246 



THE HUMAN SPECIES 



in the chamois, three times as great 

 as in man, and only half that of the 

 elephant. 



Binocular vision is the chief factor 

 in all space-perceptions (Schleich). 

 Correct localisation of objects is only 

 possible by means of stereoscopic 

 vision with two eyes. Binocular 

 vision gives each eye not identical 

 but different images ; with one eye 

 alone it is easy to be deceived as to 

 the three dimensions of the object ; 

 two eyes are necessary to appreci- 

 ate solidity. " The facts of stereo- 

 scopic vision demonstrate incontest- 

 ably that the two eyes perceive 

 independently of one another, and 

 that their perceptions are only 

 secondarily combined in a common 

 idea" (Wundt). 1 This fusion is a 

 psychical process in which tactile 

 sensibility is unconsciously aided by 

 experience and habit. " It is evident 

 that the estimation of distance and 

 of solidity of objects, even using only 

 one eye, is tolerably perfect. This 

 is worthy of remembrance, for the 

 discovery of Wheatstone has been 

 found so important that it almost 

 makes us forget what a single eye 

 can do. Two eyes, however, can 

 certainly do more. At least when 

 near objects are concerned the solid- 

 ity of the object can be estimated 

 with more certainty ; we cannot, in 

 binocular vision, so easily be de- 



1 Wundt, Beitrtige zur Theorie der Sin- 

 neswahrnehmung, 1862. Human and Animal 

 Psychology, London, 1896, p. 188. 



FIG. 124. Scheme of layers of 

 the retina. (Ranke.) a, 

 fibres of optic nerve ; b, 

 layer of ganglion cells ; c, 

 inner granular layer ; d, 

 inner nuclear layer ; e, outer 

 granular layer ; f, outer 

 nuclear layer ; g, external 

 limiting membrane ; h, rods 

 and cones. 



