Popular Science Monthly/Volume 9/October 1876/Observing the Interior of the Eye
THE retina is the point where the physical process of vision passes into the physiological process. Until it impinges upon the retina, the light which penetrates the eye has only undergone physical changes, consisting chiefly in refraction, the last perceptible result of which is the production of the image upon the retina. From this point the process passes from our immediate observation, and the difficulty of discovering its character increases at each step. The image upon the retina is reversed, and yet we see every object in the field of vision upright. This is the result of the experience, which we have acquired from childhood, in the exercise of the organ of sight. The point A (Fig. 1), which is on the right, is imprinted upon the left portion of the retina, and we, therefore, know by experience that a
ray, coming from the right, must strike the left portion of the retina; and because we always imagine the objects we see to be external to ourselves, we must do so by unconsciously following the line a A, through the optical centre k.. In this manner the eye projects a uniform field of vision, which is obtained by drawing, from every point of the retina outward, straight lines through the optical centre of the eye, which lines will terminate upon a convex surface.
This is really the manner in which the eye interprets, in all cases, its sensations of sight. For luminous appearances may be produced without our perceiving any external object, but merely a part of the eye or an inward irritation; and yet, in the same manner, we imagine them to be external to ourselves.
If we shut the eye, and press the head of a pin upon the outer edge of the eyeball, we shall see in the dark field of vision a white or colored spot of light, which has the same form as the compressing body. It will be seen upon the left side of the field if the right side is pressed, and upon the upper half if the lower is pressed, and vice versa. The retina, therefore, extends as far as the part which projects beyond the socket of the eye, and can be irritated by pressure. It is well known that when the eye is struck a cloud of sparks is seen, which is caused by the mechanical concussion of the retina. These luminous images, often perceived involuntarily, take, speaking scientifically, the form of the body producing the pressure; at the same time we observe the relation between the position of the irritation and the position of the sensation of sight. We transpose a point on the left side of the retina to the right, because we imagine that a ray of light has penetrated the eye from the right, which must fall upon the left half of the retina.
We are also able to perceive particles within the interior of the eye which are found in the transparent media. There are many persons who always see round particles or filaments, which seem to float about in the field of vision. They may be more distinctly seen when looking upon a bright surface—a cloudy sky, or through a microscope. They follow every motion of the eye, and have, moreover, a peculiar motion of their own. These particles are produced by filaments and cells, which may be found floating about in the narrow space between the hyaloid membrane and the retina. They cast their shadow directly upon the retina, which then, from experience, refers them to external objects.
It has also been discovered by more careful observation that the refracting media of the eye are not absolutely transparent, but that a kind of cloudiness is seen in places which throws a shadow upon the retina. If we look at the sky through a small hole in a sheet of paper, held a short distance from the eye, the hole will appear to be surrounded by a colored fringe. This is caused partly by a cloudiness in the vitreous humor, and partly by the peculiar radiating formation of the lens, already described. All such phenomena are called entoptic, because they deal with the perceptions of the internal portions of the eye. They are produced by the incident rays of light casting shadows of these particles upon the retina. They are best seen when an isolated pencil of light, like that admitted through a small aperture, is allow 7 ed to fall upon the eye; for, in that case, the shadows produced are distinct, while they are generally obliterated in ordinary vision, because the light penetrates the eye from all sides.
One of the most interesting entoptic phenomena is the Arborescent Figure, discovered by Purkinje. If, toward evening, we place ourselves opposite a dark wall in a dark room, and move a lighted candle to and fro before our eyes, looking, however, fixedly at the wall beyond, we shall then, after a little practice, see this arborescent figure, whose intersecting branches cover the whole of the dark space, and which is unmistakably caused by the blood-vessels in the interior of the eye. The field of vision assumes a reddish appearance, upon which the veins stand out in dark shadows. The trunk of the figure rises a little on one side of the centre, where the optic nerve enters the eye, and thence branches out after the manner of blood-vessels, which is undoubtedly a proof that in this experiment we see the blood-vessels of the retina itself. One spot alone is free from vessels: the yellow spot, which is the most sensitive to light of all parts of the retina. If, now, the candle is moved to and fro, the figure will also move and follow the direction of the light.
All these observations lead to the conclusion that we are thus enabled to perceive the shadows of the vessels of the retina. That these vessels cast a shadow behind them is clear, but that the shadow should be sufficient to cause a perception leads to the very important and interesting fact that the elements of the retina which receive the impression of light must lie behind the blood-vessels. The diagram in Fig. 2 will explain how the shadow of a vessel can produce an image. If the light is placed at a its image will be depicted upon
the retina at b. At this particular spot no vessels will be seen, because the light is too dazzling. But the image at b forms another source of light, and, if there is a vessel at v, then its shadow will be thrown upon c. Now, the retina projects the image perceived at c, outward, through the optical centre k, to d, where the vessel appears in the field of vision. If the light is now moved from a to a', then the image will move from b to b', the shadow from c to c', and the image of the vessel from d to d', thus performing the same movement as the light. We do not, however, generally perceive these retinal vessels, because usually the light falls upon the retina from all points of the pupil, and therefore no distinct shadow can he produced. In the experiment just described the light proceeds from a single point only, b, and produces a distinct shadow. Moreover, the light is an unusual one, and throws the shadows upon places which are not accustomed to receive it. This latter circumstance seems to be of some importance, for, if the light is held perfectly still, the figure gradually fades away, because the sensitiveness of the parts of the retina upon which the shadow is becomes blunted; it appears again, however, if the light is moved from side to side, so that the position o^ the shadow is changed.
A considerable amount of light penetrates the eye through the pupil, which is quite sufficient for the representation of the external world, but none of this light seems to be reflected. The pupil of the eye generally has a dark appearance, so that we cannot see farther into the eye than the iris. It is, however, possible to illuminate the eye in such a manner that all the parts of the retina may be seen. This was first done in a satisfactory manner by the celebrated physicist Helmholtz, the discoverer of the ophthalmoscope. Before describing this apparatus and its functions, we must discuss the fact of the dark appearance generally presented by the pupil.
The amount of light reflected by the background of the pupil cannot, of course, be very great; for the retina alone is able to reflect light, and as it is very transparent, and has, moreover, a dark layer of pigment immediately behind it, which absorbs all the light that has penetrated to it, the reflection must necessarily be weak. We know how difficult it is to see through a window into a room from the street. This is due to the small amount of light which comes through the window, in comparison to that which penetrates the eye from without, so that the eye is not sufficiently sensitive to perceive the weaker impression; moreover, the reflection from the panes of glass considerably increases the difficulty of perceiving objects in the interior of the room. If, however, the room is lighted up at night, we can see the interior very distinctly from the outside, although the illumination of the interior is weaker than it was in the daytime.
These circumstances also apply to the eye; but there is another circumstance which adds to the difficulty of examining the interior of the eye. The same fact makes it impossible to see the background of a camera-obscura through the lens, even when it is white. According to the laws of refraction, both the incident and emergent rays in the eye, or in a camera-obscura, have a fixed direction, while the light which proceeds from a room through the window is diffused—that is to say, emits rays in all directions. Let us suppose an image of a lighted candle to be thrown upon the retina; then, as far as the refracting media of the eye are concerned, this image may be regarded as a second object, the rays from which will take an outward, and therefore opposite direction. Now, this will be precisely the same as the path of the incident rays; for if, at the point where an image of an object has been formed by a lens, we place an exactly similar object instead of the image, then an image will be formed in the exact position of the first object, and of equal size. We see from this experiment, therefore, that the rays of light, which are emitted by an image formed upon the retina, must return to the object from which they originally proceeded.
If, therefore, a light is placed before any eye which we wish to examine, the rays will all be reflected by the eye into the light, and we are unable to intercept them by our own eye, because we should hide the light by placing ourselves between it and the eye under examination. By means, however, of a transparent plate of glass, this obstacle may be overcome, and the eye examined when illumined, in the manner represented in Fig. 3. C is the eye under observation, B
the observer's eye, and the plate of glass, S, forms an angle of 45° with the line between the two eyes. The rays emitted by the lighted candle, A, strike the glass plate, S, and are partly reflected into the eye, which they illuminate. The rays reflected by the eye, C, again strike the glass plate, which some of them penetrate, and pass into the eye of the observer, and the remainder return to the light, A. The pupil of the eye, C, may now be seen brightly illuminated, and even the illuminated retina can be seen more or less distinctly. The rays emitted by the image formed upon the retina, which pass through the glass plate, would form an image at a, which is at the same distance from the glass plate as A. The rays are, however, intercepted by the observer, B, who is thus enabled to examine a part of the retina.
In fact, a piece of window-glass placed in an oblique position, as described above, is the simplest form of an ophthalmoscope, and may easily be arranged by any one who wishes to make the experiment for himself. An ordinary piece of glass is sufficient for the purpose, if placed in the same position, relatively to the eye under observation and the light, as that shown in the figure. It is well to place a screen between the light and the person under observation, to prevent any annoyance arising from the intensity of the light. The observer must then place himself close in front of the person whose eye is under observation, hold the glass in the manner described, and move it about till the reflection of the light falls, upon the eye. The illuminated pupil will then be seen through the glass, and appear of a reddish color.
But, in order to see the separate parts of the retina distinctly, it is necessary to make use of lenses adjusted to the sight of the observer, and the refractive power of the eye under observation; and the result of such a combination is a perfect ophthalmoscope. The glass, again, has been replaced with advantage by a mirror, generally a concave mirror, with an aperture in the centre, through which the observer looks. Fig. 4 shows the method of using this apparatus, constructed after Ruete's plan. The light is placed near the person under observation, A. The rays emitted fall upon the concave mirror, d, which reflects them into the eye under observation. The observer
B, looks through the aperture in the concave mirror, and moves the two lenses, m and l, till they are in such a position that a distinct image of the retina appears.
We are now in a position, with the aid of the ophthalmoscope, to make a thorough examination of the retina. Fig. 5 gives a tolerable representation of all that we are able to distinguish of the image. The background of the whole is of a dull red, while the point where the optic nerve enters is distinguished as a round, bright spot, and we may see rising out of its midst the retinal vessels, arteries, a, and veins, b, which extend over the entire retina. The yellow spot also, the point of most distinct vision, may be distinguished as a small bright spot.
The ophthalmoscope has become an instrument of incalculable value to the oculist. Many changes in the retina and interior of the eye, which are due to disease, can be observed and examined by means of the ophthalmoscope; and, in fact, the medical treatment of the eye has made an immense advance since the discovery of this instrument.
The eyes of many animals—those of cats, for instance—exhibit a peculiar brilliancy, which is particularly remarkable in the dusk. It was formerly thought that the eyes of such animals emitted light independently, as it was also thought that light could be emitted by the human eye, under the influence of passion. This brilliancy, however, in the eyes of these animals is caused by a carpet of glittering fibres, called the tapetum, which lies behind the retina, and is a powerful reflector. In perfect darkness no light is observed in their eyes, a fact which has been established by very careful experiments; but, nevertheless, a very small amount of light is sufficient to produce the luminous appearance in them.
- From "The Five Senses of Man," No. XXI. of the "International Scientific Series."