TEE PERCEPTION OF SPACE. 175 



Euougli has now been said to show that in the education 

 of spatial discrimination the motions of impressions across sen- 

 sory surfaces must have been the principal agent in breaking 

 up our consciousness of the surfaces into a consciousness 

 of their parts. Even to-day the main function of the pe- 

 ripheral regions of our 'retina is that of sentinels, which, 

 when beams of light move over them, cry ' Who goes there ? ' 

 and call the fovea to the spot. Most parts of the skin do 

 but perform the same office for the finger-tips. Of course 

 finger-tips and fovea leave some power of direct perception 

 to marginal retina and skin respectively. But it is worthy 

 of note that such percejDtioL is best developed on the skin of 

 the most movable parts (the labors of Vierordt and his 

 pupils have well shown this) ; and that in the blind, whose 

 skin is exceptionally discriminative, it seems to have become 

 so through the inveterate habit which most of them possess 

 of twitching and moving it under whatever object may 

 touch them, so as to become better acquainted with the con- 

 formation of the same. Czermak was the first to notice this. 

 It may be easily verified. Of course movement of surf ace 

 under object is [for purposes of stimulation) equivalent to move- 

 ment of object over surface. In exploring the shapes and 



mation of both sorts of difference ; whereof the natural etfect must be to 

 produce the most perfect discrimination of all. 



A. B A. B 



Fig. 53. 



In the left-hand figure let the dark spot B move, for example, from 

 right to left. At the outset there is the simultaneous contrast of black and 

 white in Band A. When the motion has occurred so that the right-hand 

 figure is produced, the same contrast remains, the black and the white 

 having changed places. But in addition to it there is a double suc- 

 cessive contrast, first in A, which, a moment ago white, has now become 

 black ; and second in B, which, a moment ago black, has now become 

 white. If we make each single feeling of contrast = 1 (a supposition tar 

 too favorable to the state of rest), the sum of contrasts in the case of motion 

 will be 3, as against 1 in the state of rest. That is, our attention will be 

 called by a treble force to the ditierence of color, provided the color be 

 gin to move. — (Cf. also Fleisch], Physiologische Optische Notizeu, 2le 

 Mittheilung, Wiener Sitzungsberichte, 1882.) 



