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