752 



THE SENSES 



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 

 front of and over the other, as in figure 477. Thus, as we have noticed in 

 speaking of the distribution of the optic nerve fibers, the temporal portion 

 of one eye corresponds to or is identical with the nasal portion of the other 



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

 TV, 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. 



eye. The upper part of one retina is also identical with the upper part of 

 the other; and the lower parts of the two eyes are identical with each other. 

 The distribution of the optic nerve fibers corresponds with the distribution 

 of the identical points. The identical points on the upper and lower parts 

 of the retinae may also be shown by the following simple experiment. 



Pressure upon any part of the ball of the eye, so as to affect the retina, 

 produces a luminous circle, seen at the opposite side of the field of vision to 

 that on which the pressure is made. If, now, in a dark room, we press with 

 the finger at the upper part of one eye, and at the lower part of the other, 

 two luminous circles are seen, one above the other; so, also, two figures are 



