VISUAL JUDGMENTS 667 



for in no other case is it the property of corresponding nerves of the two 

 sides of the body to refer their sensations to one spot. 



Many attempts have been made to explain this remarkable relation be- 

 tween the eyes, by referring it to anatomical relation between the optic nerves. 

 The circumstance of the inner portion of the fibers of the two optic nerves 

 decussating at the commissure, and passing to the eye of the opposite side, 

 while the outer portion of the fibers continue their course to the eye of the 

 same side, so that the left side of both retinae is formed from one root of the 

 nerves, and the right side from the other root, naturally led to an attempt 

 to explain the phenomenon by this distribution of the fibers of the nerves. 

 And this explanation is favored by cases in which the entire half of one 

 side of the retina sometimes becomes insensible. 



Visual Judgments. Form and Solidity. The estimation of the 

 form of bodies by sight is the result partly of the visual sensation and partly 

 of the association of ideas. The form of the image perceived by the retina 

 depends wholly on the outline of the part of the retina affected; the sensa- 

 tion alone is adequate only to the distinction of superficial forms from each 

 other which lie in one plane, as of a square from a circle. But the idea of a 

 solid body, as a sphere, or a body of three or more surfaces, e.g., a cube, 

 can be attained only by the action of the mind in constructing it from the dif- 

 ferent 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 simul- 

 taneously 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 

 accommodation, would appear to depend largely on the definiteness of stimu- 

 lation of the rods and cones which its retinal image covers. Hence, the nearer 

 an object is to the eye, within the limits of vision, the more clearly are all 

 its details seen. Moreover, if we want carefully to examine any object, we 

 always direct the eyes straight toward it, so that its image shall fall on the 

 yellow spot, which has already been shown to be the area of most acute vision. 



In binocular vision the images of an object, while they fall in approxi- 

 mately corresponding points on the two retinae, are never absolutely the same. 



When an object is placed so near the eyes that to view it the optic axes 

 must converge, a different perspective projection of it is seen by each eye, 

 these perspectives being more dissimilar as the convergence of the optic axes 

 becomes greater. Thus, if any figure of three dimensions, an outline cube, 

 for example, be held at a moderate distance before the eyes, and viewed with 

 each eye successively while the head is kept perfectly steady, A, figure 479> 

 will be the picture presented to the right eye, and B that seen by the left eye. 



