222 HAND-BOOK OF PHYSIOLOGY. 



the object. In the sense of vision, on the contrary, the images of objects 

 are mere fractions of the objects themselves realized upon the retina, the 

 extent of which remains constantly the same. But the imagination, which 

 analyzes 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, 

 jor 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 Wheatstone and illustrated in the stereoscope, 

 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 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 com- 

 bined 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 seen. 

 Moreover, if we want carefully to examine any object, we always direct 

 the eyes straight to it, so that its image shall fall on the yellow spot where 

 an image of a given area will cover a larger unmber of cones than any- 

 where else in the retina. It has been found that the images of two points 

 must be at least y^-fjj-Q- in. apart on the yellow spot in order to be dis- 

 tinguished separately; if the images are nearer together, the points appear 

 as one. The diameter of each one in this part of the retina is about 



12ooO in ' 



Estimation of Movement. We judge of the motion of an object, 



