BINOCULAR VISION. 



351 



to point correspondence has not been determined experimentally. 

 Experiments have shown, however, that the corresponding points 

 in the upper halves of the retinas along the vertical mid-line do 

 not cover each other, that is, they do not lie in the actual anatom- 

 ical vertical meridian, but form two meridians which diverge 

 symmetrically from the mid-line so as to make an angle of about 

 2 degrees (physiological incongruence of the retinas). Within the 

 limits of our powers of observation for ordinary objects we may 

 adopt Tscherning's rule, namely, that when the images of 

 an object on the two retinas are projected to the same side of the 



Fig. 151. Perimeter chart to show the extent of the binocular visual field (shaded area) 

 when the eyes are fixed upon a median point in the horizontal plane. 



point of fixation they are seen single, their retinal images in this 

 case falling on the retina to the same side of the lines of sight ; when, 

 however, the retinal images fall on opposite sides of the lines of 

 sight and are projected to opposite sides of the point of fixation, 

 they are seen double. The doubling of objects that do not fall on 

 corresponding points (physiological diplopia) is most readily 

 demonstrated for objects that lie between the lines of sight, either 

 closer or farther away than the object looked at. If, for instance, 

 one holds the two forefingers in front of the face, in the median 

 plane, one hand being at about the near point of distinct vision 

 and the other as far away as possible, it will be noticed that when 



