SUCCESSIVE INDUCTION OR AFTER-IMAGES 101 



CHAPTER II 



SUCCESSIVE INDUCTION OR AFTER-IMAGES 



After the stimulation of an area of the retina with light, the 

 withdrawal of the stimulus does not result in the obliteration of all 

 visual sensation. The succeeding sensations vary according to the 

 nature and intensity of the primary stimulus, the condition of adapta- 

 tion of the retina, the region of the retina stimulated, etc. If a second 

 stimulus is applied to the same area of the retina during the course of 

 the after-effects of the primary stimulus the resulting sensation is 

 modified by these after-effects. The effect produced by the secondary 

 stimulus can be measured by comparing it with that obtained from 

 a retinal area not previously exposed to the primary stimulus. The 

 stimulus applied to such a resting area is called the comparison 

 light 1 . 



The sensations obtained from the persistence of the physiological 

 processes set up by a primary stimulus, whether altered by the effects of 

 succeeding stimuli, or unadulterated, i.e., observed under the condition 

 of complete exclusion of fresh stimuli, are commonly called '' after- 

 images." After looking for from 20 to 40 seconds at a white spot of 

 light and then directing the gaze upon a white surface a black spot 

 surrounded by a bright halo is seen. This is the " negative after-image." 

 If the spot or light is coloured the after-image is seen to be tinged with 

 the complementary colour. This is the " complementary after-image." 



If care be exercised a different phenomenon will be observed. Direct 

 the eyes towards a bright spot or light, but keep them carefully covered 

 with the hand for a minute or so. Remove the hand rapidly but with- 

 out any sudden or violent action and quickly replace it, so that the 

 object is momentarily seen. If the experiment is properly carried out 

 the spot will be seen as a persistent after-image in its original brightness 

 and detail. This is the " positive after-image." If the object is 

 coloured it is seen tinged with the same colour ; this is the " homo- 

 chromatic after-image." 



Although in ordinary circumstances negative after-images are more 

 easily obtained than positive McDougall 2 has shown that under suitable 



1 In Germany the effect of the primary stimulus is called "tuning" the retina 

 (Hering). The primary stimulus is called the "retuning light" (das umstimmende 

 Licht); the secondary stimulus the "reacting light" (das rengirende Licht). 



2 Mind, x. N.S. 74 sqq., 1001. 



