188 w. JAMES :

on that spot with a pencil point, he will be distinctly aware of the point's motion and vaguely aware of the direction of the motion. The perception of the motion here is certainly not derived from a pre-existing knowledge that its starting and ending points are separate positions in space, because positions in space ten times wider apart fail to be discrimi- nated as such when excited by the dividers. It is the same with the retina. One's fingers when cast upon its peripheral portions cannot be counted that is to say, the five retinal tracts which they occupy are not distinctly apprehended by the mind as five separate positions in space and yet the slightest movement of the fingers is most vividly perceived as movement and nothing else. It is thus certain that our sense of movement, being so much more delicate than our sense of position, cannot possibly be derived from it. A curious observation by Exner 1 completes the proof that movement is a primitive form of sensibility, by showing it to be much more delicate than our sense of succession in time. This very able physiologist caused two electric sparks to appear in rapid succession, one beside the other. The observer had to state whether the right-hand one or the left-hand one appeared first. When the interval was reduced to as short a time as O044" the discrimination of temporal order in the sparks became impossible. But Exner found that if the sparks were brought so close together in space that their irradiation-circles overlapped, the eye then felt their flashing as if it were the motion of a single spark from the point occupied by the first to the point occupied by the second, and the time-interval might then be made as small as 0'015" before the mind began to be in doubt as to whether the apparent motion started from the right or left. On the skin similar experiments gave similar results. Vierordt, at almost the same time, 2 called attention to certain persistent illusions which seemed to him survivals from a stage of development when motion was felt as such, but not yet discriminated as belonging to subject or object. Such feeling, he concluded, must be the primitive and undif- ferentiated form of all spatial perception. The illusions in question are, among others, these : If another person gently trace a line across our wrist or finger, the latter being sta- tionary, it will feel to us as if the member were moving in the opposite direction to the tracing point. If, on the contrary, we move our limb across a fixed point, it will be seen as if 1 Sitzb. der. Jc. Akad. zu Wien, Bd. Ixxii., Abth. 3 (1875). 2 Zeitechrift fur Biologie, xii. 226 (1876).