Bauhaus-Universität Weimar

430 
THE NEW PSYCHOLOGY. 
the horopter is a plane perpendicular to the line from 
the point of fixation to the face. Mathematically this 
plane is at an infinite distance ; practically, since the eye 
cannot distinguish minute differences, the horopter is 
a space covering all objects beyond a certain limit. 
With the point of fixation directly in front, but at a 
distance nearer than infinity, the horopter is a circle 
lying in a plane passing through the eye and the point 
of fixation and passing the point of fixation and the 
centres (for the lines of direction supposed to be drawn 
from objects to the eyes) of the two eyes, and also of a 
perpendicular line erected at the point of fixation. For 
all other positions of the point of fixation the horopter 
is the same as in this case, as long as the point is not 
raised or lowered from the horizontal plane extending 
outward from the eyes. When the point is on the 
floor directly in front the horopter line likewise lies on 
the floor. For other positions the horopter becomes 
more complicated.1 The importance of the horopter 
lies in the fact that points lying in it are more readily 
united by the two eyes. 
The “ fixation surface ” is a curved surface whose 
points occupy, in the two monocular fields, positions 
which are identical horizontally, regardless of possible 
vertical disparity. Careful experiments2 have estab¬ 
lished the law : the distance from the fixation sur¬ 
face of any point seen singly in binocular space 
depends only upon the amount of disparity between 
the points of the two fields. That is, the position of 
any point in regard to it is determined by the points 
1 For the determinations see Hering, “ Der Rauinsinn und die 
Bewegungen des Auges,” 377, in Hermann’s “ Handbuch der 
Physiologie,” and Helmholtz, “Physiologische Optik,” 2te Aufl., 
860. 
2 Hillebrand, Die Stabilität der Raumwerte auf der Netzhaut, 
“ Zt. f. Psychol, u. Physiol, d. Sinn.,” 1893, v. 1.
        

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