732 HANDBOOK OF PHYSIOLOGY. 



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 immediate!}' 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 a part of our body in which a sensa- 

 tion 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 ob- 

 jects are mere fractions of the objects themselves realized upon the 

 retina, the extent of which remains constantly the same. But the imagina- 

 tion, which analyzes the sensations of vision, invests the images of ob- 

 jects, 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 remaining unaltered. 



Estimation of Direction. The direction in which an object is 

 seen, depends on the part of the retina which receives the image, and on 

 the distance of this part from, and its relation to, the central point of 

 the retina. Thus, objects of which the images fall upon the same parts 

 of the retina lie in the same visual direction; and when, by the action 

 of the mind, the images or affections of the retina are projected into the 

 exterior world, the relation of the images to each other remains the 

 same. 



Estimation of Form. The estimation of the form of bodies by 

 sight is the result partly of the mere sensation, and partly of the associ- 

 ation 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 sen- 

 sation alone is adequate to the distinction of only superficial forms of 

 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 with 

 regard to the object, and, as shown by AVheatstone and illustrated in the 

 stereoscope, from two different perspective projections of the body being 

 present 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. 



The clearness with which an object is perceived irrespective of accom- 

 modation, would appear to depend largely on the number of rods and 

 cones which its retinal image covers. Hence the nearer an object is to 

 the eye (within moderate limits) the more clearly are all its details 



