VIII 



OCULAi; MOVEMENTS 



111 



needle with one eye, even when any amctropia is corrected, 

 though they can do it easily with both eyes. 



The fundamental condition of tri-dimensional \ision \\ith both 

 eyes consists in tlie fact that we look at things from two different 

 I mints of view, so that different perspective 

 images arc formed on the two retinae. 



When tlie images on the t\\o iviinae corre- 

 spond, and the single points of the object fall on 

 identical points, we have no tri-dimensional vision 

 of it. This happens, for instance, on looking at 

 the starry sky, or at any distant object by day- 

 light. \\ hen, on the contrary, we focus an object 

 a short distance off, with both eyes, the two retina 1 

 images differ the more in proportion as the object 

 is closer to the eyes. For instance, the trun- 

 cated pyramid shown in Fig. 194 projects very 

 different perspective images on the two retinae 

 from a short distance, as image It upon the retina of the right eye, 

 image L upon that of the left ; in the first the truncated top of 

 the pyramid is deflected to the left, in the second to the right, 

 These two images are incongruous and not coin- 



Fi<;. I'.iti. Diagram of 

 pyramid with Ha^. 

 (Bernstein.) 



as in Fig. 197. 



R' 



L' 



K 



I i' . l'.7. AW'- : 



I;- 



ami I' 1 '"' 

 . perspecti 



' pi-rspective images K L of a trmiratri] pyramid in reliH. 

 ective images "i .1 imiinw i>\],nni'l /." /.'. 



plctely superposed; if tin- big squares of the bases arc superposed, 

 the small squares of the apex will be only partially superposed, 

 and vice fersa. So that if we accommodate our eyes to distinct 

 vision of the base, we shall see the small square of the apex 

 double; if we accommodate for distinct vision of the apex, the 

 large square of the base appears double, since only identical images 

 which fall upon corresponding points can produce binocular si 



