Page:Mind (Old Series) Volume 12.djvu/198

THE PERCEPTION OF SPACE. (ll.) 185 former region, I am able to make the pain there more intense. The local differences require then an adventitious sensa- tion, superinduced upon them, to awaken the attention. After the attention has once been awakened in this way, it may continue to be conscious of the unaided difference ; just as a sail on the horizon may be too faint for us to notice until someone's finger, placed against the spot, has pointed it out to us, but may then remain visible after the finger has been withdrawn. But all this is true only on condition that separate points of the surface may be exclusively stimulated. If the whole surface at once be excited from without, and homogeneously, as, for example, by immersing the body in salt water, local discrimination is not furthered. The local- signs, it is true, all awaken at once ; but in such multitude that no one of them, with its specific quality, stands out in contrast with the rest. If, however, a single extremity be immersed, the contrast between the wet and dry parts is strong, and, at the surface of the water especially, the local- signs attract the attention, giving the feeling of a ring sur- rounding the member. Similarly, two or three wet spots separated by dry spots, or two or three hard points against the skin, will help to break up our consciousness of the latter 's bigness. In cases of this sort, where points re- ceiving an identical kind of excitement are, nevertheless, felt to be locally distinct, and the objective irritants are also judged multiple, e.g., compass-points on skin or stars on retina, the ordinary explanation is no doubt just, and we judge the outward causes to be multiple because we have discerned the local feelings of their sensations to be dif- ferent. Capacity for partial stimulation is, then, the second condition favouring discrimination. A sensitive surface which has to be excited in all its parts at once by every kind of stimulus that can be applied to it can yield nothing but a sense of undivided largeness. This appears to be the case with the olfactory, and to all intents and purposes with the gustatory, surfaces. Of many tastes and flavours, even simultaneously presented, each affects the totality of its respective organ, each appears with the whole vastness given by that organ, and appears interpenetrated by the rest. 1 1 It may, however, be said that even in the tongue there is a determina- tion of bitter flavours to the back, and of acids to the front, edge of the organ. Spices likewise affect its sides and front, and a taste like that of alum localises itself, by its styptic effect on the portion of mucous mem- brane, which it immediately touches, more sharply than roast pork, for