7O2 THE SENSES 



and at b' in the other /? will be seen single, for it affects identical parts of 

 the two retinae. The same will apply to the object 7-. 



The reason why the impressions on the identical points of the two retinae 

 give rise to but one sensation, and the perception of but a single image, 

 must either lie in the structural organization and relations of the deeper 

 or cerebral portions of the visual apparatus, or it must be the result of a 

 mental operation; 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 eyes 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 w r hich 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 sensations 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 sensation 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 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 dif- 

 ferent 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 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 stimulation 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 the 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. 



