Sensation, then, is the result of a change occurring in a centre in the brain, and yet when the skin is pinched we refer the impres sion to the skin, though it is in the brain that it is actually perceived. We think it is our ears that recognize sounds ; in reality it is only the brain that takes note of them as such. This habit of referring the sensation to the terminal organ which first received the impression is the result of education and habit. When we see a light, what we are conscious of is a change in a brain-centre, and yet we refer it outwardly. If the optic nerve be irritated by a current of electricity, or by a blow, we see flashes of light as vivid as if lights actually danced before our eyes. Impressions have reached the centres for see ing, coming along the ordinary channels, and produce the same changes that lights should do. Ln this case, however, we know the cause of the colours, and correct the conclusion we would otherwise make. The seeing centre itself may be irregularly stimulated, by some condition of the blood, for example, and the person may see things which appear as real as if they were actual external existences, but which have only a temporary existence in his disturbed brain. Thus the man in delirium tremens sees fantastic figures dancing and mak ing grotesque faces at him. His seeing centres, excited in an unhealthy way by alcohol, are of themselves producing changes which, in healthy circumstances, ought only to be aroused by real things external to him. The changes in the brain have, in this case, nothing corresponding to them outside, but the brain, nevertheless, refers them to the outside as usual, and they, therefore, appear to be real things. The man's
judgment being otherwise also warped by the alcohol, he cannot correct his impressions, and is consequently victimized by them.
Fusion of is a feature of all the senses that when they are excited by a series of impressions, in which one follows the other very quickly, they are unable to distinguish the different impressions. The series become fused together, and the sensation is of a prolonged impression. Thus, if the finger be gently pressed on the edge of a toothed wheel, with the wheel going very slowly, the contact of each separate tooth is distinctly felt; but when the wheel is made to turn rapidly one loses the sensation of the separate teeth, and the feeling is of an un interrupted kind. Again, every child knows that if a piece of string, which, has been on fire at one end, be whirled rapidly in the air, the appearance of a wheel of fire is produced. The one point of fire becomes a circle. This sen sation is produced by a rapid series of impres sions. From every point of the circle described by the glowing end of the string, from point after point of it in quick succession, an impres sion reaches the eye of the glowing point as it travels round. But all the different impressions follow so hard after one another that they are not separately distinguished in the mind, which thus becomes conscious of a continuous circle of light (p. 451). It is the same with sound. If an instrument be made slowly to emit a series of sounds, each sound of the series may be recog nized separately; but if they be produced with a certain rate of rapidity, one sound is heard before the one preceding has died out, and a continu ous instead of an interrupted sound is heard.