OBSERVING THE INTERIOR OF THE EYE. 687 



points of the pupil, and therefore no distinct shadow can he pro- 

 duced. 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 

 o^ 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 physi- 

 cist Helmholtz, the discoverer of the ophthalmoscope. Before de- 

 scribing 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 veiy 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 with- 

 out, so that the eye is not sufficiently sensitive to perceive the weaker 

 impression ; moreover, the reflection from the panes of glass consid- 

 erably 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 illumina- 

 tion 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. Ac- 

 cording 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 out- 

 ward, and therefore opposite direction. Now, this will be precisely 



