THE EYE AS A SENSORY APPARATUS. 



555 



looked at the two retinal images are different. If a truncated 

 pyramid be held in front of one eye its image will be that 

 represented at P, Fig. 161. If, however, it be held midway 



FIG. 161. 



between the eyes, and looked at with both, then the left-eye 

 image will be that in the middle of the figure, and the right- 

 eye image that to the right. The small surface, bdca, in 

 one answers to the large surface, V d f 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, instead of flat 

 pictures, solid objects, occupying three dimensions of space, 

 were looked at. Of course in many stereoscopic views the dis- 

 tribution of light and shade, etc., assist, but these are quite 

 unessential, as may be readily observed by copying the draw- 

 ings of Fig. 161 and mounting them on a card the size of a 

 stereoscopic slide, and placing it in the instrument. A solid 

 pyramid standing out into space will be distinctly perceived ; 

 if the pictures be reversed the pyramid appears hollow. The 

 pictures must not be too different, or their combination to give 

 the idea of a single solid body will not take place. Many 

 persons, indeed, fail entirely to get the illusion with ordinary 

 stereoscopic slides. The phenomena of stereoscopic vision 

 militate strongly against the view that there are any anatom- 

 ically prearranged corresponding points in the two retinas. 

 The Perception of Shine. When we look at a rippled 



