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.
[Illustration: Fig I]
[Illustration: Fig II]
[Illustration: Fig III]
[Illustration: Fig IV]
[Illustration: 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.
[Illustration: 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.