Page:Popular Science Monthly Volume 9.djvu/715

 Rh 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 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 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