1 138 VISION. 



no direct consciousness of the nature of the retinal mechanism. Localisa- 

 tion above or below, to right or to left of the fixation point, is similarly 

 held to depend directly on the nature of the retinal stimulation. In the 

 exposition of his theory Hering uses the expression " value " (" Werth "), 

 and ascribes to each retinal element a certain height, breadth, and depth 

 value. The localisation of a point relative to the fixation point {Kern- 

 punkt) is determined by these three values. Two corresponding points 

 have identical height and breadth values and opposed depth values. 

 The opposed values cancel one another, so that the external point 

 appears in the horopter. Points situated on the opposite sides of the 

 median section, but at equal distances from it, have the same depth 

 value and opposed breadth values, so that the external point is seen in 

 the line passing from the cyclopean eye through the fixation point, and 

 at a distance from the latter, depending on the nature and amount of 

 the disparation. Hering's theory has met with much opposition from 

 psychologists, because it has been held to imply innate ideas of distance. 

 As Hering points out, the anatomical mechanism of retinal disparation 

 is certainly innate, and therefore the function of that mechanism must 

 also to some extent be innate. 



Hering further supposes that retinal disparation acts as a stimulus 

 to movement, the movement being of a kind adapted to abolish the dis- 

 paration. Crossed retinal disparation will then act as a stimulus to reflex 

 convergence and accommodation, while uncrossed retinal disparation 

 will act as a stimulus to divergence and relaxation of accommodation. 

 Similarly, breadth and height values will act as stimuli to horizontal 

 and vertical movements respectively. If Hering's values are regarded 

 as " local signs," it will be seen that he would differ from Lotze in 

 regarding the sensory side of the reflex action as the local sign, while 

 Lotze supposed that the motor side was the local sign. 



Hering regards retinal rivalry, and the fact that contours prevail in 

 the rivalry, as most important factors in the mechanism of depth 

 perception. In order that contours shall prevail, it is necessary that 

 they should be sharp and well defined, and it is on this account that he be- 

 lieves contrast to be of such fundamental importance in vision (p. 1066). 



Hering denies that muscle sensations have anything whatever to do 

 with localisation of the fixation point itself. The idea of nearness 

 which may accompany convergence is regarded by him as rather the 

 cause than the consequence of the movement. The distance of the 

 fixation point is referred by him to the influence of retinal values, 

 acting together with the psychological or experience factors. So far 

 as the position of the head and eyes is of importance, localisation 

 depends, not on their actual position, but on the idea which we have of 

 their position. 



Schon's theory of the binocular mechanism. — Another view as 

 to the nature of the binocular mechanism has been advanced by 

 Schon. 1 This view only applies to indirect vision, and is based on 

 the difference in sensibility of the nasal and temporal halves of the 

 retina. According to Schon, the nasal half is much more sensitive, 

 especially to light, than the temporal half of the same or of the other 

 eye. Consequently, corresponding or slightly disparate points of the 

 two retinae will produce sensations of different intensity, when stimu- 



1 Arch. f. Ophth., 1876, Bd. xxii. Abth. 4, S. 31 ; and 1878, Bd. xxiv. Abth. 1, 

 S. 27. 



