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THE SENSES 



side of the visual axis. An examination of figure 476 will show that each 

 point in the visual field, A, B, C, Z), stimulates corresponding points, 

 a, b, c, d, a f , b f , c', d', in the retinas of the two eyes, a, b, c, d, and 

 a' b', c f , d', are corresponding points in the two retinas. When a and a' 

 are stimulated at one and the same time, the resulting sensation is attributed 

 to one object in the visual field, A, and these are corresponding points. This 

 can be shown by pressing one eye out of its normal fixation so that the axes of 



FIG. 476. Diagram Showing the Symmetrical Correspondence of the Retinal Fields. 

 N, Nodal point; F, fovea centralis. The observer is supposed to be looking down upon the 

 optical apparatus from above. Note that the line CD, which is on the lower side of the 

 object, is the upper side of the image; and that the line BD, which is the right side of the 

 object, is the left side of the image, which brings it at the inner segment of the right retina 

 and the outer segment of the left retina. 



the two eyes are not directed toward the same point. If one eye is pressed 

 lightly by the thumb while examining a given object, as soon as the pressure 

 is applied two objects will appear. This phenomenon is known as diplopia. 

 Diplopia is due to the fact that the images of visual objects do not fall on 

 corresponding points in the two retinae. 



The parts of the retinae in the two eyes which thus correspond to each 

 other in the property of referring the images which affect them simulta- 

 neously to the same spot in the field of vision, are, in man, just those parts 

 which would correspond to each other if one retina were placed exactly in 



