The Salmon’s Visual Apparatus.
There has been no end of speculation on salmon flies, for every angler has his favourite patterns in which he professes to have implicit faith. After all, these personal predilections, however strong, do not carry us very far. They are merely individual experiences, certainly of interest, but not founded on any scientific principle. Before assuming that the salmon has a liking for a particular colour, it would be more scientific to settle, if possible, whether the salmon is sensitive to colour, to discover the range of his colour perception and the effects of the refraction of water upon objects presented to his eye. Such an enquiry involves the science of anatomy as well as the science of optics; but granted an investigator adequately equipped in both departments and endowed with a little constructive imagination, we see no reason why the problem of the salmon’s vision should not be solved. There is no doubt that the proper way to go about the enquiry is for the observer to examine the salmon’s optical apparatus in comparison with man’s, to project himself in imagination to the bed of the river and applying his knowledge of optics to the refracting effect of water, to try to construct a picture of any object as it would appear to the human eye under such circumstances. When this is done it proves a very illuminative method. The two following papers show a laudable attempt to apply such principles, and if they do not say absolutely the last word on the subject, they are uncommonly suggestive, and make a valuable contribution to the solving of the problem. It is often assumed that the salmon sees a fly merely as a dark silhouette against the sky. That is now shown to be a very rare occurrence. He would seem to be sensitive to colour, and under certain circumstances has a distinct sense of the gaudiness of the lures presented to his observation.
The investigation is not without its bearing on trout-fishing, for it brings home to the angler the conditions under which, in clear water, the trout may behold him and his rod from the bank; it explains, perhaps, why, in certain conditions of air and water, the fish miss the fly, and it throws indirect light on many other mysteries that trouble the angling mind.
Is the Salmon Colour-blind?
Has a fish’s eye any sense of colour? Does it see a worm, a fly, or a minnow in all the varied colours that these creatures present to us? Are these colours blurred by the medium in which the fish lives, or are they equally brilliant below water and above, just as a cathedral window shows its tinted panes no less gorgeous in the evening light than in the noonday brightness? Or is it that the fish is colour-blind, and sees only a monotone, a grey of varied depth. Is the picture one of mere shading and devoid of colour?
These are problems worthy of an answer, especially in regard to such a fish as the salmon, to which our subsequent remarks are directed. Some fishes, we know, are blind, and their eyes are rudimentary; but such fishes live in dark caves and have no need of eyesight. On the other hand, the salmon—a denizen of both salt and fresh water—requires great keenness of vision, not only for the obtaining of its food, but for its protection from the numerous dangers that surround it.
When we ask, “Is the salmon’s eye sensitive to colour, as the human eye is?” we have brought home to us the difficulty of the question. Human perception may, for all we know, be quite different from the perceptions of fishes. The anatomist will say that we associate certain retinal structures in the human eye with the perception of colour; and if the same structures are found in the eye of a fish the conclusion is that the fish is anatomically capable of perception. This, however, although carrying great weight, is by no means conclusive, and must be supplemented from other quarters.
There are various theories of colour perception. Thomas Young developed the theory that three primary sensations of colour, red, green, and violet, can be excited in the eye by light, and that the colour of an object depends on the proportion in which each of these sensations is excited. This may or may not be true, but the experiments of Clerk-Maxwell prove that almost any colour can be matched by a combination of three colours in varying proportions. That is, so far as our perception goes, the colours are matched. Our perception, however, is somewhat imperfect. We can match the yellow of the spectrum, with red and green in combination; but while the pure tone of the spectrum cannot be broken up by the prism, the matching colour of our own creation can be broken up into its elements red and green. Our own colour perception, therefore, though quite adequate to our needs, is by no means perfect. It follows that perfect colour perception is not necessary in a fish. If the fish can discriminate between three separate wave lengths, or even two separate wave lengths, it will have a certain graduated perception, equivalent to what we recognise as colour sense. It would not see a monochrome, but a scale of colour, which, although possibly very incomplete, is still a gradation of colouring. If it has the same retinal structures that are present in the human eye, it has probably the same scale of colour sensation as man. But even if its retinal structures be inferior to those in man, the fish may still be able to discriminate the difference in wave length between, say, the blue and the red, and will have a scale of colour incomplete perhaps, but still infinitely superior to colour-blindness, in the fish significance of that term. Many human beings are partially, few totally, colour-blind. Although nothing of a conclusive kind can be proved by the microscopic anatomist, yet with the help of many circumstances that may be brought forward, a strong case can be made out for colour perception in the salmon. We propose to examine the salmon’s eye, to enquire how far it is adapted to the medium in which the fish lives, what effect that medium has on the transmitted light, and what conclusions may be legitimately drawn therefrom.
The first thing that strikes an observer in looking at the head of a salmon, more especially from above, is, that its outline is a parabolic curve, in which the eyes are placed pretty well back from the snout, but so placed that they can see objects in front, on each side, and backwards till the elliptical form of the body cuts off the view. The eyes can also look downward, but the upward view is cut off to some extent by the eyebrow or bone cavity holding the eye. The eye, although flush with the head, can be rolled to some extent in its socket, as any one who has watched a fish in an aquarium can testify. The salmon, then, has an all-round vision, as well as a downward vision, but directly overhead the range of vision is restricted. As to structure, there is no cornea proper, a clear more or less flat membrane taking its place. The pupil is large in proportion to the crystalline lens; in other words, the eye is so constructed as to admit the maximum of light. The crystalline lens is the means whereby the rays of light are brought to a focus on the retina. In the human eye this lens is bi-convex. The power of a lens is increased by deepening its convexity or by adding to the density of the materials. Refraction or bending of light varies with the density of the medium through which the light passes. Refraction, as a general rule, is proportionate to density, and the amount of refraction depends on the difference in density between the two media. The salmon lives in a medium of great density, and therefore of high refractive index. Hence, to focus the rays of light on the retina the crystalline lens in the salmon’s eye has a very deep convexity, is, indeed, almost a sphere. A spherical lens gives a sharp image, quite as sharp as a bi-convex lens. The salmon’s eye is, therefore, admirably suited to the element that surrounds it. If the fish be taken from the water it is immediately afflicted with short sight. It has been transferred from a dense to a rare medium and the refraction is disturbed, so that the rays come to a focus almost on the posterior side of the lens and form the image there instead of on the retina. To sum up, a salmon can, without moving, see in every direction except behind and directly overhead. It can see clearly with a small amount of light, and its eye is so constructed as to ensure a clear image on the retina, while the fish is in the water. The medium in which the salmon lives is of considerable density, and has a high refractive index. If pure and in small quantity the water is perfectly clear: in large mass it is blue. In a river it is more or less contaminated with mud, peat, or other matter. When the water is very muddy the fish is lost in a fog, and has only touch and smell to guide it, but when the water clears somewhat, although much light is still cut off, the fish will see within a limited area. Of course the colour of all objects will be affected by the hue of the water, white becoming brown, yellow becoming orange, and so on. The experiment described by an American writer of looking at an artificial fly in a tank through a bit of plate glass in its end, gave a very good idea of what the fly looked like through the light-absorbing water; but it did not take into account that the fish can see with less light than we can. Moreover, it altogether failed to realise the position of the fish, whose eye is immersed in the water. The observer, therefore, did not see the object under the same conditions as the fish.
Fig I
Fig II
Fig III
Fig IV
Fig V
We cannot admit that the salmon sees the fly against the bright light of the sky. This does happen at times, but such moments are the exception, not the rule. If the salmon saw the fly against the background of bright sky, the fly would undoubtedly appear black, a dark silhouette on a white ground. In that case it might well be argued that as the fish sees no colour, colour perception, being useless to the fish, is not one of its possessions. But the refraction of the light, owing to the density of the water, entirely alters the case. When a ray of light enters a body of greater density it becomes bent, and the bending always takes place in the dense body towards a line drawn perpendicular to the surface at the point of contact. This bending of the light follows a fixed law. In whatever direction the light strikes the body the sine of the angle of incidence is to the sine of the angle of refraction in a constant ratio. In the refraction from air to water the ratio is very nearly four to three. To explain more fully (see fig. I), a ray of light AC strikes the water at C and is refracted to B. With the centre C describe a circle, cutting the ray of light at A and B, and from these points draw lines perpendicular to the surface of the water AF and BG. The distance CF is to the distance CG as 4 to 3. If CF be 4 ft. then CG is 3 ft. Again, suppose the ray of light comes from a point say, near the surface (see fig. 2), then CF will be almost equal to the radius of the circle. But if CF is 4 ft. then CG is 3 ft., therefore the point G must be a foot from D, however small the angle the ray of light makes with the surface of the water at C. The converse holds true. A ray of light from B will be refracted to A; but if the ray comes from H it will not be able to get out at C, and will be reflected to K. One can always see into a dense body, but it is not always possible to see out. The angle at which one ceases to see out of a dense medium is called the critical angle. Therefore, if a fish in the water looks towards the surface so that its line of vision makes an angle with the surface somewhat less than 45°, say 42°, the fish cannot see out (see fig. 3). Now, if we take into account that when light strikes water at a very small angle with the surface a large part is reflected and, comparatively speaking, very little refracted to the fish’s eye, a fairly reasonable angle for a fish to see out of the water at is 45° or more. What follows? If our salmon is at a depth of 4 ft., then if right above its eye we describe a circle (see fig. 4) 4 ft. in radius, within this circle lie the only points in the whole river from which it is possible for the fish to see the sky. Where does the fish see the banks of the river? The line in which light enters the eye is that in which the object is seen. The banks B will be seen as if at B. All the landscape and sky will be seen within the circle, that is, within the cone whose apex is the fish’s eye, and base the circle on the surface. It may not be out of place to point out that a fish at some distance from the bank in a quiet pool may be seen distinctly by an observer, while the fish may not be able to discern him. The man on the bank sees the fish lit by all the light of the sky above and reflected to his eye. The fish, on the other hand, sees the observer only by the light reflected by his body, much of which light never reaches the fish’s eye at all, being reflected at the surface of the water. Referring to fig. 3, if the fish look at a point X outside the magic circle, it will not see out of the water. The surface acts as a reflector and all the fish can see in this direction is a picture of the bed of the river at Y. We are not considering the point whether a fish can see an object in the water at X, but whether the fish can see the sky there; all the sky that the fish can see is within the circle already mentioned. We have remarked that the fish cannot, while in its normal position, see directly above its head, therefore even in part of this circle the sky will be invisible. Suppose an object is only 3 ft. above the fish, then the area in which the fish can see it against the sky will be a circle only 6 ft. in diameter. It is only when anything drifts within the cone already described that the fish can see it against the sky. This may be illustrated in the following striking manner. Take a glass globe (see fig. 5) and fill it exactly half full of water, paint the part filled by the water any dark colour, leaving a small clear space at AB and Y. Now, if a ray XC falls on the centre C it will be refracted to Y, and then come out into the air without further refraction. If we look through Y towards C we are in the position of the fish in the water. This is true for a small pencil of light. If, however, we look towards C through the opening A, we see out of the whole surface, and can see an object above the surface. If we look from B towards C, our line of vision makes an angle of less than 45° with the surface and we cannot see out of the surface, for it then becomes a mirror and reflects the painted side of the vessel and nothing more. Drop a fly at C, and from B you will not see the fly till it actually enters the water, and you will see only that part of it which is immersed in the water, together with its reflection. The photograph (fig. 6) shows what happens.
Fig. VI.
We think we have made it perfectly clear that it is only in exceptional circumstances that the fish can see its prey against the brightness of the sky. The normal case is only where the background is the bed of the river, the reflection of that bed or a blackness, depending more or less on the mass of water between the fish and the object. The object itself is invariably lighted by rays from the surface, which rays are reflected to the eye of the fish. The amount of light reaching the fish depends on the depth of the object under the surface and on its distance from the fish as well as on the clearness of the water. The fish has, it is true, an eye suitable for a weak light, but if the fish be colour-blind, and the object be of the same tone and relative lighting as the background, how can the fish perceive the object? A pike will rush twenty feet at a fly in a piece of water only three feet deep; he cannot have seen the fly against the sky. That could happen only if the pike were at the bottom and the object not more than five feet from him. Whereas, in the case supposed the fish is near the surface and the fly is seen by reflected light. The form is probably not seen distinctly but the “colour” is certainly attractive. It is conceivable that various states of light, and various states of water favour a particular colour and make it more alluring—at one time black, at another blue, or yellow, or red, or even white. We do not assert that a fish can see colour as we see it, but we hold that a fish can hardly be colour-blind when we consider the conditions under which it lives and moves. For the salmon to see its prey against the brightness of the sky is, as we have shown, exceptional, so that the argument which insists on the uselessness of colour to a creature which sees only dark silhouettes falls to the ground. We conclude that the salmon has a colour perception; whether the scale is like that of human beings we cannot say, but it is enough that the fish recognises one colour as different from another. Further than this we cannot go as yet, but to that conclusion we think we are certainly entitled.
J. Allan Stewart, M.A.
In the case of any of the lower animals an enquiry regarding its power of appreciating different colours is usually conducted by making experiments calculated to show whether or not the animal in question behaves as if it had this power. Whether it actually sees colours as we do, or is even conscious of seeing them at all, is, necessarily, beside the question.
So far as this method of investigation carries us in the case of the salmon, there would seem, from what is mentioned elsewhere, to be trustworthy evidence that it is influenced by differences in the colour of the artificial fly. But there remains a considerable difference of opinion, I understand, as to the value of these practical observations.
That the different colours and light intensities of an object, situated within a definite area of the water surface, are theoretically visible by the fish’s eye, has been clearly shown by Mr. J. Allan Stewart, who has thus answered those who argued that any body on the surface of the water would only be seen as a dark object against the bright sky-ground.
The fact of an animal choosing one part of the spectrum in preference to another can only be accepted as evidence that it recognises some difference. Now, it is only from the facts known regarding human vision that we can draw comparative conclusions. Judging from these facts we find that certain parts of the spectrum affect us differently from other parts, not only by virtue of their colour, but also of their brightness. We find, indeed, that we have two distinct visual sensations dependent on light, colour and brightness, both influenced by differences in wave-rapidities or wave-lengths. Thus, if we gradually reduce the intensity of the spectrum, the colours all finally cease to be recognisable. In this colourless spectrum the brightest part is slightly to the violet side of the line E (corresponding to green), so that it is quite possible that this part of the spectrum would appear distinct from the rest, even in the case of an animal destitute of all colour perception. (In proof of this it may be mentioned that in a case of total congenital colour-blindness in man, this part of the spectrum is found to be recognised as different from the rest by its brightness alone.)
From arguments which I have adduced elsewhere, there is good evidence in favour of the view that light perception (brightness) is the more primitive of these two sensations, and that it is dependent upon changes induced by light in the retinal pigment epithelium. Also, it is practically proved, so far as the human retina is concerned, that this light impression is communicated to the rod-cells of our retinal neuro-epithelium and so transmitted by conducting channels in retina, nerve and ganglion, to the sight centre in the brain. In the case, therefore, of an animal with a low form of retina, in which the above-named cells are alone represented, any differences in its behaviour on exposure to different parts of the spectrum may be presumed to be explained by the amount of brightness or light effect thereby induced.
But, on the other hand, we now believe that the finer gradations of differences in stimulation due to variations in wave-length which we recognise as colours are dependent on the presence of another kind of retinal element called cone-cells. It is sufficient here merely to say that these cone-cells seem to derive the initial stimulus through some effect produced on the pigment epithelium, and that they also transmit the impulse by conducting paths to the brain in a similar fashion to that above-mentioned.
Accordingly, should the retina of an animal contain, not only pigment and rod-cells, but also cone-cells, we must admit that, so far as we know, it is anatomically and physiologically capable of being influenced by colour in a similar way to the human retina. Now, the retina of the salmon does undoubtedly possess all these anatomical elements—pigment epithelium, fine rod-cells, well-marked cones.
The whole argument for the existence of colour perception in the salmon is, therefore, undoubtedly a strong one.
(1) It behaves as if it sees differences in colour.
(2) It is possible for its eye to get a proper view of an object within a certain area on the surface of the water.
(3) Its retina is of a sufficiently highly developed type to admit of its being differently influenced by different parts of the colour spectrum.
R. Marcus Gunn, F.R.C.S.
The writer of this paper, Mr. Marcus Gunn, is an acknowledged expert as an oculist. His opinion, therefore, on the subject of the salmon’s visual apparatus is of no ordinary value and should be duly pondered. Perhaps we ought to inform our readers that the eyes of the two salmon which he examined with a view to this paper were extracted from two large fresh run fish caught in autumn by the undersigned—one on the Ythan and the other on the Dee. The eyes were extracted immediately after capture and placed, in one case in chloral, in the other in formalin. Mr. Gunn was thus enabled to make sections of both, with the results embodied in his authoritative paper.
W. Murdoch.