“In that part of the spectrum which we call yellow, the rays effect great dissimilative changes of the yellow-blue substance; but their action on the red-green substance does not lead to an excess of either dissimilation or assimilation, this substance being neutral to them; hence our sensation of yellow. The green rays, again, promote assimilation of the red-green substance, leaving the assimilation of the yellow-blue substance equal to its dissimilation; and similarly blue rays cause assimilation of the yellow-blue substance, and leave the red-green substance neutral. Finally, at the extreme blue end of the spectrum, the rays once more provoke dissimilation of the red-green substance, and by adding red to blue give violet. When orange rays fall on the retina, there is an excess of dissimilation of both the red-green and the yellow-blue substance; when greenish-blue rays are perceived, there is an excess of assimilation of both these substances; and other intermediate hues correspond to various degrees of dissimilation or assimilation of the several visual substances. When all the rays together fall upon the retina, the red-green and yellow-blue substances remain in equilibrium, but the white-black substance undergoes great changes of dissimilation, and we say the light is white.” It has been said by the same writer that this theory is tri-chromic. For my own part I do not see that it is so in the sense in which that word is used in the theory of Young. It may be a tetra-chromic, for as far as _colour_ is concerned the black-white sensation must be excluded; but it appears to me that it cannot be strictly brought under the head of tri-chromic. CHAPTER V. The readiest means of investigating the stimulation of the different sensations necessary to produce colour is evidently by eyes in which one or two sensations are absent, and this applies not only to the Young theory, but also to that of Hering.