with after-images, and may be defined as "the apparent altération of a gray or a colored surface by the previous stimulation of the same retinal area by some other sort of light." (Sanford.) Simultaneous contrast, the thème of the présent chapter, is described as "the mutual effects in respect to color and brightness which simultaneously seen but separated Visual areas hâve upon each other." (Baldwin.) The différence between the two kinds of contrast lies in the fact that one is due to successive impressions, whereas the other is due to simultaneous impressions. The effects of the two kinds of contrast are closely related. •

Consider for a moment the practical conséquences , of the phenomenon just demonstrated. Wherever surfaces of différent brightness are seen together, each modifies the brightness of the other. To see the world of lights and shades as it really is, to guide ourselves accurately in seeing form, relief, and distance, and to be able to make the proper correction wherever contrast opérâtes, we must carry in our minds an idea of the magnitude and a knowledge of the laws of brightnesscontrast. Contrast is both helpful and deceptive; it magnifies différences and therefore often helps in distinguishing surfaces; on the other hand, unless we

VISUAL CONTE AST 16

are prepared to make the correction, we are constantly deoeived as to the strength of lights and shades.*

2. Color-contrast, — Lay any two color-squares upon the background ; lay one of the gray bars upon each of the colors and cover with the tissue-paper. Observe that the gray of each bar assumes a distinct color-tinge. Record the color of each bar. Eepeat the same with other color-squares, pair by pair. If possible reduce thèse records to a law oi color-contrast.t .

The two bars are colorless — exactly the same gray, yet each shows a distinct tinge of color. Those who f ail to see one or more of the contrast-colors of thèse figures

* 8upplementary Experiment. — (Not to be performed unless dirçcted by the instructor.) The following measurement of brightness-contrast is very simple and effective and may be demonstrated if a color-wheel or a color-top is available. Revolve a black and a white disk upon the color-wheel and adjust the proportions of black and white until the resulting gray matches the gray bar on one square. Record the amount of white iu the mixture. Then measure the grayness of the other square in the same manner. The différence in the amount of white in the two measurements is a measure of the différence between the two gray bars.

By similar procédure it is easy to détermine how much of this contrast-effect is due to the white and the black fields respectively.

f A very pretty démonstration of color-contrast may be made as follows: Put a large white paper or cloth upon a table; place upon it some tall, slender object which will cast a long, narrow shadow; pull down ail window-shades except one which is left open about a foot; light a candie or any other artifîcial light and hold it a little to the side of the window. There will be two shadows of the object, one from the white daylight and the other from the yellow artificial light. The latter shadow is seen distinctly blue. In reality, or physically, it is gray; the blue is the contrast from the surroundings which are made yellow by the artiflcial light. The induced hlue is a much strongér color than the indueing yellow.

16 VISUAL CONTRAST

after a fair trial hâve a corresponding color-blindness or color-weakness, which can be measured.

Color-contrast is most effective when there îs no brightness-eontrast between the two fields. The effect would hâve been very much enhanced in the présent figures if each gray bar had been matched in brightness with the color-field upon which it rested. An otherwise strong color-contrast may be almost obliterated by the introduction of a simultaneous brightness-eontrast.*

Hère again we hâve a sweeping principle : ail colors tinge their surroundings with their complementary color. Things are not what they seem ! The colors in nature, art, fabrics — everywhere — are active and modify their environments. Flowers and foliage, grass and sky, ail play their harmonies and discords in modulations of color. The artist trusts the subjective colors as surely as he trusts the pigment on his canvas. The milliner and the modiste use contrast effectively and artistically : a dark hat and a green gown give a fair and rosy complexion. There is almost as much in the art of knowing what to avoid as in knowing what to employ.f

3. Brightness-effect on Color. — Lay the black and the white squares upon the background ; lay one of the

♦■The color-contrast may be measured on the same prîncîple as the brightness-eontrast.

f Chevreul, "The Laws of Contrast of Color and their Application to the Arts of Painting, Décoration of Buildings, Mosaic Work, Tapestry and Carpet-weaving, Calico-printing, Dress, Paper-staining, Printing, Military Clothing, Illumination, Landscape-gardening, etc.," is an interesting book, though somewhat out of date.

VISUAL CONTRAST 17

green bars upon each of thèse and cover with tissuepaper. Kecord the effect of the black and the white upon the green.

The colorless brightness of the surroundings modifies the color near it. When placed upon similar fields, the two greens are exactly alike; but when placed upon fields which differ in brightness, the greens appear to change in brightness according to the laws of brightnesscontrast. A dark field makes an adjacent color brighter, a light field makes it darker.

4. Color-effect on Color. a. Canceling. — Lay the green and the gray squares upon the white background ; lay a pale green bar upon each one and cover with the tissue-paper. Record the effect of the deep green upon the pale green.

fc. Enhancing. — Lay one green bar upon the red square and the other upon the gray and cover with the tissue-paper. Record the effect of the red upon the green.

c. Modulating. — Lay the yellow and the blue squares upon the background ; lay a green bar upon each one and cover with the tissue-paper. Record the apparent change in the color-tone of the two bars.

Thèse three variations represent the three types of effect of one color upon another. The two pale-green bars are alike, but the one upon the green tends to lose its color, while the one on the red is enhanced in color. The green square throws a contrast red upon its bar and this tends to cancel the green and produce a gray. This illustrâtes how one color tends to obliterate a neighbor-

18 VISUAL CONTRAST

ing color. Wherever shades or tints run together wîth a more saturated form of the same color, the weaker color must be made stronger to compensate for the loss by contrast with its own but stronger color.

The field of the red square induces a contrast green which reinforces the original green of the bar. The effect of the red upon the green illustrâtes the gênerai law that opposites enhance each other, which is true for both brightness and color.

The green on the yellow field looks bluish green and the one on the blue field looks yellowîsh green. Thèse are illustrations of the law that colors which are not complementary or identical hâve a modulating effect upon each other. This is by far the most fréquent of the three types of the effect of color upon color.

This condition, that every color changes adjoining colors, reduces itself, then, to a simple order and an easily applicable law. The effect can always be predicted. Knowing the séries of complementaries and the principles of color-mixing, we can always predict the resuit.

Ail thèse contrast-phenomena are very much stronger in ordinary expérience than under thèse expérimental conditions, because, in the experiment, we are not only in a more critical attitude, but we are also biased by the knowledge of the actual conditions, e.g., that the gray bars are alike and that the green bars are alike. In nature and in art, especially where we are not conscious of the contrast, the illusion has fuU play.

VISUAL CONTRAST 19

5. EfFect Immédiate, a. Direct Observation. — ^Repeat Exps. 1 and 2 and observe that the contrast is présent as soon as the object is clearly seen.

6. Comparison with After-images. — ^Détermine the shortest exposure-tiiue which will bring out a noticeable after-image of the black square. Kecord the approximate time.

This experiment proves two things: first, that the purely simultaneous contrast-effect is immédiate and is not due to adaptation; and, second, that it is very elosely related to the adaptation of successive contrast or after-images in effect, so that the greatest care must be taken to avoid complication of the two kinds of contrast as far as possible. After some training in observation one may obtain an after-image from a mère glance at the figure. We seldom see an object so quickly that adaptation does not play a rôle in the perception.

6. EfFect Marginal and Reciprocal — Lay the green, the yellowish-green, the greenish-yellow, and the yellow color-squares side by side in the order named, with edges slightly overlapping. Observe them both with and without the tissue-paper cover. Describe the changes in thèse colors which are due to the juxtaposition.

Contrast, whether of brightness or color, is always reciprocal. Which of the two or more fields shall show the greatest effect dépends upon the means of comparison, the relative area, the relative brightness, the relative saturation, the point of regard, and many other conditions. When seen by itself, each of thèse squares is of uniform color and brightness; but when seen to-

20 VISUAL CONTRAST

gether, the green brings out the yellow of the adjoining less green square by counterbalaneing the green of the. contrast red. The yellow brings out the green of the less yellow square by counterbalaneing the yellow with the contrast blue. Each color makes the différent élément in the contiguous color more conspicuous. The effect is striking even without the élimination of contour by the tissue-paper. When the effect tapers off rapidly from the margin, as in this case, it is spoken of as marginal contrast.

Marginal contrast is one aspect of the gênerai principle that the contrast-effect decreases with increase in the distance between the two contrasting surfaces. Certain conditions tend to make it prominent, as hère, but there has been opportunity for observing the fact in every experiment.

7. Area. — Place a gray bar and the gray square some distance apart upon the background and cover with the tissue-paper. Record which appears to be the darker.

The two grays are really alike, but the bar looks the darker on account of its relatively small area and the form which is favorable to the exposure from the whitè inducing field. In ail the above experiments the effects hâve been reciprocal, but we hâve directed our attention to the smaller areas because they show the greatest relative effect.

8. Contour. — Compare the force of the contrast in Exps. 1 and 2 when the squares are covered with the tissue-paper with the force when they are not covered. Record the différence ip the effect.

VISUAL CONTRAST 21

The observer bas undoubtedly been annoyed and puzzled by the constant demand for covering with tissue-paper. What can be its purpose? The tissuepaper has been employed for the purpose of eliminating contour, and that only. This has been done on the principle that contrast is very much enhanced by uncertainty in the surface. Surfaces with very sharp delineations counteract the motives for contrast.

There are other ways of eliminating contour. Nature as a rule présents more or less uncertain contour and therefore favors contrast. Distance, for example, serves the same purpose as the tissue-paper. When we admire a flower-bed or a landscape we are far enough away to get the dim outline and the vagueness of contour which are so favorable for contrast.*

9. Contrast in the After-image. — Lay one gray bar upon the black square and the other near by upon the white background; cover with tissue-paper. Fixate a point between them and secure an after-image. Record the relative brightness of the two bars in the afterimage.

The contrast is effective in the after-image.

The explanation of visual contrast in color and brightness is probably to be found chiefly in physiological terms — in the indirect stimulation of adjacent areas of the retina.

* The color-wheel serves the same purpose. If we eut three concentric gray cireles of différent brightness and place them on the wheel, the contrast will hardly be perceptible so long as the wheel is motionless; but revolve the disk and the contrast at once becomes conspicuous, because the spinning takes away the sharp rigidity of outline and surface.

22 VISUAL CONTRAST

The experiments hâve ail been devoted to color and brightness. The phenomena of vîsual contrast might hâve been equally well illustrated in visual perception of space. The gênerai law is. that opposites enhance each other although the aetual explanation for this may be différent in différent cases. As black enhances white and red enhances green, so there is a reciprocal enhancement between the long and the short, the large and the small, the narrow and the wide, the irregular and the regular, the duU and the sharp, the smooth and the rough, the straight and the crooked, the ugly and the beautiful, etc. When the tall and the short man walk together, the tall one looks taller and the short one looks shorter than otherwise. A poor penman is mortified to see his signature together with the signature of a good penman. A pocket-knife is duU in comparison with a razor, but sharp in comparison with a hatchet.

Similar illustrations might be found in the time, or duration, of visual acts. Contrast opérâtes in ail the attributes of sensation — quality, intensity, duration, and space. And we find it in ail the sensés. Indeed, one of the laws of contrast is that it is strongest in those sensés with which we make the poorest discrimination. Hence the most striking illustrations of contrast are found in the lower sensés, as in taste, smell, and température.

CHAPTEE III

THE VISUAL FIELD

For One,

The problem is to measure the field of vision for white and colors, and to détermine some characteristic color-changes in the indirect field.

Make the f oUowing préparation for the experiment : Lay a pièce of cardboard back of Fig. 1. Prick through the page with a pin at each number on the arc, at the free end of the heavy Une, and at the principal points in the light line. Trim the card by cutting according to the tracing of the light line. Insert the degree-numbers at the appropriate points. Draw the heavy line. Punch a pinhole at the free end of the heavy line, which is the center of the arc. Put a thread through the hole and tie a knot at the back of the card. Tie a knot at the other end of the thread, fifty centimeters away. Take the large black square and the small white, red, green, yellow, and blue squares f rom the book envelope. Lay the white square upon the large black square as a background; then stick a pin through the knot at the free end of the cord, through the two squares near a corner of the small one, and finally through a cork

THE VISUAL FIELD

which may serve as a handle. The quadrant and object-card thus arranged may be called a perimeter.*

FiG. 1.

The perimeter is a simple means of measuring the direction of an object in the indirect field of vision.

* There hâve been two gênerai types of instruments employed in perimetry of vision: (1) those in which the measurements are made upon a plane surface, and (2) those in which the measurements are made upon the arc of a circle centered at the eye. The former is called a campimeter, the latter a perimeter. The présent outfit works on the perimeter principle. The most effective perimeter is an instrument with colored lights in a darK room.

THE VISUAL FIELD 26

Seat yourself in good reflected light with the back toward the source of light. Mark a dot on a pièce of white paper and fasten it up sixty centimeters directly in front of the eyes. Blindfold one eye; hold the quadrant with one edge close to the other eye so that in looking straight forward at the dot, the eye sights along the heavy straight line on the quadrant. When the eye is fixed upon it, the dot becomes the fixationpoint, or point of regard, and is said to lie in the direct field of vision. The visual space around it is spoken of as the indirect field.

1. The Field for White.— Hold the quadrant in the horizontal plane in front of the right eye so that the regard-line on the quadrant points exactly toward the fixation-point when you sight along it. Keep the head upright and firm. Fixate the dot which is the point of regard and do not allow the eye to wander away from it during the actual trial. Move the object-card inward from the extrême right until the white disk can first be seen as white."^ The thread being held tant will indicate the number of degrees from the line of regard. Record this. Make five trials, and find the average and the mean variation for these.f

Proceed in the same manner and measure along the other three cardinal radii, namely, with the white entering from below, from the lef t, and from above.

* Move înward at such a rate that the destination is reaehed în about eight seconds. Take spécial care that the head does not turn or the eye wander from the point of regard, and that the point of regard is directly in front of the eye.

fThe mean variation (m. v.) is a measure of the degree of agreement in a séries of records. It represents the average of

26 THE VISUAL FIELD

To represent thèse résulta graphically, draw four radii from a point representing the point of regard — one to thé right, one to the left, one down, and one up, — and place a dot on each radius to represent the corresponding measurement on the scale of one millimeter to one degree. Mark each of thèse dots w.*

Thèse results are stated with référence to the field of vision ; they might equally well hâve been stated in terms of régions on the retina. The outer or temporal field of vision corresponds to the inner or nasal région of the retina, and the upper field of vision corresponds to the lower région of the retina. The temporal field of vision is larger than the nasal, and the lower is larger than the upper. This différence is due to the limitations placed by the nose, cheek-bone, and brow.t

the déviations of each individual record from the average of ail the records for the group, regardless of sign. Thus

Ave. 76 2.2 m. V.

•Thus, if the white square was first seen as white at fifty degrees above the eye, put a dot fifty millimeters from the oenter on the appropriate radius.

t Supplementary Exercise. — There is a totally blind spot in the nasal région of each eye. It is located at the point of entrance of the optic nerve, about fifteen degrees from the fovea, or point of clearest vision. If this exercise is assigned, the student should devise his own methods and means for one or more of three exercises: (1) to locate the blind spot; (2) to survey and détermine its shape and area; and (3) to détermine how it is fiUed eut in perception.

THE VISUAL FIELD 27

2. The Ficlds for Colors. — Substitute the red square for the white and détermine the lîmits * for red on the two meridians, right and down, making five trials for each.t Compute the average and mean variation and insert the averages on the two corresponding radii of the chart for Exp. 1 and mark them r,

Measure and record in the same manner for yellow, blue, and green.

Assuming that the results for the meridians selected are typical of what we should find in the other meridians in regard to the order of limits, this and the foregoing experiments demonstrate that the color-fields are ail smaller than the field for white; and, in the normal eye, the colors supplied hâve fields which vary in magnitude in the order yellow, blue, red, and green. There is such a large différence that the field for green has less than one-f ourth the area of the field for yellow.

How does this afïect our ordinary perception of color in the indirect field of vision? If we look steadily at one flower in a flower-bed and attempt, without movement of the eyes, to see the coloration of the whole bed, we observe that, outside of a certain narrow limit, the leaves do not look green; beyond a somewhat larger limit, no flowers are seen red, although the blue and yellow ones look brilliant; and in the outermost parts of the bed ail flowers and leaves

•The observer knows what the color is to be, and the task îs to discover the limit at which he can identify the disk as being of that particular color. The inward movement should begin just clearly outside the field and be made at such a rate that the limit is reached in from five to ten seconds.

f If desired, thèse color squares may be eut smaller, but the size must be recorded.

28 THE VISUAL FIELD

look gray. This is literally true, but not alarmîng, because we do not usually look at flower-beds in that way. When we want to see the color of a bed of flowers, a painting, or a sky, we regard (that is, pay attention to), or apperceive, only the color of the direct field at and around the point of regard ; our eyes sweep back and forth over the object automatically with astonishing swiftness and take a séries of snapshots, as it were, on the central portion of the retina, and then we combine thèse into a whole in memory, although the process is almost instantaneous and seems to be a single act of perception. We hâve the feeling that the colorimpressions from the direct and indirect fields are simultaneous, but the fact is that we hâve memoryimages of impressions from différent parts of the object and the simultaneous impressions from the whole object become merely a sort of plat on which we unconsciously distribute thèse impressions and reconstruct the true color-relations.

If we had used the four truly fundamental colors,* the absolutely complementary pairs, yellow and blue, and red and bluish green, and had reduced them to the same brightness and saturation, we should hâve found that the limits for blue and yellow coincide and those for red and green coincide. In Fig. 2 the inner curve represents the red-green boundary and the outer the yellow-blue boundary. The shading shows the limit for

. * The colors which answer for this purpose are very difficult to obtain. Those furnished are neither fully complementary nor of the right hue as fundamental colors. They also differ much in brightness.

THE VISUAL FIELD

white and should be compared with the record in Exp. 1.

Thus, the retina may be conoeived of as being divided înto three zones: (1) a central zone over which ail colors and brightnesses can be seen; (2) a middle zone over which only blues and yellows and their derivatives can be seen; and (3) an outermost zone over which ail objects, colored and uncolored, appear gray. Let us

FiG. 2.

note some of the most significant applications of this law of distribution.

Color-vision is made possible by the existence in the retina of the so-called color-elements. There are many théories in regard to the probable number of thèse. The close coïncidence of thèse two pairs of colors tends to support the conjecture that the retina contains either

30 THE VISUAL FIELD

two doubly-functioning or two pairs of color-elements, one for red-green and one for yellow-blue. The psychological chart of the fields of color-vision must theref ore be used as one of the criteria of a physiologîcal theory of the nature and distribution of the color-elements.

The three zones probably indicate as many epochs in the évolution of color-vision. The primitive eye was sensitive only to brightness ; some of thèse éléments in the central portion of the retina differentiated and became sensitive to yellow and blue, and thèse éléments spread gradually, during evolutionary âges, from the fovea to the periphery of the retina and hâve now reached the expanse outlined by the yellow-blue curve in the chart ; very much later, a higher differentiation of the éléments at the fovea resulted in the development of sensitiveness to red and green and thèse éléments are now spreading toward the periphery but hâve not yet reached farther than the limits which correspond to the red-green curve in Fig. 2.*

This arrangement also becomes an explanation for the well-known fact about color-blindness, that by far the most numerous cases of color-blindness are of the red-green type. Why should this be so? There is a well-known biological law to the effect that, in gênerai, the last acquired structure is the first to become def ective or to be lost. Now, the red-green éléments are the least stable because they hâve been acquired most recently, and therefore a person is much more likely

•Ail colors other than the fundamental are mixtures of two or more of the fundamental colors. Hence the four color-elements can produce ail the experienced color-effects.

THE VISUAL FIELD 31

to be red-green blind than yellow-blue blind, if his colorvision is def ective. •

Thèse zones, as determined in the experiments upon which Fig. 2 is based, are not fixed except for the particular conditions specified. Among the conditions which détermine the limits of the zones the foUowing may be noted :

a. Color tone. — The four colors in this experiment go by the same name as the four upon which Fig. 2 is based, but the two sets of colors differ very much in tone and the résulta vary accordingly.

b. Saturation or Purity, — As a rule, the purer the color the larger the field.

c. Brightness. — ^The brighter the color or the gray the larger will be its field. One of the chief causes for the différence between the record in this experiment and in the curves in Fig. 2 is that of brightness; in a standard experiment the colors are ail reduced to the same brightness, while in the common tones as hère used there is a great différence in brightness. As the illumination of the colored stimulus is increased the field is enlarged.

d. Area or Magnitude, — Within certain limits, the larger the colored object the larger is its field.

e. Background, — The background has profound influence upon the color. Numerous laws of the relation of the peripheral color to its background bave been worked out. In gênerai for mixed colors (ail pigment colors) the field is largest when the contrast between the background and the color is the greatest.

/. Adaptation and Fatigue. — It makes a différence

32 THE VISUAL FIELD

whether the eye has become adapted to darkness or to some kind of liglit before a trial. 'The rested eye is more efficient and gives a larger field than the nonrested eye. Retinal fatigue shows itself more rapidly in a retinal area the farther the area is away from the fovea. We hâve only to recall what we learned in the two foregoing chapters to realize what a large variable this is. It is necessary to make comparatively rapid movements in order to seciire the best results.

g. Practice. — It is probable that the extension of the color-fields as the resuit of practice is not an increase of sensitivity in the retinal éléments, but merely a development in capacity for observation.

h. Age. — Color-fields enlarge with âge. The différence between the field of a boy of ten years and one of twenty is large ; but we do not know how much of this is due merely to the former's lack of power in application and skill in observation.

i. Disease. — Certain nervous diseases are characterized by peculiar changes in the fields of color. There may be, for example, gênerai contraction of the colorfields, loss of vision in one half of the retina, impairment in the vision of certain colors, or central colorblindness. This last-named defect often results from nicotine-poisoning ; it is discovered by the fact that, in order to see colors, the patient must view the object by indirect vision.

y. Color-blindness, — This is a large topic by itself. About one per cent of ail women and more than four per cent of ail men are distinctly blind to some colors ; and indeed the most récent and most efficient tests of

THE VISUAL FIELD 33

color-blindness make it seem probable that thèse figures are underestimations. No one can hâve corne successfiilly to the présent stage in this course without discovering whether or not his color-vision is defective.

h. Arhitrary Limits, — A color cornes into its field gradually ; the magnitude of the field theref ore dépends upon what degree of resemblance to the color as seen in direct vision the observer has set himself as a standard for the purpose of the measurement. The incoming color is never exactly like the color as known through direct vision.

3. Transition Colors. — Starting to the right, outside the color-field, and going by ten-degree steps toward the point of regard, observe* and record the color of the red square as it appears at the first momentary impression in each step. Make three independent séries of trials.*

Record three séries of observations for yellow in the same manner.f

According to the Hering theory of color-vision, there are four "Urfarben", that is, primary or fundamental colors. AU other colors may be built up from thèse, and to them the color éléments in the retina correspond. They are the two pairs of perfect complementaries spoken of in Exp. 2. They undergo no change when passed outward through the indirect field and are therefore called stable colors. Ail other than the stable

* If the rîght eye shows fatigue, this and the following experiments may be made with the left eye. The practice gained, however, makes it easier to use the right eye.

fThe record must necessarily be a crude description of the changes in color and brightness.

84 THE VISUAL FIELD

colors undergo more or less radical changes wheu passed through the indirect field, as in this experiment. The foUowing is a typical record, though much abbreviated:*

/t^/^ — ..^ i- Transition eolars : the aameeolorwhen

ih^t ^M P^*^<* inward through the indirect

IM atreei jieta. ^gi^ appeared succesnvely as foUows:

Deep red Yellowish, yellow, orangish yellow, yellow-

orange, orange-red, red.

Reddish orange Yellowish, orangish yellow, reddish orange.

Orange-yellow Yellowish, yellow, orangish yellow.

Green Yellowish, yellow, greenish yellow, green.

Blue Blue. (This happened to be the stable blue. )

Violet Bluish, blue, violet.

Purple (red end) ...Yellowish, orange-yellow, yellow-orange, orange-red, red, purplish red, reddish purpie, purple.

This record expresses a gênerai law: the colors of the red end of the spectrum first appear as yellowish or yellow in the outer régions of the field of vision, while those of the blue end first appear as bluish or blue. Red, orange, yellow, and green corne in with a yellowish tinge, while blue and violet corne in with a bluish or blue tinge.

Every color enters the field of vision as a gray. Within the yellow-blue zone ail colors appear in some aspect of yellow or blue, if they are seen as colors. Only within the central zone can the reds and greens and their derivatives reveal their true color. A comparison of this statement with the above law makes what is otherwise an apparently chaotic condition of affairs seem natural and intelligible.

•From Baird's "The Color-aensitivity of the Peripheral Retina,'' Publication 29, Carnegie Institution. The présent chapter is based maînly upon this monograph.

THE VISUAL FIELD 35

The pigment colors of papers and fabrics are always mixed colors. The reds, oranges, and greens, for example, contain considérable yellow, and it is this yellow which becomes dominant and represents those colors within the yellow-blue zone. In the same way, it is the blue of the violet end which represents violet outside of the red zone.

Ail colors also undergo characteristic changes in brightness in passing through the indirect field.

4. Adaptation Colors — Hold the red square at 25° on the right meridian for about ten seconds, being careful to maintain a steady fixation on the point of regard, and observe the changes the color goes through in that time. Record from memory. Make three trials, allowing adéquate rest

Eepeat the same for yellow.

Constant stimulation of a given area of the peripheral retîna produces definite cycles of change in brightness and color. The foUowing is a typical record of the changes for three seconds by a trained observer :

„ . . Adaptation coton : the mccéssUm

Coloras *tenin of colors this ot^ect revsaled in a

thé direct field. three-second exposure at Su» Hght.

Red Red, orange, yellow, white, blue.

Orange Rich orange, yellowish orange, yellow.

Yellow Orange-yellow, yellow, gray, bluish.

Green Yellow-green, yellow, gray, bluish. (26**.)

Blue Blue, gray, yellowish.

Violet Blue ?, blue, bluish.

Purple Red, orange-red, orangish. (26**.)

From records of this kind we dérive a law which expresses a cycle of three stages: the colors of the red

36 THE VISUAL FIELD

end of the spectrum pass toward yellow, and those of the violet end toward blue; then follows a momentary gray ; and then appears the complementary color of the one just before the gray.

In the above fragment of a record, thèse three stages are represented in the case of red, yellow, green, and blue; the other colors did not complète their cycles within the time limit.

The reasons for this law of adaptation are as follows : * The peripheral retina is not much used and

* Dr. Baird describes this process in a personal letter as follows: "The reasons for the existence of thèse characteristie phenomena of adaptation or exhaustion are thèse : The peripheral régions of the retina contain but a relatively scant supply of color-sensing substance; and they, too, are very seldom employed in the vision of every-day life. Consequently the advent of retinal exhaustion must be relatively rapid when thèse régions are stimulated. Now, the pigment colors which we ordinarily see are not pure, but mixed colors; green leaves and red ribbons contain a considérable admixture of yellow, orange, blue, etc. Hence, when a colored object stimulâtes the peripheral retina, its stimulation is not confined to a single color-sensing substance, as would be the case if certain pure colors were employed. The impure red of the ribbon may affect both the red-sensing substance and the yellow-sensing substance of the retina. Reasons for believing that the yellow-sensing substance is more stable and persistent in function than the red-sensing substance hâve already been cited. So long as both thèse substances continue to function in approximately equal degree, a red (or yellowish red) ribbon will be seen. But in proportion as the red-sensing substance becomes exhausted — ^by reason of its scantiness or as a resuit of its lesser tenacity of function — the function of the yellow-sensing substance becomes dominant in the visual process; the ribbon which originally seemed red now appears yellowish or even yellow. This marks the completion of the first phase of the color process of indirect vision, and the beginning of the second phase. If the ribbon still continues to stimulate the same retinal région, the yellow-sensing substance will in turn become exhausted; and the ribbon will appear gray so long as the black-white substance continues to function alone. When this third stage of adaptation is reached the retinal région is completely exhausted. But this peculiar condition of adapta-

THE VISUAL FIELD si

therefore fatigues with embarrassing rapidity. The colors we ordinarily see are ail mixed, and the yellow or blue élément in a mixture is more stable than the red or green, for reasons explained under Exp. 2 ; hence the latter fatigues more quickly and falls out, leaving the other dominant. This is the goal of the first stage. Then follows the process of adaptation, as in afterimages, when the stable color élément becomes f atigued, and leaves a blank or gray. Then follows the last stage, whieh corresponds to the after-image of the strongest élément in the first stage.*

The fact of order in thèse changes is perhaps as astonishing as their rapidity. Our common expérience of thèse changes in daily life has resulted simply in a distrust — both conscious and unconscious — of indirect color-vision. When we think of it, we think of it as chaotic; when we inadvertently follow it, we feel uneasy; in normal perception we automatically neglect it.

Thèse four experiments may sufiice to give a glimpse of the complexity and wonderful arrangement of the color-fields and to point the way in which the scientific attitude is rewarded by révélation of System and reason

tion does not last long. A process of régénération takes place Mithin the organ; and so long as the retinal régénération continues, one sees color whether the colored object be présent or not. This fourth stage of the visual process is analogous to that which you studied in your experiments with after-images. The Visual sensation which now appears in indirect vision is complementary to the color which was présent in the second stage." *It is remarkable that after-images are never observed in the periphery of the dark-adapted retina, and very rarely more than 30° from the fovea. They may be observed, however, in the periphery of the light-adapted retina.

38 THE VISUAL FIELD

in it ail. Many of the problems of indirect color-visîon hâve not even been mentioned, and conditions and variables hâve been stated in the barest way possible. *

In conclusion, let us notice a beautiful biological arrangement. If we compare the direct with the indirect field with référence to sensitiveness, we find that we are most sensitive to color and form in the direct field, and to light and movement in the indirect field. That is, we can see color and form most accurately when the image f ails upon the central région of the retina, but we can detect movements and changes in brightness more readily when the outer-lying portions of the retina are stimulated.*-

This is a story of adjustment and it su^ests to us the real office of the indirect field. The central région of the retina, corresponding as it does to the direct field, is the organ of attention, of concentrated mental activity, which represents the environment in tenus of space and color; while the indirect field is merely accessory. Its functions are tbose of a scout or guardian. The life-preserving value of this arrangement is clear. Consciousness is warned of the présence or approach of an object beneficent or noxious to life, by impressions of luminosity or movement in the indirect 'field. If then the signal is heeded, the eye quickly tums so as to bring the object of scrutiny into the direct field where its true nature can be seen accurately, by the most efficient and economical expenditure of energy.