172 PSYCHOLOGY. 



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 slight- 

 est movement of the fingers is most vividl}- 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 * completes the proof that move- 

 ment 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 re- 

 duced to as short a time as 0.044" the discrimination of 

 temporal order in the sparks became impossible. But 

 Exner found that if the sparks were brought so close to- 

 gether in sjjace 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 from the left. On the skin similar ex- 

 periments gave similar results. 



Vierordt, at almost the same time,f called atttention to cer- 

 tain persistent illusions, amongst which are these : If another 

 person gently trace a line across our wrist or finger, the 

 latter being stationary, it will feel to us as if the mem- 

 ber 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 the point were moving as well. 

 If the reader will touch his forehead with his forefinger 

 kept motionless, and then rotate the head so that the skin 

 of the forehead passes beneath the finger's tip, he will have 



*Sitzb. der. k. Akad. Wien, Bd. lxxii., Abth. 3 (1875). 

 f Zeitschrift fur Biologic, xii. 226 (1876). 



