556 



VISION 



on symmetrical points. If so, these asymmetrical points would nevertheless 

 be corresponding points. But suppose by a slight operation, the squinting 

 eye be given its proper position; at once double vision results, which though 

 for a time very disturbing, gradually disappears, either because the person 

 learns to disregard one image, or it may be, by a new arrangement of the 

 corresponding points of the two retinae (Wundt). 



If we take two patterns ruled in different directions (as in Fig. 247) and 

 look through cylindrical tubes at one with the left eye and at the other with 

 the right, we should expect, if the corresponding points of the two eyes were 

 connected with the same ganglion cells, to get a double pattern ruled both 

 ways. But instead, when the vertical lines of one pattern are seen clearly, 



the horizontal lines of the other are indistinct and 

 vice versa: If the eyes be moved in the vertical 

 direction, the vertical lines stand out more and 

 more prominently if in the horizontal direction 

 the horizontal lines. 



Because the two fields of vision appear thus 

 to contend for the supremacy, this phenomenon 

 is known as the rivalry of the retince. 



The question whether the correspondence of the 

 retinae is inborn or acquired has been answered in 

 very different ways. At all events we may be sure 

 that the nerve connections for the movements of 

 the two eyes and for keeping them in their natural 

 positions are established before birth. For this 

 reason the portrayal of an object on certain parts 

 of the retina is especially favored. Since now the 

 bilateral connection of the optic fibers with the 

 cerebrum is, for the most part at least, inborn also, 

 there must exist from the earliest moment of extra- 

 uterine life onward very favorable conditions for 

 the correspondence of the retina. Hence it will 

 be relatively easy for the child in the formation 

 of his visual sensations to relate the two retinal 

 images, together with the tactile impressions, to a 

 single object and thus gradually to develop a cor- 

 respondence of the two retinae. 



C. PERCEPTION OF DEPTH 



FIG. 248. Schema illustrating 

 the formation of images in 

 the two eyes, of an oblique 

 line in the median plane. 



The principal significance of vision with two 

 eyes is that it enables us to estimate distance in 

 the sagittal direction more exactly and to obtain 

 an idea of the solidity of objects. 



It is true that one can estimate distances with one eye, but he can do so 

 much more accurately with two. 



The factors which figure in the perception of depth with one eye are the 

 following: (1) visual angle; (2) accommodation; (3) convergence of the 

 lines of vision. 



