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Fusion of the signals from both eyes 1s a necessity for undisturbed 
vision — as the limitations of the nervous information processing 
DD capacity do not allow seeing of two images at a time. 
    
  
  
  
  
  
   
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NEE |, Fusion is more a neural operation than a thinking 
— process, it is more a matter of information flow (and 
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correlation) than of mental activity, more brain than 
mind, more physiology than psychologye 
Binocular fusion occurs if a real object is seen by 
both eyes. Infants learn to fuse both impressions by 
viewing to exactly the same details of the object. 
Convergence of the eyes and accomodation are developed 
in the first 10 weeks to operate in tandem, as are the scanning 
movements of the eyes and fixation (Vernon 1962, p. 16, 121). These 
visual activities are possibly performed best by developing a leading 
and a lazy eye, if only the latter is not becoming too lazy to stop its 
activity completely. 
Stereoscopic fusion has a high relevance to photo-interpretation. 
It is a completely new task for beginners. Although the proverb says man 
is never too old to learn, one may well ask whether a person can be too 
old to decouple convergence and accomodation. 
The precision with which points can be compared and transferred between 
equal prints is of the order of a few seconds of arc, but deteriorates 
quickly by non ideal image properties. On the subject of fusion - in 
technical terms "image correlation", in photogrammetry "parallax 
clearance" - it is still possible to write a doctorate's thesis, even if 
depth perception is left out completely by working with two prints from 
the same negative. A number of simple tolerances and engineering aspects 
are to a large extent still to be researched and compared with those for 
automatic image correlation equipment. 
[Some of the following questions were already outlined in the 
research programme by Anson (1959), studied by Palmer (1960, 1964), 
and others, but were not solved completely. 
Does the "Lazy" eye take over the command if the presented image 
is an order better in image quality than the print offered to the 
stronger eye” 
Is the inequality of the eyes the reason for systematic errors 
in stereoscopic measurements, which amounts up to 1/3 of the 
accidental error (Zorn, 1966, and private comm. 1968). 
Do operators have the same tolerances for the disturbing 
y-parallaxes, thus for rotation of one image w.r.t. the other, for 
magmification differences, maximum x-parallaxes, etc.? This is of 
practical value for training and for the evaluation of instruments.