In the first stage, one response, white, was made to light of whatever wave-length. In the second stage, this single response divided into two, one aroused by the long waves and the other by the short. The response to the long waves was the sensation of yellow, and that to the short waves the sensation of blue. In the third stage, the yellow response divided into one for the longest waves, corresponding to the red, and one for somewhat shorter waves, corresponding to the green. Now, when we try to get a blend of red and green {222} by combining red and green lights, we fail because the two responses simply unite and revert to the more primitive yellow response; and similarly when we try to get the yellow and blue responses together, they revert to the more primitive white response out of which they developed. But, since no one can pretend to _see_ yellow as a reddish green, nor white as a bluish yellow, it is clear that the just-spoken-of union of the red and green responses, and of the yellow and blue responses, must take place _below the level of conscious sensation_. These unions probably take place within the retina itself. Probably they are purely chemical unions. [Illustration: Fig. 37--The Ladd-Franklin theory of the evolution of the color sense. (Figure text: Stage 1--white, Stage 2--yellow blue, Stage 3--red green blue)] The _very first_ response of a rod or cone to light is probably a purely chemical reaction. Dr. Ladd-Franklin, carrying out her theory, supposes that a light-sensitive "mother substance" in the rods and cones is decomposed by the action of light, and gives off cleavage products which arouse the vital activity of the rods and cones, and thus start nerve currents coursing towards the brain. In the "first stage", she supposes, a _single_ big cleavage product, which we may call W, is split off by the action of {223} light upon the mother substance, and the vital response to W is the sensation of white. In the second stage, the mother substance is capable of giving off two smaller cleavage products, Y and B. Y is split off by the long waves of light, and B by the short waves, and the vital response to Y is the sensation of yellow, that to B the sensation of blue. But suppose that, chemically, Y + B = W: then, if Y and B are both split off at the same time in the same cone, they immediately unite into W, and the resulting sensation is white, and neither yellow nor blue. [Illustration: Fig. 38.--The cleavage products, in the three stages of the color sense. The "mother substance" is not represented in the diagram, but only the cleavage products which, according to the Ladd-Franklin theory, are the direct stimuli for the color sensations. (Figure text: 1--white, 2--yellow blue, 3--red green blue)]