SECTION V 



TEMPORAL EFFECTS 

 CHAPTER I 



RECURRENT VISION ; THE TALBOT-PLATEAU LAW ; 

 THE FLICKER PHENOMENON 



We have already had evidence (p. 57) from stimulation of the dark- 

 adapted retina by light of low intensity that two mechanisms are 

 involved. One, which may be called the " scotopic mechanism," in 

 the condition of maximum dark adaptation, responds to light of low 

 intensity by a colourless light sensation, no matter what the nature of 

 the light stimulus so long as it be an " adequate " stimulus (v. p. 19). 

 The other, which may be called the " photopic mechanism," responds to 

 light of higher intensity (which in itself induces a relatively high degree 

 of light adaptation) by a light sensation of greater intensity and greater 

 complexity, the sensation being that of white or colour according to 

 the nature of the stimulus. It may at once be admitted that theoretical 

 conceptions of definite mechanisms subserving these diverse though 

 allied functions are here introduced. They will be allowed to intrude 

 as little as possible in the description of the facts, but the comprehen- 

 sion of the description is facilitated by permitting their use. 



The sensational response to a single short-lived stimulus is not a 

 single, equally short-lived light sensation. Except in the case of a very 

 feeble stimulus it is " a series of pulses of sensation of diminishing 

 intensity rapidly succeeding one another 1 ." Attention was early called 

 to these recurring responses by C. A. Young in 1872 2 . He found that 

 when a discharge from a powerful electric machine momentarily illumi- 

 nates a room the objects may be seen, not once only, but two, three or 

 four times in rapid succession, although the spark is single. He called 



1 McDougall, Brit. JL of Psychology, i. 78, 1904. 



2 Nature, v. 512, 1872 : Philos. Mag. XLIII. 343 ; Sill. JL in. 2(i2. 



