Fig. 31.— Simultaneous Contrast.
The shading of the two V’s is exactly similar; but the figure in half-tone on black appears brighter than the figure in half-tone on a white ground.
All the senses show a tendency to rebound after activity, exhibiting contrast-phenomena; but the contrasts of vision are more marked and varied than those of the other senses, as everyone who is curious in the observation of his own sensations is aware. Negative after-images are generally referred to the retina; but various other kinds of after-image and contrast-phenomena must be attributed to the judgments passed by the mind upon the sensations which it receives; and not to physical changes in sense-organs. Positive after-images are well-marked appearances, although less common, perhaps, than the phenomena of reversal of sensation of which we have just written. On waking in the morning, one looks at the window; shifting the gaze to the ceiling, an after-image of the window appears, just as one saw it, with bright panes and dark frame. The “dark adapted eye,” being exceptionally sensitive, yields the same persistent positive after-image as the eye in its usual condition yields, after being directed towards the sun at mid-day. Movement-after-images can be explained only by referring them to misdirection of judgment. If the gaze is fixed on a rock close beside a waterfall, then shifted to a bank covered with grass or bushes, the part of the bank which occupies the lateral part of the field of vision appears to rush upwards, reversing the movement of the water. When the gaze has been fixed upon falling water—a narrow stream sparkling in sunlight—a central strip of the field moves upwards, the margins remaining stationary. If one stares at the spot on the surface of a basin of water on which drops are falling from a tap, and then looks at the floor, it is seen to contract towards the spot looked at, reversing the movement of the ripples in the basin. These observations reveal a fact of great importance in the physiology of vision. It is, probably, impossible truly to fix the gaze. The muscles of the eyeball keep the retinal field in constant movement—larger movements with minute oscillations superposed. When, as in watching a waterfall, movement has for a time taken a definite direction, its cessation is judged to mean reversal.
The anatomical unit of sensation is a cone. The fovea centralis, the only part of the retina capable of receiving sensations sufficiently discrete for reading, contains cones alone. If the gaze be directed but a very few millimetres on to the white margin of the page, letters lose their form. In the fovea the centre of one cone is 3·6 µ distant from the centre of the next. Two stars are visible as separate stars if they subtend an angle of at least 60 seconds with the eye. Their images on the retina are then 4 µ apart. Parallel white lines ruled on black paper, held at such a distance as causes them to subtend angles of 60 seconds with the eye, appear not straight but wavy, showing that their images are taken up, not by a continuous substance, but by the mosaic of cones. So far the explanation of the visual unit is strictly anatomical; but it must be added that trained observers can recognize the separateness of objects which subtend angles of much less than 60 seconds—not more than 5 or 6 seconds. This can be accounted for only on the hypothesis that images far closer together than the width of a cone produce a specific effect in passing across the anatomical unit.
In 1807 Thomas Young, the physicist, formulated a theory to account for colour-vision. He supposed that the retina contains three kinds of apparatus—a, b, and c—each especially responsive to a particular kind of light, all three slightly stimulated by rays of all colours. (Young imagined three kinds of nerve, but modern supporters of his theory suppose three different substances chemically changed by light.) A prism spreads out the rays which are combined in white light into a band in the order of their wave-lengths—those which have the longest wave-length (0·8 µ) and the slowest rate of vibration (381 billions to the second) at one end, those which have the shortest wave-length (0·4 µ) and the most rapid vibration (764 billions to the second) at the other: between these two extremes every intermediate grade of length and rapidity. These are a mere fraction—a small group—of the waves which the æther transmits, but they are all that we can see. The long, slow vibrations give rise to sensations which we describe as red; the short, rapid vibrations we describe as violet. Our names for the tints which intervene are singularly old-fashioned and unsatisfactory, but all persons agree that they recognize in the spectrum a certain number of definite colours. Some normal-sighted persons say twelve, others eighteen. It is largely a question of terminology.
Many considerations show that it is quite unnecessary to imagine that the retina is affected in a different kind of way by every kind of light, or by each of several groups of waves. If the red of the spectrum is mixed with yellow, we receive an impression of orange, which is identical with the impression produced by waves of the mean length of red and yellow; orange and green give yellow; yellow and blue, green. Any two complementary colours yield white. By taking three colours—say, red, green, and violet—we obtain, when they are duly mixed, not white light only, but light of any other tint, although not of spectral purity, since it is mixed with white. Young considered that all the conditions of colour-vision would be satisfied, all our various sensations provided for, if the retina contain three kinds of apparatus which light, according to its quality, affects in varying degrees; and with this theory of three kinds of apparatus—a, b, and c—the theory of three elementary or fundamental colour-sensations is indissolubly linked. The colour x produces its intensest effect when a is stimulated, with the least possible stimulation of b and c; y is the reaction of b, z of c. Recent studies of the curves of intensity give us the tints of x, y, and z as carmine-red, apple-green, and ultramarine blue.
The blending of sensations is illustrated with the well-known colour-top. But perhaps the most striking proof that three elementary colour-sensations are adequate to produce our visual world is afforded by photographs taken with the three-colour method. Three plates are exposed—(a) behind a red screen, (b) behind a greenish-yellow screen, (c) behind a blue screen. They are fixed in such a way that the portions acted upon by light are rendered insoluble, whereas the rest of the film can be dissolved away; a is then stained red, b greenish yellow, c blue. The three are superposed, and the result appears to the eye as an exact reproduction of the subject of the photograph in all its hues. It shows every shade of orange and green and violet. It is as bright—that is to say, as full of white light—as the original.
Various objections may, however, be brought against Young’s theory. Of these, the most weighty are: (1) The retina does not contain three kinds of apparatus, as Young supposed; nor can we find three kinds of photochemical substances, as required by the theory in its modern form. If we could find them, a fresh difficulty would arise; for we have no reasons for supposing that one and the same nerve-ending can receive stimuli of three different kinds. (2) The theory offers no explanation of negative after-images—the complementary colours experienced when the eye is closed after staring at a brightly coloured object. (3) It does not adequately account for the various deficiencies of colour-blindness.
It is well recognized that there are various degrees of colour-blindness, and that the colour-vision of persons considered normal presents different grades of refinement. Nevertheless, the abnormalities of colour-blind persons are so marked that cases fall into definite classes. Those whose cones do not function—which means that their yellow spots are either undeveloped or diseased—see all things grey. They are totally colour-blind. Excluding these, the colour-blind may be grouped in one or other of two divisions—(a) those who confuse red and green, (b) those who confuse yellow and blue. One person out of every thirty-five is red-green blind. The proportion is even higher if males only are considered, showing how very unfortunate is our choice of warning signals. A man who is red-green blind cannot tell the port from the starboard light. Blue-yellow blindness is, on the other hand, extremely rare. According to Young’s theory, colour-blindness is due to the absence of one of the three sets of visual apparatus. But cases do not altogether conform to this hypothesis. We knew an amateur water-colourist, since deceased, who derived intense pleasure from the beauties of Nature, and showed no mean skill in reproducing them with his brush, notwithstanding the fact that he was red-green blind. Each night his sister arranged his paint-box for him, and only rarely did he use vermilion to fill in a foreground of lush green grass. But this mistake, when he made it, did not destroy his own satisfaction in the picture. It was clear that red had a value for him, although he confused it with green. It is impossible for a normal person to see through the eye of one who is colour-blind, and there is no other means of comparing his sensations with our own. The mistakes which the colour-blind make in sorting coloured objects and in naming mixtures of light selected from various parts of the spectrum show the range of their deficiency, but give us no information regarding the qualities of the sensations which they retain.
The test of colour-sensitiveness usually employed is the grading of a large number of wools of different tint. The order in which the colours should be arranged is not a matter of opinion. They must be placed in the order in which they occur in the spectrum—i.e., arranged according to their wave-lengths. In the cases of colour-blindness which are most frequently met with the defect may be described as due to an absence of the sense of redness, or as an absence of the sense of greenness. The two conditions can be distinguished. But since the eye is not dark for red (although in certain cases vision is very weak for the red end of the spectrum) or dark for green, the abnormality cannot be adequately accounted for on structural grounds. It is not explicable on the hypothesis that one of three sets of responsive sense-organs (or nerve-fibres) or photochemical substances is absent from the eye. Again, it is generally agreed that the sensations of white, yellow, and blue of the red-green colour-blind are similar to those of normal persons. This is not in harmony with the theory of the omission from their eyes of one of three pieces of colour-apparatus.