The Ladd Franklin Hypothesis of Colour Vision.

As the incompatibilities of the Hering and Young hypotheses of colour vision became more obvious, there occurred to more than one experimenter the idea of devising some intermediate theory which would avoid the pitfalls which the older hypotheses appeared to have encountered. In this way the theories of McDougall, Schenck, and others came into being. Mrs. Ladd Franklin's hypothesis may be quoted as another example. She regards the fundamental and essential feature of the Young hypothesis to be the fact that all known shades of colour can be matched by mixtures of the three pure spectral colours-red, green and blue. As the fundamental fact of the Hering theory she regards the existence of " opposite " pairs of colours, red and green, yellow and blue, black and white. On this foundation she erects the following structure There was initially in the earliest stage of development of the organ of vision but one sensation-that aroused by white light and its negation. This underwent gradual elaboration, first into two sensations, one of short wave length, blue light, and the other into long wave length, yellow light. Later, a further elaboration of the yellow sensation occurred by its partial differentiation into sensations for green and red. So that we have present in the foveal cones of normal-sighted individuals, according to Mrs. Ladd Franklin, five sensations, those for white, blue, yellow, red and green. In the periphery of the normal-sighted retina, and in the fovea of certain colour-blind persons, the process of development has not been completed-that is, that in her opinion colour blindness is atavistic. Having thus briefly stated the theory, I propose to discuss three things: 1. Its ability to fit in with all the known facts of colour vision. 2. Its novelty; and 3. Its utility.