Popular Science Monthly/Volume 24/April 1884/Why the Eyes of Animals Shine in the Dark
By SWAN M. BURNETT, M. D.
THAT the eyes of some animals, particularly the cat, are luminous when they are in the dark, is a fact established from time immemorial. It is surprising, however, to find the exact nature of the phenomenon entirely misunderstood even by scientists whose lines of investigation lie in the particular field to which it belongs. In conversing, not long ago, for instance, with one of the first physicists of this country, who is at the same time an ardent sportsman, he gave me a graphic description of a "still hunt" for deer. This method of hunting, as is well known, consists in placing a bright light in the bow of a boat and propelling it noiselessly through the water. The deer is attracted by the light and goes toward it, but is prevented by its glare from seeing his enemies who are concealed in the shadow. The hunter, looking straight ahead, sees in the outer darkness—rendered Egyptian by contrast with the bright light immediately in front of his own eyes—two large, luminous bodies, like balls of fire. These are the eyes of his victim; and, making his calculation as to the distance from the eyes down to the breast, the valiant sportsman (who probably is also a strong anti-vivisectionist) fires, intending to send his bullet through the heart. The eminent physicist, in speaking of this luminosity, referred to it as due to the phosphorescence of the eyes, in that final way in which we are accustomed to speak of things beyond dispute.
But it is hardly less surprising to read in the article "Light," in the ninth edition of the "Encyclopædia Britannica," the following remarkable statement by Professor P. G. Tait, on the sources of light: "3. A third source [of light] is physiological; fire-flies, glow-worms, medusæ, dead fish (?)—the eye of a cat" (vol. xiv, p. 379).
If these are the opinions of acknowledged authorities in optics, we can hardly expect the mass of even ordinarily intelligent and informed persons to have more correct ones, and should expect thorough credence to be given to the story of the man who claimed that he was able to recognize an antagonist who struck him in the dark by means of the light emitted from his own eye as the result of the blow. The fact is, there is no phosphorescence in the eyes of animals—at least, so far as my knowledge extends, none has been demonstrated; and, that it is absent from the eyes of the cat, Professor Tait can demonstrate conclusively for himself, by taking a cat, be it ever so black (and these I believe are supposed to have the luminous power in the greatest degree), into a completely dark room where there can come no ray of extraneous light, and he will find that the eyes can not generate enough light to make even the darkness visible.
The real cause of the luminosity of the eyes of animals in the dark is now thoroughly understood by physiological opticists and by many practical oculists, and depends upon the well-demonstrated laws of the refraction and reflection of light. For a clear apprehension of the phenomenon, however, it is necessary to understand the properties of the eye as an optical instrument.
The office of the eye as an optical instrument, pure and simple, is to bring rays of light to a focus on the membrane at the back part known as the retina, in such a manner that small and inverted images of external objects shall be formed there. For this purpose there is a general plan, which is subject, f however, to more or less variation in different animals. The basis of this plan is the camera-obscura, in which the box is represented by the hollow globe or ball of the eye, the small aperture through which the light enters, by the pupil, and the lens by which the inverted and reduced images of external objects are formed, by the refracting surfaces of the eye, which are usually two—the cornea, or clear part of the front of the eye, and the crystalline lens.
Now, the eye, in its capacity of optical instrument, is obedient to the same laws as any other apparatus reflecting and refracting light. It may astonish some to be told that the eye reflects the light passing into it. It was for a long time believed that all light that entered the eye was in some manner consumed there, and that none ever found its way out again. It was considered one of the functions of the choroid or pigmented coat of the eye to absorb such light as was not used in the formation of the image. The basis of this opinion was that, under ordinary circumstances, no matter how bright the light may be in which the eye is looked at, the pupil always appears black. But no fact is more clearly demonstrated now than that the eye does throw back a large part of the light which enters its pupil.
One of the fundamental principles of optics is what is called the law of conjugate foci. This is readily understood by means of the accompanying diagram (Fig. 1). If the object is at a, the lens I will form an image of that object at c. The law of conjugate foci is that the image can exchange places with the object and the object with the image, and the result be still the same. That is to say, if the object were placed at c, its image would be formed at a. Or, expressing it in another way, the rays of light follow the same lines, whether going from the image to the object or from the object to the image.
Let us now apply this law to the case of the eye. We will suppose the eye to be in a normal optical condition; that is, that the retina on which the image is formed is to be found exactly at the focus of
the lenses by which the light is refracted. By consulting Fig. 2, we can follow the course of the rays of light in both directions. We have rays going from a in the flame a b, which after refraction by the lenses of the eye are brought to a focus at c, and form the lower end of the inverted image; whereas, these going from b are united again at d. But, since the bottom of the eye is a reflecting surface, and sends back a part, at least, of the light which falls on it, some of these rays pass out again, but, in accordance with the law of conjugate foci, they must follow the same lines as in entering; therefore, the rays from c will come back to a, and those from d will come
back to b. If we could place our eye at a b, then we would catch some of these rays, and the bottom of the eye would appear illuminated just as any other surface from which light was reflected. But our eye and the candle can not occupy the same place at the same time, and if we place it behind the candle, the flame itself cuts off the rays of light, and if we place it in front, our head obstructs the passage of the light to the eye to be observed. So, under these circumstances, it is impossible for an eye, at 0, for instance, to get any of the light that is constantly coming from the bottom of an eye which has been illuminated. But, if we were able by any contrivance to place our eye in the position of the source of light, we would be able to catch the rays coming from the bottom of the observed eye, and it would appear illuminated. Now, there is such a contrivance, and it is called the ophthalmoscope, and it owes its existence to the genius of Professor Helmholtz. The principle of its construction is so simple that the wonder is that no one ever thought of it before; but never, until the year 1851, had any one ever seen in anything like detail the interior of a living eye. If you take a piece of bright tin and punch a small hole in it, and, placing the hole directly in front of your own pupil, throw the light from a lamp into the eye of a child, the pupil, instead of appearing black as it usually does, will be of a beautiful yellowish-red color. This is because you have, to all intents and purposes, put your eye in the place of the source of light. For the light reflected from the surface of the tin is that which passes into the eye, and it must come back to it after reflection. The eye placed behind the hole catches the small quantity which would fall on that part, and therefore sees the surface from which it comes, illuminated. This is the principle of illumination of the bottom of the eye, and, when you have your object sufficiently well lighted, it is only a matter of optical appliance to see it distinctly and in great detail. This digression is designed to show that, when we have favoring circumstances, by the action of well-known optical laws, the eyes of animals appear illuminated, and that it is not necessary to call in the supposition of phosphorescence to account for the phenomenon.
But, in the case of some animals, the eyes appear to shine without the intervention of any optical means, however simple. This, however, is only apparent, for the principle of illumination is applicable here as in the other cases.
In the case we have supposed, the retina, which in this instance is the reflecting surface as well as the membrane on which the image is formed, was found at the focus of the refracting surfaces of the eye. But this condition is met with only in what is accepted as the perfect optical state of the eye. As can be readily understood, the retina may lie either in front or behind the focus of the refracting media—that is, the eye may be too long or too short for its focus, and unfortunately such conditions are but too common. When an eye is too long, it is said to be near-sighted or myopic; when too short, it is far-sighted or hypermetropic.
The change in the position of the retina, then, must exercise an influence on the direction of the rays that are reflected from it. From the well-demonstrated properties of lenses we know that, when rays of light coming from a point at the focus of a lens pass through it, they are rendered parallel; when they come from a point within the focus, they are spread out, or rendered divergent; and when from beyond the focus, they are rendered convergent, or brought toward another focus.
In accordance with these laws, therefore, we must expect the rays of light to take a different course in coming out of an eye according as it is near-or far-sighted. The course of the rays coming from the far-sighted or hypermetropic eye is shown in Fig. 3.
If the retina lay in the focus of the refracting surfaces of the eye at E, then the light from the inverted image c d of the flame would travel back, in the same direction in which it came, to the flame a b
itself. If, however, it meets the reflecting surface of the retina within the focus at H, then the rays from the confused image e i would come out in a divergent manner, and form a cone of light, F G, like that from the head-light of a locomotive.
It is now easy to see that if an observing eye is placed anywhere in the vicinity of the source of illumination, as at o, it will take in some of the rays coming from e i, and see it illuminated. There are very few human eyes so accurately adjusted as to their focus that all the rays come back to the source of light; some of them are scattered, and by a very simple arrangement it is possible to catch them in sufficient number to show the bottom of the eye illuminated.
Place a child (because the pupils of children are large), and by preference a blonde, at a distance of ten or fifteen feet from a lamp which is the only source of light in a room, and cause it to look at some object in the direction of the lamp, turning the eye you wish to look at slightly inward toward the nose. Now, put your own eye close behind the lamp-flame, with a card between it and the flame. If you will then look close by the edge of the flame covered by the card into the eye of the child, you will see, instead of a perfectly black pupil, a reddish-yellow circle. If the eye happens to be hypermetropic, you will be able to see the red reflex when your own eye is at some distance to one side of the flame. This is the true explanation of the luminous appearance of the eyes of some animals when they are in comparative obscurity. It is simply the light reflected from the bottom of their eyes, which is generally of a reddish tinge on account of the red blood in the vascular layer of the choroid back of the semitransparent retina, and not light that is generated there at all. This reflection is most apparent when the animal is in obscurity, but the observer must be in the light, and some-what in the relative position indicated in the above-described experiment—that is, the eye of the. observer must be on the same line with the light and the observed eye. The eyes of nearly all animals are hypermetropic, most of them very highly so, so that they send out the rays of light which have entered them in a very diverging manner.
The circumstances under which the phenomena of luminosity are usually seen are, it will be noted, those most favorable for the success of the experiment. The animal is always in an obscure corner, under a table or chair, as in the case of the cat, while the deer is in the outer darkness of the night. It is well known that the pupils dilate when in the dark, and they often attain an immense size in the eyes of those animals with nocturnal habits, and the size of the cone of light is governed by the size of the pupil, since its circumferential boundary is formed by it.
In making some experiments on dogs and cats, for the purpose of determining the size of this cone of light, I found that it had actually about twice the diameter it should have theoretically, from the amount of hypermetropia present, as determined by means of the ophthalmoscope. This I can account for only by the great dispersion of light at the periphery of the lens and cornea, rendered possible by the immense dilatation of the pupil; and this I think, too, is the reason why the phenomenon is not more frequently observed in the higher animals affected with hypermetropia. The pupil in man never attains the size, under the same circumstances, as that of the cat, for example; and, moreover, it is most likely that the surfaces of the cornea and lens are more regular in their curve, even at their more peripheral parts, and consequently disperse the light in a very much less degree.