โ Scribed by C.
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ILLUSlON.
--A number of years ago, I observed the following illusion. When the eye watches for some time, in complete darkness, a motionless object of small diameter, and feebly lighted, it very often happens that this object appears clearly to move with a certain rapidity in a specific direction. This phenomenon, when it takes place, is very striking, and almost all upon whom I have tried to reproduce it have verified it. It is an appearance analogous to that of a shooting-star, but less rapid. The angular rapidity of the apparent displacement of this object has appeared to me to be between 2 ยฐ and 3 ยฐ per second. The direction of displacement is variable. The object appears oftenest to progress in a curved line upwards or outwards, but this direction may be quite different from either. Sometimes the object follows successively different courses; it may describe curves or zigzags. The total extent of the displacement varies; it may attain and exceed, 3 oยฐ. In reflecting upon the possibilities of this illusion, I have thought at first of an unconscious movement of the eye. This explanation must be rejected: the eye remains fixed as well as the object. Indeed, the form of a group of little points very near together may be given to the object, and these cannot be seen except by the centre of the retina. Now, under these conditions the illusion persists; besides, the experiment succeeds with both eyes, and a simultaneous displacement, exactly symmetrical and quite extensive, of the two eyes would not pass unperceived. The illusion, then, does take place when an eye at rest looks fixedly at an object at rest. I have studied, moreover, to see whether the point of departure was not a special position of the eye in relation to the head, which would produce an abnormal, tonic contraction of the muscle corresponding to the apparent sense of displacement of the object ; but the different positions I have given to the eye have not prevented the phenomenon from appearing, and have not introduced anything special int~t. A fact which may throw some light on the production of this illusion, is that it is possible to provoke a voluntary, apparent displacement of the object in a given direction. It suffices for this, to think of seeing another object, or of accomplishing an act in the desired direction. For example, with my eye at the eye-piece of the dark chamber, which serves roe for these experiments, I think of picking up a pin or book on the floor, and I see the object not always, but generally, move down: if I think of the chimney, on the roof which faces my window, the object rises, etc. I repeat it, and there comes a distinct sensation which everybody can obtain. It has been objected that my eye accomplished the supposed movement itself, but this is incorrect ; I have assured myself of the contrary. Moreover, it is easy to see that, as the eye looked up, the object must, on the contrary, appear to descend, and vice-versa. Under what conditions do we ordinarily feel that an object moves when the body and head are immovable and we have no
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