When we look into the eye the pupil appears quite black, no matter in what position we place the light. The reason of this is that the retina can only be made visible by the light reflected outward from it, and that the portion of the rays which is reflected by the retina is so refracted in passing out of the eye that it occupies exactly the same path as that traveled by the light on its way from the point of illumination to the eye. Consequently, unless the eye of the observer be placed directly in this path, none of these reflected rays can reach it to enable him to see the fundus.
That is to say, the lens and other refractive media that bend the rays of the ingoing cone of light to a focus on the retina also bend those of the outgoing cone reflected from the retina to a focus at the point of illumination.
The fact that the blackness of the interior of the eye is caused by the lens, etc., can be shown by a simple experiment.
Fig. 229. Diagram showing the effect of a lens on the rays of light reflected from the paper (retina) in the experiment given in the text. E. Observer's eye. C. Point of illumination. On the left the reflected rays diverge, and some pass to E. On the right they are refracted by the lens to form a cone.
Blacken the inside of a pill box (about an inch deep), paste printed paper on the bottom, and cut a round hole half an inch in diameter in the lid. By illuminating the interior of the box obliquely the print can be easily recognized. If a convex lens of one inch focus be placed behind the opening, the paper cannot even be seen, and the opening looks black like the pupil with any position of the light. The paths traversed by the rays in this experiment may be seen in Fig. 229.
* " How to Use the Ophthalmoscope," by Edgar A. Brown, p 32.
In the left-hand figure of the above wood-cut the first case is illustrated. Here the divergent rays passing from the candle C to the surface P P' are reflected in various directions from it; those which strike the blackened interior of the box are absorbed; others emerge through the hole in the lid, and reaching the eye placed at E, enable the print at P to be seen.
The second case is shown in the right-hand figure. Here, instead of diverging till they reach the bottom of the box, the rays are refracted by the lens to a focus at the point P', from which they pass back through the lens, and are thereby bent to a cone converging to the source of light. No rays pass in the direction E, so the interior of the box looks quite black.
In attempting, then, to view the fundus, the observer must either place his head in the line of light, or the light in the line between the observed eye and his own; in short, his eye must lie in the line of reflection, in order to see the fundus. If we could see through the source of light, the above object would be accomplished. Helmholtz, by reflecting light into the eye by means of transparent glass plates, originally succeeded in seeing through the plates some of the rays reflected from the fundus. In this method, however, the power which enables the glass plates to reflect the luminous rays toward the eye also robs the observer of much of the light sent back from the retina by reflecting it toward the source of light, and the remaining rays which penetrate the glass cannot give a clear image of the retina.
A simple instrument, the ophthalmoscope, is now in general use for examining the retina. This consists of a concave mirror of short focal distance, which is substituted for the transparent reflecting plates. The rays converging from the mirror to the eye are brought to a focus on the retina, and thence some are reflected outward, and converged by the dioptric media to the hole in the centre of the mirror, behind which the observer's eye is placed to receive the cone of converging rays.
If the observer place his eye and the mirror at a distance of about 3 cm. from the observed eye, and the refraction of both eyes be normal, he can see an enlarged virtual image of the fundus. If the refraction of either eye be abnormal, it must be corrected by a suitable lens placed behind the aperture in the mirror. This is called the direct method of examination.
To overcome the inconvenience and difficulty of this mode of examination the indirect method is usually employed. In it a convex lens of 20 or 40 diopters is used in addition, enabling the observation to be made at a more convenient distance. When the eye has been illuminated, the lens is placed at its proper focal distance (2 or 1 inches respectively) in front of the eye. By the converging power of the lens a real inverted image of the fundus is formed in the air a couple of inches to the observer's side of the lens, and can be seen by him through the aperture in the mirror, if he hold his head at a distance to suit his refraction.
Fig. 230. Ophthalmoscopic view of fundus of eye, in which the central artery (g and c) and the corresponding veins (k and d) are seen coursing through the retina from the optic disc (A).
With this instrument a round whitish part is seen a little to the nasal side of the axis of the eye, where the nerve pierces the dark choroid coat. This is called the optic disc. The fundus now, when lighted up, does not look black, but is of a lurid red color, owing to the great vascularity of the choroid coat. Over this red field are seen a number of blood vessels, which start from the centre of the optic disc, and radiating over the fundus send branches to the most anterior parts that can be seen. These are the branches of the vessel which runs in the centre of the nerve. In the very axis of the eye a peculiar depression, free from branches of the blood vessels, can be seen. This central depression (fovea centralis) differs a little in color from the neighboring parts during life, and turns yellow at death, and hence has been called the "yellow spot." The retina is so transparent that we cannot see it with the ophthalmoscope, but the radiating vessels (central arteries and veins of the retina) lie in it and belong to the nervous structure only.
The ophthalmoscope has proved of inestimable value not only to the ophthalmologist, but also to the physician, as a means of arriving at an accurate knowledge of disease. Hence, it has became more a pathological than a physiological instrument.