INTRODUC TION 
Ideally, the art of the photogrammetrist ought to be insensitive to 
the distortions of binocular perceptual space created by his instruments. 
In practice, the existence of these distortions may be quite troublesome. 
Distortions of binocular visual space are inherent in stereoscopic instru- 
ments and there is no completely perfect way of correcting them as we shall 
see. 
In training binoculars upon an approaching sailboat, for example, we 
may find the vessel oddly distorted; hardly longer from stem to stern than in 
beam. To determine the cause of this grotesque foreshortening we note that 
the glass itself gives 6 x magnification of the frontal field, but that the distance 
between the centers of the objective lenses only multiplies the interocular dis- 
tance by 2, approximately. In other words, the means by which we perceive 
depth, the separation of the eyes, has not been increased in proportion to the 
increase in visual breadth. It would seem that the thing to do is to multiply the 
  
interocular separation by an amount equal to the magnification. This is indeed 
a very effective correction, but it is not the whole story. 
2. THE EFFECTIVE BINOCULAR STIMULUS 
The problem we have posed will be better understood if we consider pre- 
cisely what information is utilized by the organism in binocular space perception. 
It would seem that information about those muscular activities of the eyes which 
control their optical accommodation and rotation is used only to a minor degree, 
if at all. Retinal stimulation is the primary source for spatial impressions [3]. 
For monocular vision this has certain simple implications. It will simpli- 
fy our presentation to treat the eye as though it had pinhole optics located at the 
center of rotation. This is actually a sufficiently good approximation for our pur- 
pose at distances greater than some 10 or 15 ocular diameters, and the approxi- 
mation improves with distance. For the purposes of this paper it will be sufficient 
  
to confine our attention to stimuli which lie in a plane containing the axis joining 
the centers L and R ofthe left and right eyes respectively. We assume in 
what follows that the head is fixed although the eyes are not constrained to constant 
fixation. The stimulus also is assumed to be fixed, not moving. These are pre- 
cisely the conditions appropriate to viewing through a stereoscope. 
Consider a monocular stimulus consisting of points Po: Pi, Po, «.. and their 
respective images Qo Qi; Q»; ... on the retina. The points Qi (k = 0,1, 2, ...), 
- 3.