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.
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Fig II