SECTION XII 

 BINOCULAR VISION 



BINOCULAR vision may be defined as the co-ordinated employment of two 

 separate visual organs in order to produce a single mental impression. The 

 advantages of binocular as opposed to monocular vision are : 



1. Optical defects of one eye are less important, since they are masked 

 by the well-defined images of the other eye. 



2. Defective vision in parts of the -visual fields of both eyes is hidden so 

 long as the defects do not affect the same parts of both fields. Thus the blind 

 spots do not obtrude themselves under ordinary circumstances, because the 

 corresponding field of the other eye contains normal retina. 



3. The combined fields of the two eyes are larger than either alone because, 

 while the features restrict the nasal halves of the fields of both eyes, the 

 combined field contains the unrestricted temporal areas of both retinae. 



4. Binocular vision under certain circumstances provides a very accurate 

 perception of depth, size and distance, which is called stereoscopic vision. 



In order that there should be binocular vision the following conditions 

 s-hould be complied with : 



1. The fields of the two eyes must overlap. Animals in which the eye 

 axes are parallel have the greatest overlap, and therefore possess the 

 completest binocular vision. 



2. Approximately similar images must be formed on the retinas, because 

 if this condition is not satisfied, antagonism between the images will occur, 

 as described above, and first one image and then the other will be presented 

 to consciousness. 



3. The retinae must possess physiologically corresponding points in 

 order that similar images formed on them may produce one conscious 

 impression. 



4. The external eye muscles must so adjust the visual axes that the centres 

 of the fields of the two eyes coincide with the images of one and the same 

 object. This adjustment is called fixation. It is sometimes described as 

 the intersection of .the visual axes at the point fixated. 



5. The oblique muscles must rotate the eyes about their axes until 

 corresponding retinal points occupy corresponding meridians. 



The rotation adjustment is necessary because otherwise identical points 

 of the retinae might not correspond, even when the centres did, so that one 

 image would appear tilted at an angle with the other. Fixation is partly 

 a voluntary act and partly a reflex process. The former is shown by the 

 fact that the eyes may be directed towards an imaginary object a short 



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