In the normal human eye, only the central parts of the retina are sensitive to colour, the peripheral parts are practically colour-blind. Anæmia of the retina, which may be produced by pressure on the eye-ball, will render the retina, first colour-blind and then insensitive to light. To me it appears that colours in relation to each other assume a grey tone, and the sensation of black and white disappears last.
The great difference which I have been able to observe between the human retina and the retina of the trout is, that while the human retina contains a layer of rods and cones, the retina of the trout only contains cones, or if it does contain rods, contains very few, as I have not found any as yet. There exists also at the back of the retina of the trout a “tapetum,” which extends over almost the whole of its posterior surface. This does not exist in the human eye, but is found in the eyes of some of the vertebrates. It consists of a layer of “guanin” crystals, and, presenting as it does a metallic appearance, and having great power of reflecting light, probably plays an important part in the visual power of the trout, particularly, I should think, in a dim light.
The fact that the rods are absent from the trout’s retina does not bear the important significance that one would imagine on first realising it. The fovea centralis of the human retina is the seat of most acute vision, and in the fovea centralis there are no rods. The cones in the retina of the trout are very closely arranged, so that they are practically in contact with each other, and their outer limbs are rather longer and finer than in the case of man. This layer of cones extends to the periphery of the retina, and the cones are just as closely arranged as far as they extend. These facts should lead us to believe that the vision of the trout is probably extremely acute, in fact, as we find in the retina of the trout, no material difference from the fovea centralis of the human retina, we have no reason to suppose that the visual powers of the whole of the retina of the trout, should differ in any way from the visual powers possessed by the fovea centralis, the seat of most acute vision both as to colour and light in the human retina. The retinæ of other fishes which I have examined (none of them were Salmonidæ) contained only cones; but these cones were some distance from each other.
The layer of pigment epithelium which is present in the human eye, is present also in that of the trout. It occupies the same position between the layer of rods and cones, or cones only, and the choroid. As in the human eye, it adheres sometimes to the choroid and sometimes to the retina, when the retina is removed, though perhaps it most often adheres to the retina.
My space is too limited to enter into any of the theories as to the possibility of the pigment cells playing a part in colour vision. It is quite sufficient to state that they undoubtedly do play some part in our sense of sight, and that they are contained in the eye of the trout.
The retina of a colour-blind person does not show any organic difference from the normal eye, so we cannot say to what cause colour-blindness is due; but so far as our knowledge goes, there is no reason to suppose that the trout is normally colour-blind.
As Michael Foster so ably put it, “No man can tell what are the sensations of his fellow-man,” still less I think can man say what are the sensations of a trout. All we can do with regard to this question of colour vision, is to find out all the facts we can relating to it, and working on comparisons, arrive, not at conclusions, but at probabilities.
The only thing of which I am sure is that we shall find it safe and comparatively easy to imitate flies in colours, but to make a monochromatic imitation of one, which would accurately represent it to a normally monochromatic eye (about which we know nothing), in a medium of which we know very little, is practically impossible.