Astronomers have long recognized that one of the smaller stars which catches the attention when they are not looking directly at it may be invisible when the gaze is directed to the spot where it ought to be. It was visible when focussed on rods, but it is not visible when focussed on cones. In most birds the retina shows cones alone. To anyone who for the first time enters a dovecote at night the experience is very curious. A candle is for him a sufficiently strong illuminant, but it does not give light enough to enable the pigeons to see. Although evidently alarmed by the noise made by the intruder, they allow themselves to be taken down from their perches without making any attempt to escape. If, startled by the touch of a hand, they take to flight, they fly against the wall. Pigeons are night-blind. The retina of an owl bears chiefly rods, the outer limbs of which are exceptionally long.
The outer limbs of the rods are coloured reddish-purple. This colour is quickly bleached by light. If a frog which has been kept for a short time in the dark be decapitated, its head fixed for ten minutes in a situation in which a window is in front of it, then carried to a photographic dark-room, where an eye is taken out by red light, opened, and the retina removed, a print of the window will be seen upon it. Such an optogram may be fixed by dipping the retina in alum.
The retina is easily detached from its pigment-layer. If it has been bleached by exposure to light, it regains its “visual purple” when again placed in contact with its pigment. Evidently the visual purple is renewed from the pigment which lies behind (and around) the rods.
From the cells of the pigment-layer a fringe of streaming processes depends amongst the outer limbs of the rods and cones ([Fig. 30]). In a dull light the processes hang but a short way down; in a bright light they react almost to the outer limiting membrane. They supply pigment to the rods, but their relation to cones is not understood. It is clear, however, that the cones, although they are not coloured, are dependent upon the pigment-fringe, since they always remain in contact with it. Their inner limbs elongate in the dark, lifting them to the pigment, and shorten in bright light. These movements may merely indicate that the cones require a backing of pigment, but it would seem more probable that, like the rods, they absorb a substance which is sensitive to light, although we cannot recognize it by its colour.
The responsiveness of the rods to light is due to visual purple. As every lady is aware, colours, especially mauves and lilacs, are bleached by light. The chemical change affected by light in the colour of the outer limbs of the rods is the stimulant which originates impulses in the nerve-fibres connected with them, and it is generally believed that cones—the more highly specialized sensory cells—are stimulated in the same way. Visual purple is particularly abundant in all animals that range at night, with the exception of the bat. But its absence in the bat does not militate against the theory that it is the cause of night-vision, for it has been shown that a blind bat flies with almost as much freedom, and avoids obstacles—even threads stretched across the room—with as much skill as one that can see. It is guided by the bristles of its cheek. So, too, is the cat, which has the reputation of being able to see in the dark. Undoubtedly a cat’s eye is an exceptionally efficient organ in dim light, just as it is exceptionally sensitive to sunshine—it is provided with an iris which contracts the pupil almost to a pinhole—but the cat trusts to the bristles of its cheek for information regarding the things which block its path.
Most of the peculiarities which distinguish the reactions of the eye from those of other sense-organs can be explained by its mode of stimulation—the initiation of a nerve-current by a chemical change. No stimulus, if sufficiently strong, can be too brief. The retina reacts to an electric spark in the same way as a photographic plate; but, unlike the plate, the retina is restored to its previous condition of sensitiveness in about one-tenth of a second. A visual sensation lasts about one-tenth of a second. This prolongation of the sensation is, however, a mental, not a retinal, effect. The mind continues to see an object which has been illuminated by a flash until the retina is again in a condition to send brainwards a second impulse. Were our sensations coincident in duration with the stimulation of our sense-organs, we should live in a flickering cinematograph. When one is watching a moving point of light—the glowing end of a match, for example—the prolongation of sensation has its disadvantages; the moving point is interpreted as a streak of light. If the illumination be very brilliant, the object seen may give rise to a prolonged after-image. A glance at the sun leaves in the mind for seconds, or even for minutes, the image of a glowing disc. Sensations due to stimulation of the yellow spot last longer than those which originate in the peripheral retina. If, in a train, one is being carried at a certain pace, past a fence composed of upright palings, one sees the separate slats until the eyes are directed towards them, when they fuse into a continuous screen.
The phenomena of negative or complementary images are of retinal origin. The bright image of the sun, if the stimulus has not been too violent, gives place to a black disc. If one closes the eyes after staring at a window, a black surface crossed by bright lines is seen in place of a white surface with dark frames to the panes. If, after staring at a red surface, one looks at the ceiling, a green patch is seen; after yellow, blue. Every colour has its complement, which may be determined in this way. There is much uncertainty as to the exact terms in which this phenomenon is to be accounted for, but little doubt as to its being due to the peculiar mode of reaction of the retina to light. Chemical substances which have been used up have to be restored, and during the period in which they are coming back to what may be termed a neutral condition the retina delivers to the brain impulses of the opposite sign.
Contrasts which are experienced simultaneously are more difficult to understand than those which appear successively. In [Fig. 31] the half of the grey cross which is surrounded by black appears brighter than the half which lies on white paper. A grey cross on a red background looks green; on a green background, red; on yellow, blue; on blue, yellow. If green is on red, it looks greener than if it is on white or black. These simultaneous contrasts are seen best when the strength of the colours is reduced by covering them with tissue-paper. It is as if activity of any one part of the retina is accompanied by activity of the opposite sign in the remainder. But it is unsafe, in explaining our various sensations, to lay too much stress on the mode of stimulation. The mind judges sensations in the light of previous experience. In anatomical language, the effect of sensations upon the personality depends upon the paths which impulses follow in the brain, and the associations which have been established by previous impulses which have followed the same paths. The retina enables us to distinguish tone and colour. By the variations in tone, the juxtapositions of light and shade, we recognize form. All streams of impulses which do not present tone-variations—do not, that is to say, reproduce the details of a scene—are interpreted in terms of colour. Every child discovers that the tedium of the intervals during which it is proper that his eyes should be closed may be relieved by pressing his knuckles against the lids. Although the world is shut out, a phosphene offers itself for his consideration—a yellow or white disc of irregular form with a red margin, changing into lilac bordered with green, and then into yellowish-green with a blue edge. Such, if my recollection can be trusted, were the pictures which I used to see as a boy; but no adjustment of pressure calls them forth with anything like the same vividness now.
