STEREOSCOPIC VISION. 



533 



the eyes; it will be seen double. For every given position of 

 the eyes there is a surface in space, all objects on which 

 produce images on corresponding points of the two retinas: 

 this surface is called the lioropter for that position of the 

 eyes: all objects in it are seen single; all others in the visual 

 field, double. 



The Perception of Solidity. When a solid object is 

 looked at the two retinal images are diiferent. If a trun- 

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

 that represented at P 9 Fig. 142. If, however, it be held 

 midway 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' c a, in the other. This may be readily observed by 



FIG. 142 



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 

 so as to sec 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 by the left, and the other when looked at with the 

 right eye. These are then mounted for the stereoscope so 

 that each is seen by its proper eye, and the scene or object 

 is seen in distinct relief, as if, instead of flat pictures, 

 solid objects were looked at. Of course in many stereo- 

 scopic views the distribution of light and shade, etc., assist, 

 but these are quite unessential, as may be readily observed 

 by enlarging the middle and right outline drawings of Fig. 



