THEORIES OF COLOUR- VISION. 1117 



Simultaneous contrast, and the other phenomena which I have included 

 under spatial induction, are explained physiologically by Hering on the 

 same lines as the phenomena of temporal induction. When a retinal area 

 undergoes katabolism under stimulation, the process of katabolism may be 

 regarded as acting as a stimulus to anabolism in the same area ; it is due 

 to this tendency of allonomous katabolism to promote autonomous ana- 

 bolism that allonomous equilibrium occurs. In the same way, allonomous 

 anabolism in a retinal area may be regarded as acting as a stimulus to 

 autonomous katabolism in that area. Contrast is simply explained by 

 supposing that this tendency to promote the opposite kind of change is not 

 limited to the area stimulated, but extends to adjoining areas, and that this 

 action is most marked at the line of junction of the reacting areas, and 

 diminishes from here outwards. Hering formulates the nature of the 

 physiological reciprocity of retinal areas, advocated by Johannes Mliller, by 

 making an assumption which is perfectly consistent with his general theory. 



The strong point of Bering's explanation is, that it brings temporal 

 and spatial induction into the closest relations with one another ; each of 

 the two processes helps to make the occurrence of the other more intelli- 

 gible. In both kinds of induction we may see the working of a general 

 law, that the occurrence of destructive metabolism at once determines an 

 increase in constructive metabolism, which hinders the living substance 

 from undergoing too great or too rapid decomposition ; and that, on the 

 other hand, constructive metabolism determines an increase in the 

 tendency to spontaneous decomposition, which sets a limit to assimilation, 

 and prevents it from reaching a degree beyond the needs of the organism. 



Specific brightness. Hering's treatment of the subject of bright- 

 ness may be considered here. It is interesting, as the only feature 

 in which the theory has suffered any important modification since first 

 formulated. In the " Lehre zum Lichtsinn " Hering thought it probable 

 that the brightness of a visual sensation was wholly due to its white 

 component, and that if we were able to see the four primary colours in a 

 state of purity they would appear of equal brightness. For this assump- 

 tion has now been substituted the idea of specific brightness of the 

 colours. 1 The two colours, red and yellow, which are due to katabolism, 

 are supposed to contribute positively to the total brightness, while the 

 contribution of green and blue is negative and diminishes the total 

 brightness of the sensation. This assumption is especially applied to 

 explain the changes in the spectrum with lowered intensity. In the eye 

 adapted to the dark, the black-white substance will have been raised to a 

 condition of high potential, while the chromatic substances will not be 

 appreciably affected; consequently, the "weight" of the colourless 

 component of the sensation will be relatively much increased, and with 

 slight stimulation the coloured components will remain under the 

 threshold. The chromatic substances will also cease to contribute to the 

 brightness of the sensations, and the curve of luminosity of the spectrum 

 will be that of the black-white substance. According to Hering's theory, 

 then, the spectrum of low intensity is the spectrum of the black-white 

 substance. With increase of intensity the specific brightness of the 

 chromatic processes begins to affect the curve of luminosity; red and 

 yellow contribute positively, green and blue negatively, and the conse- 

 quence is a shifting of the maximum brightness of the spectrum towards 

 the former, until yellow again becomes the brightest part. From this 



1 Hillebrand, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1889, Bd. xcviii. Abth. 3, S. 70. 



