crease toward the periphery, and it is very difficult to get any image from the extreme periphery of the retina. In this respect the after-image behaves like the primary image in perception.

12. Periodicity.-Fixate the black square until it begins to blur and then project the after-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 recurrence is a fundamental law. Sometimes the usual sequence of positive and negative 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 all its transitions through brightness and color follows the course of this reaction in the retinal elements.*

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

CHAPTER II

VISUAL CONTRAST

For One.*

EVERY sensation is different from what it would have been if it had been experienced together with, or in sequence 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 sense of sight.

These experiments should be performed in good diffused daylight. Unless otherwise directed, the object must invariably be viewed through 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 all 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 book containing the millimeter scale for a background, and cover the colors laid upon it with the facing sheet of tissue-paper.

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

The two gray bars are exactly alike in brightness; but, by contrast with their backgrounds, one becomes 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 alteration 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 theme of the present chapter, is described as "the mutual effects in respect to color and brightness which simultaneously seen but separated visual areas have upon each other." (Baldwin.) The difference 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 consequences of the phenomenon just demonstrated. Wherever surfaces of different 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 operates, 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 differences and therefore often helps in distinguishing surfaces; on the other hand, unless we

are prepared to make the correction, we are constantly deceived 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. Repeat the same with other color-squares, pair by pair. If possible reduce these records to a law of color-contrast.†

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

*

Supplementary Experiment.-(Not to be performed unless directed 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 in the mixture. Then measure the grayness of the other square in the same manner. The difference in the amount of white in the two measurements is a measure of the difference between the two gray bars.

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

A very pretty demonstration 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 all window-shades except one which is left open about a foot; light a candle or any other artificial 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 artificial light. The induced blue is a much stronger color than the inducing yellow.

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

Color-contrast is most effective when there is no brightness-contrast between the two fields. The effect would have been very much enhanced in the present 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-contrast.

*

Here again we have a sweeping principle: all 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, all 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.†

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 principle as the brightness-contrast.

Chevreul, "The Laws of Contrast of Color and their Application to the Arts of Painting, Decoration 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.