THE SENSES. 613 



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 asso- 

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

 depends wholly OH 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 stereo- 

 scope, from two different prospective projections of the body being pre- 

 sented 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 sur- 

 face; 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 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 number of cones 

 than anywhere else in the retina. It has been found that the images of 

 two points must be at least Tsfanr i n - a P art on the yellow spot in order 

 to be distinguished separately; if the images are nearer together, the 

 points appear as one. The diameter of each cone in this part of the ret- 

 ina is about T^-JoT in. 



Estimation of Movement. 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 fol- 



