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Page:Popular Science Monthly Volume 19.djvu/543

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show that it may also coexist with all the appearances of good health and sober judgment.

A convenient distinction is made between hallucinations and illusions. Hallucinations are defined as appearances wholly due to fancy; illusions, as misrepresentations of objects actually seen. There is, however, a hybrid case which deserves to be specifically classed, and arising in this way: Vision, or any other sensation, may, as already stated, be a "direct" sensation excited in the ordinary way through the sense-organs, or it may be an "induced" sensation excited from within. We have, therefore, direct vision and induced vision, and either of these may be the ground of an illusion. So we have three cases to consider, and not two. There is simple hallucination, which depends on induced vision justly observed; there is simple illusion, which depends on direct vision fancifully observed; and there is the hybrid case of which I spoke, which depends on induced vision fancifully observed. The problems we have to consider are, on the one hand, those connected with induced vision, and, on the other hand, those connected with the interpretation of vision, whether the vision be direct or induced.

It is probable that much of what passes for hallucination proper belongs in reality to the hybrid case, being an illusive interpretation of some induced visual cloud or blur. I spoke of the ever-varying patterns in the field of view; these, under some slight functional change, might easily become more consciously present, and be interpreted into phantasmal appearances. Many cases, if space allowed, could be adduced to support this view.

I will begin, then, with illusions. What is the process by which they are established? There is no simpler way of understanding it than by trying, as children often do, to see "faces in the fire," and to carefully watch the way in which they are first caught. Let us call to mind at the same time the experience of past illnesses, when the listless gaze wandered over the patterns on the wall-paper and the shadows of the bed-curtains, and slowly evoked faces and figures that were not easily laid again. The process of making the faces is so rapid in health that it is difficult to analyze it without the recollection of what took place more slowly when we were weakened by illness. The first essential element in their construction is, I believe, the smallness of the area upon which the attention is directed at any instant, so that the eye has to move much before it has traveled over every part of the object toward which it is directed. It is as with a plow, that must travel many miles before the whole of a small field can be tilled, but with this important difference—the plow travels methodically up and down in parallel furrows; the eye wanders in devious curves, with abrupt bends, and the direction of its course at any instant depends on four causes: on the most convenient muscular motion in a general sense, on idiosyncrasy, on the mood, and on the associations current at