754 THE SENSES 



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 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 

 atter 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. 



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. 

 Wheatstone has shown that on this circumstance depends in a great measure 

 our conviction of the solidity of an object, or of its projection in relief. If 

 different perspective drawings of a solid body, one representing the image 

 seen by the right eye, the other that seen by the left, for example, the drawing 

 of a cube, A, B, figure 479, be presented to corresponding parts of the two 

 retinae, as may readily be done by means of the stereoscope, the mind will 



