io 7 o VISION. 



which follows intense stimulation of the retina (p. 1059). It is here 

 supposed that periods of stimulation alternate with periods of complete 

 absence of stimulation. Flicker also occurs when periods of greater 

 and less excitation alternate, and the point of fusion is then lowered by 

 diminution of the difference of intensity in the two periods. 



There are different opinions as to the influence of the relative 

 duration of the periods of excitation and non-excitation. Charpentier l 

 found that increase of duration of the stimulation raised the point of 

 fusion, while Marbe 2 found the point of fusion lowered when the more 

 intense stimulus was prolonged in relation to the less intense. 



FIG. 387. Sherrington. 



Sherrington 3 has recently shown that the factors described in the 

 previous sections as spatial and temporal induction, have an enormous 

 influence on the point of fusion. Using a yellow, blue, and black disc 

 (shown in Fig. 387), in which Bl and Bk were of the same objective 

 brightness as Bl' and Bk', he found that the rate at which flicker ceased 

 in the two rings differed very greatly ; Bl Bk f requiring a much higher 

 rate than Bl' Bk, the brightness of Bl and the darkness of Bk' being 

 increased by simultaneous contrast, or spatial induction. By means of 

 this and similar experiments, he shows conclusively that the point of 

 cessation of flicker depends on what one may call the " physiological " 

 intensity, and not on the physical intensity, and also furnishes a conclusive 

 proof that simultaneous contrast rests upon a physiological basis. By 

 means of the disc shown in Fig. 388, Sherrington further shows that 

 temporal induction influences the point of fusion. If this disc be 

 rotated in the same direction as a clock hand, flicker persists longer 



1 Arch, d'opht., Paris, 1890, tome x. p. 340. 



2 Phil. Stud., Leipzig, 1896, Bd. xii. S. 279. 



3 Journ. PhysioL, Cambridge and London, 1897, vol. xxi. p. 33. 



