of the complementary color of the small square with the color of the ground; and, by recollecting this, we can easily retain this class of facts in the memory.
There is another similar experiment, which is simpler than those just described, but which nevertheless is instructive: A small square of black paper is to be placed on a sheet of red paper, and the attention in this case is to be directed to a mark on the edge of the former (see Fig. 6). When the black square is suddenly removed, the observer sees in place of it a more luminous spot, which in the case before
| Fig. 6.—Red Ground with Black Paper. | Fig. 7.—Red Ground with Intense Red Image. |
us will, of course, be red; but what is remarkable is the circumstance that this red image will be more intense or saturated in color than the rest of the ground. The rest of the sheet of red paper will look as though gray had been mixed with its color (Fig. 7). This experiment will, of course, succeed with paper of any bright color, and Helmholtz has found that the same effect can be obtained with the pure colors of the prismatic spectrum. The explanation, according to our theory, runs about thus: While we are in the act of looking at the edge of the black square, red light is passing into the eye, and is fatiguing all those portions of the retina that are not protected by the presence of the black square; it thus happens that the ability of the larger portion of the retina to receive the sensation of red is considerably diminished; the ability of the protected portion, of course, suffers meanwhile no such change. When the black square is suddenly removed, the unfatigued portion of the retina receives a powerful impulse from the red surface, but the effect produced upon the rest of the retina is inferior in degree. This accounts for the fact that the image of the square is brighter or more luminous, and we can easily understand why it is at the same time more intense or saturated in color, if we remember, as explained in Chapter IX., that red light excites into action not only the red nerves, but to a lesser extent the green and violet nerves. Now, as the red nerves begin to be fatigued, the action of the other two sets will be relatively more powerful than at first, so that gradually the sensations of green and violet begin to add themselves to that of red, or, what is the same thing, the sensation
