The Sense of Sight

Of the tremendous number and variety of visual sensations, the great majority are certainly compounds. Two sorts of compound sensation can be distinguished here: _blends_ similar to those of taste or smell, and _patterns_ which scarcely occur among sensations of taste and smell, though they are found, along with blends, in cutaneous sensation. Heat, compounded of warmth, cold and pain sensations, is an {205} excellent example of a blend, while the compound sensation aroused by touching the skin simultaneously with two points--or three points, or a ring or square--is to be cla.s.sed as a pattern. In a pattern, the component parts are spread out in s.p.a.ce or time (or in both at once), and for that reason are more easily attended to separately than the elements in a blend. Yet the pattern, like the blend, has the effect of a unit. A spatial pattern has a characteristic shape, and a temporal pattern a characteristic course or movement. A rhythm or a tune is a good example of a temporal pattern.

Visual sensations are spread out spatially, and thus fall into spatial patterns. They also are in constant change and motion, and so fall into temporal patterns, many of which are spatial as well. The visual sensation aroused, let us say in a young baby, by the light entering his eye from a human face, is a spatial pattern; the visual sensation aroused by some one's turning down the light is a pure temporal pattern; while the sensation from a person seen moving across the room is a pattern both spatial and temporal. Finding the elements of a visual pattern would mean finding the smallest possible bits of it, which would probably be the sensations due to the action of single rods and cones, just as the smallest bit of a cutaneous sensation would be due to the exciting of a single touch spot, warmth spot, cold spot or pain spot.

a.n.a.lyzing a visual blend is quite a different job. Given the color pink, for example, let it be required to discover whether this is a simple sensation or a blend of two or more elementary sensations.

Studying it intently, we see that it can be described as a whitish red, and if we are willing to accept this a.n.a.lysis as final, we conclude that pink is a blend of the elementary sensations of white and red. Of the thousands and thousands of distinguishable hues, shades {206} and tints, only a few are elements and the rest are color blends; and our main problem now is to identify the elements. Notice that we are not seeking for the physical elements of light, nor for the primary pigments of the painter's art, but for the elementary _sensations_. Our knowledge of physics and painting, indeed, is likely to lead us astray. Sensations are our responses to the physical stimulus, and the psychological question is, what fundamental responses we make to this cla.s.s of stimuli.

Suppose, without knowing anything of pigments or of the physics of light, we got together a collection of bits of color of every shade and tint, in order to see what we could discover about visual sensations. Leaving aside the question of elements for the moment, we might first try to _cla.s.sify_ the bits of color. We could sort out a pile of reds, a pile of blues, a pile of browns, a pile of grays, etc., but the piles would shade off one into another. The salient fact about colors is the gradual transition from one to another. We can arrange them in _series_ better than we can cla.s.sify them. They can be serially arranged in three different ways, according to brightness or intensity, according to color-tone, and according to saturation.

The _intensity series runs from light to dark_. We can arrange such a series composed entirely of reds or blues or any other one color; or we can arrange the whole collection of bits of color into a single light-dark series. It is not always easy to decide whether a given shade of one color is lighter or darker than a given shade of a different color; but in a rough way, at least, every bit of whatever color would have its place in the single intensity series. An intensity series can, of course, be arranged in any other sense as well as in sight.

The _color-tone series_ is best arranged from a collection consisting entirely of full or saturated colors. Start the {207} series with any color and put next to this the color that most resembles it in color-tone, i.e., in specific color quality; and so continue, adding always the color that most resembles the one preceding. If we started with red, the next in order might be either a yellowish red or a bluish red. If we took the yellowish red and placed it beside the red, then the next in order would be a still more yellowish red, and the series would run on to yellow and then to greenish yellow, green, bluish green, blue, violet, purple, purplish red, and so back to red.

The color-tone series returns upon itself. It is a circular series.

[Ill.u.s.tration: Fig. 33.--The color circle. R, Y, G and B, stand for the colors red, yellow, green and blue. The shaded portion corresponds to the spectrum or rainbow. Complementary colors (see later) lie diametrically opposite to each other on the circ.u.mference.]

A _saturation series_ runs from full-toned or saturated colors to pale or dull. Since we can certainly say of a pale blue that it is less saturated than a vivid red, etc., we could, theoretically, arrange our whole collection of bits of color in a single saturation series, but our judgment would be very uncertain at many points. The most significant saturation series confine themselves to a single color-tone, {208} and also, as far as possible, to a constant brightness, and extend from the most vivid color sensation obtainable with this color-tone and brightness, through a succession of less and less strongly colored sensations of the same tone and brightness, to a dead gray of the same brightness. Any such saturation series terminates in a neutral gray, which is light or dark to match the rest of the particular saturation series.

White, black and gray, which find no place in the color-tone series, give an intensity series of their own, running from white through light gray and darker and darker gray to black, and any gray in this series may be the zero point in a saturation series of any color-tone.

A three-dimensional diagram of the whole system of visual sensations can be built up in the following way. Taking all the colors of the same degree of brightness, we can arrange the most saturated, in the order of their color-tone, around the circ.u.mference of a circle, put a gray of the same brightness at the center of this circle, and then arrange a saturation series for each color-tone extending from the most saturated at the circ.u.mference to gray at the center. This would be a two-dimensional diagram for colors having the same brightness.

For a greater brightness, we could arrange a similar circle and place it above the first, and for a smaller brightness, a similar circle and place it below the first, and we could thus build up a pile of circles, ranging from the greatest brightness at the top to the least at the bottom. But, as the colors all lose saturation when their brightness is much increased, and also when it is much decreased, we should make the circles smaller and smaller toward either the top or the bottom of the pile, so that our three-dimensional diagram would finally take the form of a double cone, with the most intense white, like that of sunlight, at the upper point, with dead black at the lower point, {209} and with the greatest diameter near the middle brightness, where the greatest saturations can be obtained. The axis of the double cone, extending from brightest white to dead black, would give the series of neutral grays. All the thousands of distinguishable colors, shades and tints, would find places in this scheme.

[Ill.u.s.tration: Fig. 34.--The color cone, described in the text.

Instead of a cone, a four-sided pyramid is often used, so as to emphasize the four main colors, red, yellow, green and blue, which are then located at the corners of the base of the pyramid. (Figure text: white, black, R, B, G, Y)]

Simpler Forms of the Color Sense

Not every one gets all these sensations. In _color-blindness_, the system is reduced to one or two dimensions, instead of three. There are two princ.i.p.al forms of color-blindness: total, very uncommon; and red-green blindness, fairly {210} common. The totally color-blind individual sees only white, black, and the various shades of gray. His system of visual sensations is reduced to one dimension, corresponding to the axis of our double cone.

_Red-green blindness_, very uncommon in women, is present in three or four percent of men. It is not a disease, not curable, not corrected by training, and not a.s.sociated with any other defect of the eye, or of the brain. It is simply a native peculiarity of the color sense.

Careful study shows that the only color sensations of the red-green blind person are blue and yellow, along with white, black and the grays. His color circle reduces to a straight line with yellow at one end and blue at the other. Instead of the color circle, he has a double saturation series, reaching from saturated yellow through duller yellows to gray and thence through dull blues to saturated blue. What appears to the normal eye as red, orange or gra.s.s green appears to him as more or less unsaturated yellow; and what appears to the normal eye as greenish blue, violet and purple appears to him as more or less unsaturated blue. His color system can be represented in two dimensions, one for the double saturation series, yellow-gray-blue, and the other for the intensity series, white-gray-black.

Color-blindness, always interesting and not without some practical importance (since the confusions of the color-blind eye might lead to mistaking signals in navigation or railroading), takes on additional significance when we discover the curious fact that _every one is color-blind_--in certain parts of the retina. The outermost zone of the retina, corresponding to the margin of the field of view, is totally color-blind (or very nearly so), and an intermediate zone, between this and the central area of the retina that sees all the colors, is red-green blind, and delivers only blue and yellow sensations, along with white, black and gray. Take {211} a spot of yellow or blue and move it in from the side of the head into the margin of the field of view and then on towards the center. When it first appears in the margin, it simply appears gray, but when it has come inwards for a certain distance it changes to yellow. If a red or green spot is moved in similarly, it first appears gray, then takes on a faint tinge of yellow, and finally, as it approaches the center of the field of view, appears in its true color. The outer zone gets only black and white, the intermediate zone gets, in addition to these, yellow and blue, and the central area adds red and green (and with them all the colors).

[Ill.u.s.tration: Fig. 35.--Color cones of the retina. F is the fovea, or central area of clearest vision. (Figure text: all colors, white-black & yellow-blue, white-black)]

Now as to the question of elements, let us see how far we can go, keeping still to the sensations, without any reference to the stimulus. If a collection of bits of color is presented to a cla.s.s of students who have not previously studied this matter, with the request that each select those colors that seem to him elementary and not blends, there is practically unanimous agreement on three colors, red, yellow and blue; and there are some votes for green also, but almost none for orange, violet, purple, brown or any other colors. {212} except white and black. That white and black are elementary sensations is made clear by the case of total color-blindness, since in this condition there are no other visual sensations from which white and black could be compounded, and these two differ so completely from each other that it would be impossible to think of white as made up of black, or black of white. Gray, on the other hand, appears like a blend of black and white. In the same way, red-green blindness demonstrates the reality of yellow and blue as elementary sensations, since neither of them could be reduced to a blend of the other with white or black; and there are no other colors present in this form of color vision to serve as possible elements out of which yellow and blue might be compounded. That white, black, yellow and blue are elementary sensations is therefore clear from the study of visual sensations alone; and there are indications that red and green are also elements.

Visual Sensations as Related to the Stimulus

Thus far, we have said nothing of the stimulus that arouses visual sensations. Light, the stimulus, is physically a wave motion, its vibrations succeeding each other at the rate of 500,000000,000000 vibrations, more or less, per second, and moving through s.p.a.ce with a speed of 186,000 miles per second. The "wave-length", or distance from the crest of one wave to the crest of the next following, is measured in millionths of a millimeter.

The most important single step ever taken towards a knowledge of the physics of light, and incidentally towards a knowledge of visual sensations, was Newton's a.n.a.lysis of white light into the spectrum. He found that when white light is pa.s.sed through a prism, it is broken up into all the colors of the rainbow or spectrum. Sunlight consists of a {213} mixture of waves of various lengths. At one end of the spectrum are the long waves (wave-length 760 millionths of a millimeter), at the other end are the short waves (wavelength 390), and in between are waves of every intermediate length, arranged in order from the longest to the shortest. The longest waves give the sensation of red, and the shortest that of violet, a slightly reddish blue.

Outside the limits of the visible spectrum, however, there are waves still longer and shorter, incapable of arousing the retina, though the very long waves, beyond the red, arouse the sensation of warmth from the skin, and the very short waves, beyond the violet, though arousing none of the senses, do effect the photographic plate. Newton distinguished seven colors in the visible spectrum, red, orange, yellow, green, blue, indigo and violet; but there is nothing specially scientific about this list, since physically there are not seven but an unlimited number of wave-lengths included in the spectrum, varying continuously from the longest at the red end to the shortest at the violet; while psychologically the number of distinguishable colors in the spectrum, though not unlimited, is at least much larger than seven. Between red and orange, for instance, there are quite a number of distinguishable orange-reds and reddish oranges.

If now we ask what differences in the stimulus give rise to the three kinds of difference in visual sensation that were spoken of previously, we find that color-tone depends on the wave-length of the light, brightness on the energy of the stimulus, i.e., on the amplitude of the vibration, and saturation on the mixture of long and short wave-lengths in a complex light-stimulus--the more mixture, the less saturation.

These are the general correspondences between the light stimulus and the visual sensation; but the whole relationship is much more complex.

Brightness depends, not only on the energy of the stimulus, but also on wave-length. The {214} retina is tuned to waves of medium length, corresponding to the yellow, which arouse much brighter sensation than long or short waves of the same physical energy. Otherwise put, the sensitivity of the retina is greatest for medium wavelengths, and decreases gradually towards the ends of the spectrum, ceasing altogether, as has been said, at wavelengths of 760 at the red end and of 390 at the violet end.

Saturation, depending primarily on amount of mixture of different wave-lengths, depends also on the particular wavelengths acting, and also on their amplitude. So, the red and blue of the spectrum are more saturated than the yellow and green; and very bright or very dim light, however h.o.m.ogeneous, gives a less saturated sensation than a stimulus of medium strength.

Color Mixing

Color-tone depends on the wave-length, as has been said, but this is far from the whole truth; the whole truth, indeed, is one of the most curious and significant facts about color vision. We have said that each color-tone is the response to a particular wave-length. But any color-tone can be got without its particular wave-length being present at all; all that is necessary is that wave-lengths centering about this particular one shall be present. A mixed light, consisting of two wave-lengths, the one longer and the other shorter than the particular wave which when acting alone gives a certain color-tone, will give that same color-tone. For example, the orange color resulting from the isolated action of a wave-length of 650 is given also by the combined action of wave-lengths of 600 and 700, in amounts suitably proportioned to each other.

A point of experimental technique: in _mixing colored lights_ for the purpose of studying the resulting sensations, we do not mix painter's pigments, since the physical {215} conditions then would be far from simple, but we mix the lights themselves by throwing them together either into the eye, or upon a white screen. We can also, on account of a certain lag or hang-over in the response of the retina, mix lights by rapidly alternating them, and get the same effect as if we had made them strike the retina simultaneously.

By mixing a red light with a yellow, in varying proportions, all the color-tones between red and yellow can be got--reddish orange, orange and yellowish orange. By mixing yellow and green lights, we get all the greenish yellow and yellowish green color-tones; and by mixing green and blue lights we get the bluish greens and greenish blues.

Finally, by mixing blue and red lights, in varying proportions, we get violet, purple and purplish red. Purple has no place in the spectrum, since it is a sensation which cannot be aroused by the action of any single wave-length, but only by the mixture of long and short waves.

To get all the color-tones, then, we need not employ all the wave-lengths, but can get along with only four. In fact, we can get along with three. Red, green and blue will do the trick. Red and green lights, combined, would give the yellows; green and blue would give the greenish blues; and red and blue would give purple and violet.

The sensation of white results--to go back to Newton--from the combined action of all the wave-lengths. But the stimulus _need_ not contain _all_ the wave-lengths. Four are enough; the three just mentioned would be enough. More surprising still, two are enough, if chosen just right. Mix a pure yellow light with a pure blue, and you will find that you get the sensation of white--or gray, if the lights used are not strong.

[Footnote: When you mix blue and yellow _pigments_, each absorbs part of the wave-lengths of white light, and what is left after this double absorption may be predominantly green. This is absolutely different from the addition of blue to yellow light; addition gives white, not green.]

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Lights, or wave-lengths, which when acting together on the retina give the sensation of white or gray, are said to be _complementary_.

Speaking somewhat loosely, we sometimes say that two _colors_ are complementary when they mix to produce white. Strictly, the colors--or at least the color sensations--are not mixed; for when yellow and blue lights are mixed, the resulting sensation is by no means a mixture of blue and yellow sensations, but the sensation of white in which there is no trace of either blue or yellow. Mixing the stimuli which, acting separately, give two complementary colors, arouses the colorless sensation of white.

Blue and yellow, then, are complementary. Suppose we set out to find the complementary of red. Mixing red and yellow lights gives the color-tones intermediate between these two; mixing red and green still gives the intermediate color-tones, but the orange and yellow and yellowish green so got lack saturation, being whitish or grayish. Now mix red with bluish green, and this grayishness is accentuated, and if just the right wave-length of bluish green is used, no trace of orange or yellow or gra.s.s green is obtained, but white or gray. Red and bluish green are thus complementary. The complement of orange light is a greenish blue, and that of greenish yellow is violet. The typical green (gra.s.s green) has no single wave-length complementary to it, but it does give white when mixed with a compound of long and short waves, which compound by itself gives the sensation of purple; so that we may speak of green and purple as complementary.

What Are the Elementary Visual Sensations?

Returning now to the question of elementary sensations, which we laid aside till we had examined the relationship of the sensations to the stimulus, we need to be on our guard against physics, or at least against being so much impressed with the physics of light as to forget that we are concerned with the _response_ of the organism to physical light--a matter on which physics cannot speak the final word.

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Fig. 36.--(After Konig.) The color triangle, a map of the laws of color mixture. The spectral colors are arranged in order along the heavy solid line, and the purples along the heavy dotted line. The numbers give the wave-lengths of different parts of the spectrum.

Inside the heavy line are located the pale tints of each color, merging from every side into white, which is located at the point W.

Suppose equal amounts of two spectral colors are mixed: to find from the diagram the color of the mixture. Locate the two colors on the heavy line, draw a straight line between these two points, and the middle of this line gives the color-tone and saturation of the mixture. For example, mix red and yellow: then the resulting color is a saturated reddish yellow. Mix red (760) and green (505): the resulting yellow is non-saturated, since the straight line between these two points lies inside the figure. If the straight line joining two points pa.s.ses through W, the colors located at the two points are complementary.

Spectral colors are themselves not completely saturated. The way to get color sensations of maximum saturation is first to stare at one color, so as to fatigue or adapt the eye for that color, and then to turn the eye upon the complementary color, which, under these conditions, appears fuller and richer than anything otherwise obtainable. The corners, R, G, and B, denote colors of maximum saturation, and the whole of the triangle outside of the heavy line is reserved for super-saturated color sensations.