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A", B", C" farther from the observer and convex toward him. The three con- 
figurations separately give the same impressions of straightness and position 
relative to the observer. Experimental evidence in support of this conciusion is 
given elsewhere [1]. 
It is clear from these considerations that the organism does not utilize 
all the available geometric elements of the physical stimulus to form its binocular 
spatial perceptions. We confine our attention to those elements alone which are 
essential to perception and shall consider them as constituting the effective bin- 
ocular stimulus. The principle we have enunciated is that the effective binocular 
stimulus consists of those geometrical characteristics and only those which are 
invariant under rotations of the separate monocular optical images about their 
respective ocular centers. 
We introduce special coordinates to describe these invariant characteristics. 
Let P be any point of the plane and consider the circle drawn through P and the ocu- 
lar centers R and L (Fig. 3). The angle y = x LP R is the angle between the visual 
axes when the eyes are fixed on P. The angle y is called the convergence and is 
constant for all points of the circle. Let A denote the anterior intersection of the 
median line with the circle. The angle = ¥ PR A= x P L À is the mean angular 
displacement (or version) of the eyes from the sagittally forward direction and is 
called bipolar azimuth. The position of the point P in the plane is uniquely deter- 
  
mined by the angles (y,9) and we introduce these angles as bipolar coordinates. 
The stated binocular principle is equivalent to the assertion that in altering the co- 
ordinates (y, 9) of each point P of the stimulus to values (y', P') according to the 
transformation rule, 
(1) y! =yv+u ÿ'=0+v 
where p and v are constants, we leave the visual perception unchanged. Another 
way of stating the case is that differences in convergence and bipolar azimuth (these 
are invariant under the transformation rule) are the effective clues for space per- 
ception. 
3. THE DISTORTIONS OF THE EFFECTIVE BINOCULAR STIMULUS 
PRODUCED BY BINOCULAR INSTRUMENTS AND THEIR CORRECTION. 
Until we describe the visual perception itself we shall not be in a position 
to describe the distortions produced by binocular instruments. Nonetheless, since 
we have obtained a description of the effective binocular stimulus we already know 
what needs to be done to correct that distortion: it is necessary to modify the sepa- 
rate clues provided monocularly to each eye separately so as to present the same 
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