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OCULAK MOVEMENTS 



411 





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

 though they can do it easily with both eyes. 



The fundamental condition of tri-dimensional vision with both 

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

 points of view, so that different perspective 

 images are formed on the two retinae. 



When the images on the two retinae 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. \V hen, on the contrary, we focus an object 

 a short distance off, with both eyes, the two retinal 

 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 ft 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, 

 as in Fig. 197. These two images are incongruous and not coin- 



FIG. 196. Diagram of 

 pyramid with flag. 

 (Bernstein.) 



FIG. 197. Above ; right and left perspective images R L of a truncated pyramid in relief. 

 Below ; perspective images of a hollow pyramid R' L'. 



pletely superposed ; if the big squares of the bases are superposed, 

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

 and vice versa. 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 single 



