BINOCULAR VISION. 



1 127 



images, but there is a further distinction that the single image receives a 

 different localisation, and is seen either nearer to or further from the eye 

 than the single image of corresponding points. This difference of 

 localisation was first observed by Wheatstone, 1 and, as will be seen later, 

 there is a close connection between disparation and spatial localisation. 



Panum 2 proposed a modification of the doctrine of corresponding 

 points, in which it was supposed that to each point in one eye there 

 corresponded a definite sensory region (Umpjindungskreis) in the other. 



The horopter. — The horopter is the name applied to the sum of 

 those points of space which, in a given position of the eyes, fall on 



Fig. 402.— Hillebrand. 



corresponding points in the two retime. The form of the horopter has 

 chiefly been determined mathematically, but is also to a certain extent 

 capable of experimental investigation. According to J. Miiller, the points 

 forming the horopter lie in a circle passing through the fixation point and 

 the nodal points of the two eyes, and also in a line at right angles to the 

 plane of the circle, passing through the fixation point. In this theo- 

 retical construction it is assumed that corresponding points are identical ; 

 that no wheel rotation occurs in movements of the eyes ; that the retina is a 

 spherical surface ; and that the centre of rotation of the eye coincides with 

 the nodal point. None of these assumptions are correct, with the doubtful 

 exception of the first, and consequently the theoretical horopter changes 



1 Loc. cit., p. 374. 



2 "Physiol. Untersuch. iiber das Sehen mit zwei Augen," Kiel, 1858, S. 62. 



