CHAPTER III.

Recognition of the Interior Portions of the Eye-The Ophthalmoscope.

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. 10), 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 coloured 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 coloured fringe. This is caused partly by a cloudiness in the vitreous humour, 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 allowed to fall upon the eye; for, in that case, the shadows produced are distinct, whilst 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, towards 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 unmistakeably caused by the bloodvessels 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. 20 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, outwards, through the optical

G

a

d

centre k, to d, where the vessel appears in the field of vision. If the light is now moved from a to d, 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 be 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

a

Fig. 20.

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 of 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 further 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 satis