BINOCULAR VISION 



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3. BINOCULAR VISION 



The study of vision with two eyes is of very great interest for physiological 

 psychology, and has been treated by many excellent authorities. Here, how- 

 ever, we must limit ourselves to the most important points and shall only 

 discuss the conditions of single vision and the perception of depth. 



A. CORRESPONDENCE OF THE TWO RETINAE 



It is a matter of everyday experience that a distant object regarded with 

 both eyes in their ordinary position looks single, but that if one eye be pushed 

 out of line the object then looks double. One condition of single vision with 

 two eyes, therefore, must be that the images fall on parts of the two retinae 

 which exactly correspond to each other. Those points of the retinae upon 

 which the same parts of the two images fall are called corresponding points. 



On purely optical grounds it is evident that only two points can correspond, 

 for with any given position of the eye a luminous point can be pictured at 



FIG. 247. The rivalry of the retinae. 



but one definite spot in each eye. The centers of the two fovae centrales 

 represent corresponding points. The exact position of others can be deter- 

 mined experimentally by means of an instrument known as the haploscope 

 (Hering). It is evident on reflection that the nasal side of one retina must 

 correspond to the temporal side of the other, since light from, say, the right 

 side of the field of vision must strike the left side of both retinas and vice 



versa. 



B. SINGLE VISION WITH TWO EYES 



It might be supposed in explanation of the remarkable fact of single vision 

 with two eyes, that the optic nerve fibers proceeding from corresponding points 

 of the two retinae end in the same ganglion cell of the brain. But this is not 

 true, for the independence of each eye is much greater than it would be on this 

 hypothesis. There is a form of squint i. e., pathological deviation in the 

 positions of the eyes which is due to an abnormal shortening of the eye 

 muscles (muscular strabism). The line of vision of the squinting eye devi- 

 ates by a certain angle from the proper position. Now it happens that the 

 person so affected sees single with two eyes in which the images do not fall 



