STATEMENT OF THE THEORY 255 



complete exclusion of light from the eye, which formed the great 

 difficulty of the fatigue theory, are here referred to autonomous changes. 

 Hering has formulated the nature of the active opposed change, which 

 Plateau had previously suggested as the basis of after-image in 

 distinction from the passive condition advocated by Fechner and 

 Helmholtz. 



" The absence of colour in all kinds of light at low intensity is, 

 according to Hering, a function of the dark adaptation of the eye rather 

 than of the low intensity of the stimulus, and is due to the fact that 

 in the dark the white-black substance rises greatly in potential, while 

 the chromatic substances remain in the condition of autonomous 

 equilibrium. Consequently, the weight of the chromatic is very small 

 compared with that of the colourless elements, and the former remain 

 under the threshold." 



Hering ascribes the positive after-image produced by closing the 

 eyes after gazing at a bright object to exhaustion of assimilation during 

 fixation. As a consequence all that remains is a feeble process of 

 dissimilation due to internal stimulation. The fact that a positive 

 after-image becomes negative if a surface, the brightness of which is 

 greater than that of the positive after-image, is fixated, negatives this 

 theory. 



Simultaneous contrast is explained by Hering in the same manner 

 as successive contrast. Allonomous katabolism in a retinal area acts 

 as a stimulus to autonomous anabolism in that area, but the effect 

 is not limited to the area. It extends to adjoining areas, being most 

 marked at the junction of the reacting areas. 



Hering attributes the bright halo, which under certain conditions 

 surrounds complementary after-images (v. p. 101), to a process of succes- 

 sive induction. When, for example, a white square upon a black back- 

 ground has been for some time fixated, the assimilation process, w r hich 

 according to Hering has been especially active just beyond the contrast- 

 ing edges of the white square, reaches such a height that finally a 

 process of simultaneous induction is set up ; assimilation gives way to 

 dissimilation. When now the eyes are turned away from the white 

 square, this dissimilation process continues. It is owing to this succes- 

 sive induction that a bright halo appears around the dark after-image 

 of the white surface. And the dark after-image results from the con- 

 trasting assimilation process, which is evoked by the dissimilation 

 process that produces the halo. 



The dark halo, which under other conditions surrounds an 



