THE EYE AS A SENSORY APPARATUS 253 



face, b d c a, in one answers to the large surface, b' d' c' a', in the 

 other. This may be readily observed by holding a small cube in 

 front of the nose and alternately looking at it with each eye. In 

 such cases, then, the retinal images do not correspond, and yet we 

 combine them in consciousness so as to see one solid object. This 

 is known as stereoscopic vision, and the illusion of the common 

 stereoscope depends on it. Two photographs are taken of the same 

 object from two different points of view, one as it appears when 

 seen from the left, and the other when seen from the right. These 

 are then mounted for the stereoscope so that each is looked at by 

 its proper eye, and the object appears in distinct relief, as if, in- 

 stead of flat pictures, solid objects, occupying three dimensions of 

 space, were looked at. 



