all, the decomposition of the visual purple and visual yellow, just like that of the white substance, sets energy free (of course, to a less extent in the case of the visual yellow than in that of the more highly complex visual purple). This brings about a neural stimulation, and we become in the end conscious of its results, just as in the previous instance, as sensations of brightness. The sensation occasioned by the continuous decomposition of the white substance is simply intensified by this, since the nerve possesses no capacity of discriminating from what source the energy exciting it proceeds. But, at the same time, the nervous excitation in this case has a peculiar secondary character, the nature of which is unknown, but which we term provisionally the rhythmatization of the stimulation or excitation. The brightness-sensations occasioned by the decomposition processes receive in consequence of it an especial toning; if it is the visual purple which is decomposed, a toning towards yellow; if the visual yellow, a toning towards blue. The two excitation-rhythms are not compatible with one another; they contain antagonistic moments, and mutually interfere with each other. If, therefore, visual purple and visual yellow are simultaneously decomposed (if, i. e., we see a mixture of yellow and blue light), the one colored toning weakens the other. Blue and yellow are antagonistic colors. If the two excitations are present together in a certain definite quantitative relation, the chromatic character of the sensation disappears completely. The energy set free in the decomposition is, however, not affected by this; so that the brightness-sensations contained in the yellow and blue persist undisturbed. That is, in a suitable mixture of the two antagonistic colors we see simply the sum of their brightnesses (proceeding partly from the decomposition of the white substance) as white or grey.—A third substance (red-green substance) is present in man only in the external members of the cones. It has, therefore, the least range of distribution, but it is somewhat more readily decomposable than is the visual purple. It is naturally green in color, and may possibly exist in isolation in the green rods of the frog's retina. Since its color is almost complementary to that of the visual purple, the two substances neutralize each other's tint, where they occur together, and the external members of the cones appear for that reason colorless. The physical properties of this red-green substance, and its significance for vision, must be conceived of quite analogously to the properties and significance of the visual purple. Under the influence of suitable light-rays (rays of longest and shortest wave-length),
