BINOCULAR VISION 753 



seen when pressure is made simultaneously on the outer or the inner sides 

 of both eyes. But if pressure be made with the fingers upon both eyes 

 simultaneously at their lower part, one luminous ring is seen at the middle 

 of the upper part of the field of vision. If the pressure be applied to 

 the upper part of both eyes, a single luminous circle is seen in the middle 

 of the field of vision below. So, also, if we press upon the outer side of one 

 eye and upon the inner side of the other eye, a single luminous spot is pro- 

 duced, and is apparent at the extreme right of the field of vision. The 

 hemispheres of the two retinae may, therefore, be regarded as lying one over 

 the other, as in C, figure 477. If the axes of the eyes, A and B, figure 478, 



FIG. 477. FIG. 478. 



FIG. 477. Diagram to Show the Corresponding Parts of the Retinae. 

 FIG. 478. Diagram to Show the Simultaneous Action of the Eyes in Viewing Objects 

 in Different Directions. 



be so directed that they meet at a, an object at a will be seen singly, for 

 the point a of the one retina and a' of the other are identical. So, also, if 

 the object /? be so situated that its image falls in both eyes at the same dis- 

 tance from the central point of the retina. namely, at b in the one eye 

 and at b f in the other /? will be seen single, for it affects identical parts of 

 the two retinae. The same will apply to the object f 



The reason why the impressions on the identical points of the two retinae 

 give rise to but one sensation, and the perception of but a single image, 

 must either lie in the structural organization and relations of the deeper 

 or cerebral portions of the visual apparatus, or it must be the result of a 

 mental operation; for in no other case is it the property of corresponding 

 nerves of the two sides of the body to refer their sensations to one spot. 



Many attempts have been made to explain this remarkable relation be- 

 tween the eyes, by referring it to anatomical relation between the optic nerves. 

 The circumstance of the inner portion of the fibers of the two optic nerves 

 decussating at the commissure, and passing to the eye of the opposite side, 

 while the outer portion of the fibers continue their course to the eyes of the 



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