factory 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 day-time.

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, whilst 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

Fig. 21.

B

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 illuminated, in the manner represented in

fig. 21. 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

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d 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 anyone 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 colour.

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

The glass,

combination is a perfect ophthalmoscope. 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. 22 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 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. 23 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, whilst 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.