iii6 VISION. 



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, accord- 

 ing 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. The altered distribution of brightness of the colourless 

 spectrum will be considered later. 



While the ordinary negative or complementary after-image may easily 

 be explained by the adoption of Hering's views of metabolism, the same 

 cannot be said of the prolonged positive image which occurs after looking 

 at the sun or other strong illumination. Hering proposes the following 

 explanation. 1 When looking at the sun there is not only very violent 

 katabolism, but also a considerable increase of anabolism, so that not 

 only is the visual substance used up, but also the material for anabolism. 

 Consequently, in the darkened eye, the sensation depends on very feeble 

 katabolism due to intrinsic stimuli and still feebler anabolism. The 

 substance is at a very low potential, but the sensation is one of bright- 

 ness. If now the retina is illuminated, the increase of katabolism in 

 the blinded area will be very slight, but its anabolism will be very 

 much increased by the katabolism going on in the rest of the retina (see 

 Explanation of Contrast). Anabolism will be in excess of katabolism, 

 and the image will appear dark. Given the assumption that the material 

 for anabolism is used up, this explanation follows perfectly from the 

 theory ; but it is not quite easy to understand how this consumption can 

 take place during the very short exposure necessary for a blinding- 

 image. The phenomenon clearly borders on the pathological, and is of 

 essentially the same nature as the persistent scotoma which has 

 in many cases followed direct vision of the sun. Its explanation 

 belongs to the pathology rather than the physiology of metabolism. 



The relation of Hering's theory to the question of fatigue is a 

 problem belonging to general physiology, but it may be briefly con- 

 sidered here. Some of the objections which have been most strongly 

 urged against the theory have involved this question. Donders pointed 

 out that during the day the eye is exposed for hours to light which gives 

 a more intense sensation than that of the mean grey, and that the eye 

 should consequently become fatigued. The answer to this is the occur- 

 rence of allonomous equilibrium, and from one point of view this may 

 be regarded as fatigue, since the substance is then less capable of work. 

 This condition, however, must be rigorously distinguished from such a 

 condition as that resulting from exhaustion of the material for anabo- 

 lism and also from accumulation of metabolic products. During the 

 continuous daylight of a northern summer, the visual substance oscillates 

 about a condition of equilibrium at a low potential, but the supply of 

 new and the removal of waste material go on with a regularity suited to 

 the needs of the substance at its comparatively low level of potential. 



1 "Zur Lehre vom Lichtsinn." sS. 109.. 



