D URA TION OF STIMUL US AND SENS A TION. i o 7 1 



in the ring corresponding with the white space between the two 

 black teeth, and with the black tooth between the two white notches 

 than in the two rings enclosing it ; on reversing the direction of rotation, 

 nicker ceases sooner in this middle ring. In this experiment, the white- 

 ness arid blackness of the teeth and notches are intensified by contrast, 

 and the effect of changing the direction of rotation shows that the fusion 

 takes place less readily when ordinary white precedes intensified black, 

 and ordinary black precedes intensified white, than in the opposite order. 

 In the one case the intensified black and white have been preceded 

 by ordinary black and white respectively, which will, by lowering sensi- 



Fig. 388.— Sherrington. 



bility, have lessened their intensification ; while, in the other case, the 

 intensified black and white will have been preceded by white and black 

 respectively, and this, by increasing sensibility, will further augment 

 the intensification due to spatial induction, and bring about an increased 

 difference in the physiological intensities of the successive stimuli. 



The influence of contrast or induction thus demonstrated by 

 Sherrington gives a clue to a difficulty in regard to flicker which has 

 been much discussed. Filehne l found that if discs of black and white 

 sectors were rotated, the number of sectors on the disc was of influence. 

 If a disc with four sectors be rotated n times per second in order to 

 produce fusion, a disc with sixty-four sectors might be expected to 

 require -^ rotations to eliminate flicker. A higher number is, however, 

 found necessary. His results have been confirmed by other observers. 

 The size of the aperture through which the rotatory disc is observed 

 is found to be of influence. The phenomenon has been explained by 

 the fact that in the slower rotation necessary with the larger number of 

 sectors, the movement of the contours is of influence (Marbe), 2 or that 



1 Arch.f. Ophth., 1885, Bd. xxxi. Abth. 2, S. 20. 2 Loc. tit., p. 288. 



