TI28 



VISION. 



with every change of convergence, and does not exist in asymmetrical 

 positions of the eyes. In the experimental investigation of the horopter, it 

 is not sufficient to determine the points in space which in any given position 

 of the eyes are seen singly, since single vision occurs not only with corre- 

 sponding but also with slightly disparate points. Hering 1 determines 

 the form of the horopter by finding the points which are seen singly and 

 in the same vertical plane as the point fixed. He distinguishes between 

 the vertical and the horizontal horopter ; the vertical horopter being 

 made up of lines which fall on corresponding vertical sections of the 

 retina, and the horizontal on corresponding horizontal sections. The total 

 horopter is the part common to both vertical and horizontal horopters. 

 In the determination of the vertical horopter, Hering uses vertically 

 A c B suspended threads, the ends of 



which are not exposed, seen 

 against a uniform background. 

 He finds that if these threads 

 are viewed with both eyes, and 

 arranged so that they appear to be 

 in a plane parallel to the frontal 

 plane, at a certain distance, less 

 than 2 metres from the eyes, 

 they form a plane ; nearer than 

 this they form a curved surface 

 concave to the face, the concavity 

 increasing with approach to the 

 eyes ; beyond this distance they 

 form a surface convex to the face. 

 Helmholtz 2 also found that threads 

 apparently in a plane really 

 formed a concave cylindrical sur- 

 face near the eyes, and became a 

 plane surface only at a certain distance, which he found to vary in different 

 individuals ; and he explained the facts partly by erroneous estimation of 

 degrees of convergence. He also found that if small objects were fixed 

 on the threads, these were arranged in a plane 

 and not a curved surface ; but Hillebrand 3 has 

 shown that this is not the case if the objects 

 are at irregular distances from one another ; 

 Hillebrand also found that the arrangement as 

 described by Hering occurs with any degree 

 of convergence. According to Hering, the 

 curved surfaces are the real form of the 

 horopter at different distances from the eyes, 



and depend on the special kind of retinal q ~q' 



incongruence described by Kundt and him- F 404 _ Hillebrand 

 self (see p. 1125). 



Hillebrand illustrates the form of the horopter by Figs. 402, 403, and 

 404, in which and 0' represent the nodal points of the two eyes. In all 

 three figures the angle a is less than the angle /3, corresponding to the 

 incongruence in which a distance on the nasal half of the retina is greater 



1 "Beitr. z. Physiologic," Leipzig, 1864, Heft 5, S. 296. 



2 "Handbnch d. physiol. Optik," 1867, S. 654. 



3 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1893, Bd. v. S. 1. 



Fig. 403.— Hillebrand. 



