666 



THE SENSES 



seen when pressure is made simultaneously on the outer or the inner sides 

 of both eyes. But if pressure be made with the fingers upon both eyes 

 simultaneously at their lower part, one luminous ring is seen at the middle 

 of the upper part of the field of vision. If the pressure be applied to the upper 

 part of both eyes, a single luminous circle is seen in the middle of the field of 

 vision below. So, also, if we press upon the outer side of one eye and upon 

 the inner side of the other eye, a single luminous spot is produced, and is appar- 



A jb 



FIG. 478. 



FIG. 477. Diagram to Show the Corresponding Parts of the Retinae. 



FIG. 478. Diagram to Show the Simultaneous Action of the Eyes in Viewing Objects in Dif- 

 ferent Directions. 



ent at the extreme right of the field of vision. The hemispheres of the two 

 retinae may, therefore, be regarded as lying one over the other, as in C, figure 

 477; so that the left portion of one eye lies over the identical left portion of 

 the other eye, the right portion of one eye over the identical right portion of 

 the other eye; and with the upper and lower portions of the two eyes, a lies 

 over a', b over b', and c over c'. The points of the one retina intermediate 

 between a and c are again identical with the corresponding points of the other 

 retina between a' and c'; those between b and c of the one retina, with those 

 between V and c' of the other. If the axes of the eyes, A and B, figure 478, 

 be so directed that they meet at a, an object at a will be seen singly, for the 

 point a of the one retina and a' of the other are identical. So, also, if the 

 object ft be so situated that its image falls in both eyes at the same distance 

 from the central point of the retina namely, at b in the one eye and at b' in 

 the other ft will be seen single, for it affects identical parts of the two retinae. 

 The same will apply to the object y. 



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 of the deeper or cerebral por- 

 tions of the visual apparatus, or it must be the result of a mental operation; 



