;8S 



THE BYE AND VISION. 



[CH. LV. 



The action of the sense of vision in relation to external objects 

 is, therefore, quite different from that of the sense of touch. The 

 objects of the latter sense are immediately present to it ; and our 

 own body, with which they come in contact, is the measure of 

 their size. The part of a table touched by the hand appears as 

 large as the part of the hand receiving an impression from it, for 

 the part of our body in which a sensation is excited, is here the 

 measure by which we judge of the magnitude of the object. In 

 the sense of vision, on the contrary, the images of objects are 

 mere fractions of the objects themselves, realised upon the retina, 

 the extent of which remains constantly the same. But the 

 imagination, which analyses the sensations of vision, invests the 

 images of objects, together with the whole field of vision in the 

 retina, with very varying dimensions ; the relative size of the 

 image in proportion to the whole field of vision, or of the affected 

 parts of the retina to the whole retina, alone remains unaltered. 



Fig. 599. Diagrams to illustrate how a judgment of a figure of three dimensions is obtained . 



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 superficial 

 forms from each other, as of a square from a circle. But 

 the idea of a solid body like a sphere, or 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 

 with regard to the object, and, as shown by Wheatstone and 

 illustrated in the stereoscope, from two different perspective pro- 

 jections of the body being presented simultaneously to the mind 

 by the two eyes. Hence, when, in adult age, sight is suddenly 

 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. 



Thus, if a cube is held at a moderate distance before the 



