But these observations are still insufficient to reduce the great number of simple, qualities contained in each of the classes mentioned, to a limited number of primary qualities and their mixtures. Finally, it has been observed that many odors neutralize each other, so far as the sensation is concerned, when they are mixed in the proper intensities. This is true not only of substances that neutralize each other chemically, as acetic acid and ammonia, but also of others, such as caoutchoue and wax or tolu-balsam, which do not act on each other chemically outside of the olfactory cells. Since this neutralization takes place when the two stimuli act on entirely differerent olfactory surfaces, one on the right and the other on the left mucous membrane of the nose, it is probable that we are dealing, not with phenomena analogous to those exhibited by complementary colors (22), but with a reciprocal central inhibition of sensations. Another observed fact tells against the notion that they are complementary. One and the same olfactory quality can neutralize several entirely different qualities, sometimes even those which in turn neutralize one another, while among colors it is always only two fixed qualities that are complementary.
13 . Sensations of taste have been somewhat more thoroughly investigated, and we can here distinguish four, distinct primary qualities . Between these there, are all possible transitional tastes, which are to be regarded is mixed sensations. The primary qualities are sour, sweet, bitter, and saline . Besides these, alkaline and metallic are sometimes regarded as independent qualities. But alkaline qualities show an unmistakeable relationship with saline, and metallic with sour, so that both are probably mixed sensations, (alkaline made up perhaps of saline and sweet, metallic of sour and saline). Sweet and saline are opposite qualities. When these two sensations are united in proper intensifies, the result is a mixed sensation (commonly known as "insipid"), even though the stimuli that here reciprocally neutalize each other do not enter into a chemical combination. The system of taste-sensations is, accordingly, in all probability to be regarded as a two-dimensional continuity, which may be geometrically represented by a circular surface on whose circumference, the four primary, and their intermediate, qualities are arranged, while the neutral mixed sensation is in the middle, and the other transitional taste-qualities on the surface, between this middle point and the saturated qualities on the circumference.
13a. In these attributes of taste-qualities we seem to have the fundamental type of a chemical sense. In this respect taste is perhaps the antecedent of sight. The obvious interconnection with the chemical nature of the stimulation, makes it probable even here that the reciprocal neutralization of certain sensations, with which the two-dimensional character of the sensational system is perhaps connected, depends, not on the sensations in themselves, but on the relations between the physiological stimulations, just as in the case of sensations of hot and cold (p. 48). It is well known that very commonly the chemical effect of certain substances can be neutralized through the action of certain other substances. Now, we do not know what the chemical changes are that are produced by the gustatory stimuli in the taste-cells. But from the neutralization of sensations of sweet and saline we way conclude, in accordance with the principle of the parallelism of changes in sensation and in stimuli (p. 45), that the chemical reactions which sweet and saline substances produce in the sensory cells, also counteract each other. The same would hold for their sensations for which similar relations could be demonstrated. In regard to the physiological conditions for gustatory stimulations, we can draw only this one conclusion from the facts mentioned, namely, that the chemical processes of stimulation corresponding to the sensations which neutralize each other in this way, probably take place in the same cells. Of course, the possibility is not excluded that several different processes liable to neutralization through opposite reactions, could arise in the same cells. The known anatomical facts and the experiments of physiology in stimulating single papillae separately, give lie certain conclusions in this matter. Whether we are here dealing with phenomena that are really analogous to those exhibited by complementary colors (v. inf. 22) is still a question. D. SENSATIONS OF LIGHT.
14. The system of light-sensations is made up of two partial systems: that of sensation of achromatic light and that of sensations of chromatic light. Between the qualities in these two, all possible transitional forms exist.