A Beginner's Psychology

by Edward Bradford Titchener

All lights and colours are psychologically simple. Paints may be mixed on a palette, and colour-stimuli may be mixed in all sorts of ways; we learn in physics that white daylight is a mixture of all the rays that are seen separately in the rainbow. Yet a white, considered just as a look, is perfectly simple; and the looks of orange and yellow-green and green-blue are equally simple. There are no compound colours, to correspond with compound tones. Hence the number of light and colour sensations is very large, at least ten times as large as the number of simple tones.

The organ of vision is the eye; and the eye is a little photographic camera, with shutter, iris-diaphragm, self-adjusting lens, dark chamber, and self-renewing sensitive film. We are concerned only with the film, that is, with the retina or nervous network that lines the posterior half of the eyeball. It seems that the retina is really made up of three interfused films; for simplicity’s sake you may consider them as lying upon one another, just as three saucers might do if you piled them together. The oldest and largest film, the bottom saucer, gives us the sensations of black and white; the middlemost, somewhat smaller, gives us blue and yellow; and the topmost and smallest gives us a purplish-red and a bluish-green. The existence and size of the three films can be shown by experiment; for we are all totally colour-blind at the edge of the field of vision, and are blind to reds and greens for some distance further in toward the centre. There are also cases of inherited colour-blindness, in which the eye is blind either for all colours (total colour-blindness) or for red and green alone (partial colour-blindness); the latter form is fairly common, as is natural,—for the red-green film, being the last to come, might be expected to be the first to go. Partial colour-blindness was first brought to scientific notice by the English chemist John Dalton in 1798. Dalton was a Quaker, but made no objection to wearing the scarlet gown of a doctor of laws, because, as he said, “to me its colour is that of nature—the colour of those green leaves”; it is needless to remark that he did not see green either! The defect is practically important for pilots and signalmen, who have to distinguish red and green lights.

From these three films we get all the lights and colours that we see in the daytime, with the single exception of neutral grey; and this appears to come, not from the eye at all, but from the brain. It may be seen even when the retina is quite blind, provided that the rest of the nervous apparatus is in working order; and it may be seen by night as well as by day; it is mixed, physiologically, with all our sensations of light and colour, though we cannot by psychological analysis pick it out from the lights and colours. Strange enough! but we shall understand better as we go on. The German physiologist Ewald Hering has shown that the processes which take place in the films are, in all probability, chemical processes of an antagonistic or reversible kind; that is why we never see a bluish-yellow, or a greenish-red; if we throw on the same part of the retina, at the same time, equal amounts of black and white, or of blue and yellow, or of purplish-red and bluish-green, the chemical processes go on in opposite directions and cancel each other, with the result that we see just nothing. This antagonism can be proved, under the right experimental conditions, for blue-yellow and for red-green; if these pairs are fittingly thrown together on the retina we see, in fact, only neutral grey; so that our seeing of the same grey, when black and white stimuli are acting together, does not necessarily mean that grey is a retinal mixture of black and white; the black and white may also cancel each other, and leave only the brain-grey to be seen.

We have, then, the three films in each eyeball, and we have the brain-grey behind them. More than this: we have a night or twilight eye. When colours fade out, as twilight deepens, another retinal film comes into play; the lights that we still see come, not from the black-white film, but from a fourth film, of the same size, whose only sensation is a slightly bluish-white. Of course, this white is always mixed, physiologically, with the brain-grey; we never see it by itself; but we owe to it, among other things, the silvery look of blues in the twilight. The very centre of the twilight eye is totally blind; if on a moonless night you want to see a faint star or a distant street-lamp you must not look directly at it, but just to one side of it. Children’s fear of the dark is partly due to the fact that they cannot see what they turn their gaze upon; there had seemed to be something there, but when they looked at it, it eluded them; and if they think they see it again, and look in the new direction, again it is gone.

Now suppose that you are looking out, in daylight, over a variegated landscape. Somewhere or other you see a patch of light grey. You get this sensation from the black-white film and the brain-grey; the white-process is stronger than the black-process in the film, and the excess of white, added physiologically to the brain-grey, shows as light grey. Or again, you see a patch of dark purple. This sensation comes from the red-green film (excess of red); from the blue-yellow film (excess of blue); from the black-white film (excess of black); and from the brain-grey. All the lights and colours of the landscape can be accounted for in the same way.

Not quite correctly, however!—there are still other factors at work. The film-processes are antagonistic, for instance, even when they go on in different parts of a film; lights and colours contrast with one another; if you lay a strip of grey paper on red, it looks greenish; on blue, yellowish; on white, blackish; make the trial with your own papers. So all the various lights and colours of the landscape stand out, by contrast, against one another; the eye makes their differences greater than they ought physically, from the nature of the stimuli, to appear. Black, indeed, is wholly a contrast-sensation; it has no physical stimulus; and you see deep black only in strong illumination.

Contrast is effective at once, the moment you cast your eyes on the landscape. As time goes on, however, the opposed film-processes tend to settle down into a state of balance or equilibrium; so that actually, if you stared at the landscape long enough, without moving your eyes, you would finally see nothing but the brain-grey. This levelling down of all lights and all colours toward neutral grey is called adaptation. Stand up two strips of black and white paper, side by side, and stare at their line of junction for a minute or two; even in that short time you will find that they tend toward a uniform grey. If, now, a stimulus to which you are wholly or partly adapted is suddenly removed, the antagonism of the film-processes shows itself once more; you see an after-image. Lay a disc of red on grey; stare at it for half a minute; flick it away, keeping the eyes steady, and look at the grey background; you see a corresponding disc of green. White leaves a black after-image, black a white; blue a yellow after-image, and yellow a blue.

It is clear, then, that the lights and colours of the landscape depend on many things beside the stimuli there presented; they depend on contrast, on the previous adaptation of the eye, on the presence or absence of after-images. The main reason that we do not notice all these influences is that we ordinarily view the landscape, not for itself, but for what it means; it shows us the familiar trees and stream and houses, and we take their stability for granted. That is the main reason; it is not the only one. We have said, for instance, that the normal retina is totally colour-blind along its outer edge, and partially colour-blind for some distance in toward the centre; the edge of the landscape ought therefore to be colourless, and a certain outlying portion of it ought to appear simply as blue and yellow. There is no hint of these differences; and the explanation is that we are accustomed to turn our eyes directly towards what we want to see, and therefore to view it with all three of the daylight films; head and eyes move so easily, and we see so much better with the centre of the retina, that we totally disregard the altered look of things seen ‘out of the corner of the eye.’ Even if we do not, we are likely to remember how the things appear in direct vision; we paint them over, so to speak, with memory-colours, colours that represent their natural or average appearance at the centre of the visual field; indeed, we may paint these colours over the whole landscape, and in that way correct the changes due to contrast or adaptation. We always talk of a certain book as brown; we recognise it in all lights, and in all states of the eye, by its brown colour; we see it, in memory-colour, as brown; whereas, if that same brown were shown us in all the different circumstances without our knowing it to be the same, it might give us sensations of yellow, of pale brown, of deep brown, of black. These two factors, movement of the eyes and memory-colour, lead us to overlook, in great part, the actual variation of lights and colours in the landscape.

A final word may be added regarding the likeness of sight and smell. Odours and colours fade out by adaptation; odours, like lights and colours, contrast, and even cancel one another; and smell-stimuli as well as sight-stimuli mix to produce new and simple sensations. It is highly probable that the sensory cells of smell are the seat of only a few chemical processes, by whose combination all the wealth of odours is created, just as the cone-cells of the retina are the seat of those three reversible processes (black-white, blue-yellow, red-green) whose combination endows us with the variety of daylight vision. We have as yet, however, no such definite grounds for hypothesis as we have in the case of sight; we cannot even guess what these processes are, or how many of them are taking place in the smell-membrane.