658 THE SENSE OF SIGHT. 



retina are projected into the exterior world, the relation of 

 the images to each other remains the same. 



The estimation of the form of bodies by sight is the result 

 partly of the mere sensation, and partly of the association 

 of ideas. Since the form of the images perceived by the 

 retina depends wholly on the outline of the part of 

 the retina affected, the sensation alone is adequate to the 

 distinction of only superficial forms from each other, as of 

 a square from a circle. But the idea of a solid body, as a 

 sphere, or a body of three or more dimensions, e.g., a cube, 

 can only be attained by the action of the mind constructing 

 it from the different superficial images seen in different 

 positions of the eye witK regard to the object; and, as 

 shown by Mr. Wheatstone and illustrated in the stereo- 

 scope, from two different perspective projections of the 

 body being presented simultaneously to the mind by 

 the two eyes. Hence, when, in adult age, sight is sud- 

 denly restored to persons blind from infancy, all objects in 

 the field of vision appear at first as if painted flat on one 

 surface; and no idea of solidity is formed until after 

 long exercise of the sense of vision combined with that of 

 touch. 



We judge of the motion of an object, partly from the 

 motion of its image over the surface of the retina, and 

 partly from the motion of our eyes following it. If the 

 image upon the retina moves while our eyes and our body 

 are at rest, we conclude that the object is changing its 

 relative position with regard to ourselves. In such a case 

 the movement of the object may be apparent only, as when 

 we are standing upon a body which is in motion, such as 

 a ship. If, on the other hand, the image does not move 

 with regard to the retina, but remains fixed upon the same 

 spot of that membrane, while our eyes follow the moving 

 body, we judge of the motion of the object by the sensa- 

 tion of the muscles in action to move the eye. If the 

 image moves over the surface of the retina while the mus- 



