PSYCHOLOGY 51 touch and smell : thus the pungency of pepper and the dryness of wine are tactual sensations, and their spicy flavours are really smells. How largely smells mingle with what we ordinarily take to be simply tastes is best brought home to us by a severe cold in the head, as this temporarily prevents the access of exhalations to the olfac- tory surfaces. The difference between the smooth feel of a polished surface and the roughness of one that is unpolished, though to direct introspection an irresolvable difference of quality, is probably due to the fact that several nerve-terminations are excited in each case : where the sensation is one of smoothness all are stimulated equally ; where it is one of roughness the ridges compress the nerve-ends more, and the hollows compress them less, than the level parts do. The most striking instance in point, however, is furnished by musical timbre (see EAR, vol. vii. p. 593). We find other evidence of the complexity of our existing sensations in the variations in quality that accompany variations in intensity, extensity, and duration. With the exception of spectral red all colours give place, sooner or later, to a mere colourless grey as the intensity of the light diminishes, and all in like manner become indis- tinguishably white after a certain increase of intensity. A longer time is also in most cases necessary to produce a sensation of colour than to produce a sensation merely of light or brightness : the solar spectrum seen for a moment appears not of seven colours but of two only faintly red towards the left side and blue towards the right. Very small objects, again, such as coloured specks on a white ground, though still distinctly seen, appear as colour- less if of less than a certain size, the relation between their intensity and extensity being such that within certain limits the brighter they are the smaller they may be with- out losing colour, and the larger they are the fainter in like manner. Similar facts are observable in the case of other senses, so that generally we seem justified in regard- ing what we now distinguish as a sensation as probably com- plicated in several respects. In other words, if psychical magnification were possible, we might be directly aware that sensations which we now suppose to be both single and simple were both compound and complex that they consisted, that is, of two or more sensational elements or changes, alike or different in quality, of uniform or variable intensity, and occurring either simultaneously or in regular or irregular succession. It is interesting to note that all possible sensations of colour, of tone, and of temperature constitute as many groups of qualitative continua. By continuum is here meant a series of presentations changing gradually in quality, i.e., so that any two differ less the more they approximate in the series. We may represent this rela- tion among presentations spatially, so long as the differences do not exceed three. In this way our normal colour-sensa- tions have been compared to a sphere, in which (a) the maximum of luminosity is at one pole and the minimum at the other ; (6) the series of colours proper (red to violet and through purple back to red), constituting a closed line, are placed round the equator or in zones parallel to it, according to shade ; and (c) the amount of saturation (or absence of white) for any given zone of illumination in- creases with distance from the axis. The several musical tones, again, have been compared to an ascending spiral, a given tone and its octaves lying in the same perpen- dicular. Temperatures similarly might be represented as ranging in opposite directions, i.e., through heat or through cold, between a zero of no sensation and the organic sensa- tions that accompany the destructive action of heat and cold alike. As we frequently experience a continuous range of intensity of varying amount, so we may experience continuous variations in quality, as in looking at the rain- bow, for example. Still it is not to be- supposed that colours or notes are necessarily presented as continua : that they are such is matter of after-observation. The groups of sensations known as touches, smells, tastes, on the other hand, do not constitute continua : bitter tastes, for instance, will not shade off into acid or sweet tastes, except, of course, through a gradual diminution of inten- sity rendering the one quality subliminal followed by a gradual increase from zero in the intensity of the other. This want of continuity might be explained if there were grounds for regarding these groups as more complex than the rest, in so far as tertiary colours or vowel -sounds, say, are complex and comparatively discontinuous. But it might equally well be argued that they are simpler than the rest and, as simple and different, are necessarily dis- parate, while the continuity of colours or tones is due to a gradual change of components. Our motor presentations contrast with the sensory by their want of striking qualitative differences. We may divide them into two groups, (a) motor presentations proper and (6) auxilio-motor presentations. The former answer to our " feelings of muscular effort " or " feelings of innerva- tion." The latter are those presentations due to the strain- ing of tendons, stretching and flexing of the skin, and the like, by which the healthy man knows that his efforts to move are followed by movement, and so knows the position of his body and limbs. It is owing to the absence of these presentations that the anaesthetic patient cannot directly tell whether his efforts are effectual or not, nor in what position his limbs have been placed by movements from without. Thus under normal circumstances motor pre- sentations are always accompanied by auxilio-motor ; but in disease and in passive movements they are separated and their distinctness thus made manifest. Originally we may suppose auxilio-motor objects to form one imper- fectly differentiated continuum, but now, as with sensa- tions, movements have become a collection of special continua, viz., the groups of movements possible to each limb and certain combinations of these. Perception. In treating apart of the differentiation of our sensory Mental and motor continua, as resulting merely in a number of synthesis distinguishable sensations and movements, we have been or V? te ~ compelled by the exigencies of exposition to leave out of sight another process which really advances pari passu with this differentiation, viz., the integration or synthesis of these proximately elementary presentations into those complex presentations which are called perceptions, in- tuitions, sensori-motor reactions, and the like. It is, of course, not to be supposed that in the evolution of mind any creature attained to such variety of distinct sensations and movements as a human being possesses without making even the first step towards building up this material into the most rudimentary knowledge and action. On the contrary, there is every reason to think, as has been said already incidentally, that further differentiation was helped by previous integration, that perception prepared the way for distincter sensations, and purposive action for more various movements. This process of synthesis, which is in the truest sense a psychical process, deserves some general consideration before we proceed to the several complexes that result from it. Most complexes, certainly the most important, are consequences of that principle of subjective selection whereby interesting sensations lead through the intervention of feeling to movements ; and the movements that turn out to subserve such interest come to have a share in it. In this way which we need not stay to examine more closely now it happens that, in the
