Page:Mind (Old Series) Volume 12.djvu/200

 THE PEECEPTION OF SPACE. (ll.) 187 now. 1 There is, however, one quality of sensation which is particularly exciting, and that is the feeling of motion over any of our surfaces. The erection of this into a separate ele- mentary quality of sensibility is one of the most recent of psychological achievements, and is worthy of detaining us a while at this point. Psychologists generally have assumed the perception of motion to be impossible until the positions of terminus a quo and terminus ad quern are severally cognised, and their successive occupancies by the moving body are perceived to be separated by a distinct interval of time. 2 As a matter of fact, however, we cognise only the very slowest motions in this way. Seeing the hand of a clock at XII. and afterwards at VI., we judge that it has moved through the interval. Seeing the sun now in the east and again in the west, I infer it to have passed over my head. But we can only infer that which we already generically know in some more direct fashion, and it is experimentally certain that we have the feeling of motion given us as a direct and simple sensation. Czermak long ago pointed out the difference between seeing the motion of the second-hand of a watch, when we look directly at it, and noticing the fact of its having altered its position when we fix our gaze upon some other point of the dial-plate. In the first case we have a specific quality of sensation which is absent in the second. If the reader will find a portion of his skin the arm, for example where a pair of compass-points an inch apart are felt as one impres- sion, and if he will then trace lines a tenth of an inch long 1 1 tried on nine or ten people, making numerous observations on each, what difference it made in the discrimination of two points to have them alike or unlike. The points chosen were (1) two large needle-heads, (2) two screw-heads, and (3) a needle-head and a screw-head. The distance of the screw-heads was measured from their centres. I found that when the points gave diverse qualities of feeling (as in 3), this facilitated the discrimination, but much less strongly than I expected. The difference, in fact, would often riot be perceptible twenty times running. When, however, one of the points was endowed with a rotary movement, the other remaining still, the doubleness of the points was much more evident. To observe this I took an ordinary compass with one point blunt, and the movable leg replaced by a metallic rod which could, at any moment, be made to rotate in situ "by a dentist's drilling machine, to which it was attached. The compass had then its points applied to the skin at such a distance apart as to be felt as one impression. Suddenly rotating the drill-apparatus then almost always made them seem as two. 2 This is only one example of what I have called ' the psychologist's fallacy 'thinking that the mind he is studying must necessarily be con- scious of the object after the fashion in which the psychologist himself is conscious of it.