BINOCULAR VISION. 373 



images of the point observed shall fall upon symmetrical parts of the 

 two foveae; the lines of sight of the two eyes converge upon and 

 meet in the point looked at. If, while observing an object, we press 

 gently upon one eyeball with the end of the finger, two images are 

 seen at once, and they diverge farther and farther from each other 

 as the pressure upon the eyeball is increased. Experiment shows, 

 also, that, in a general way, portions of the retina symmetrically 

 placed to the right side of the foveae in the two eyes are cor- 

 responding, and the same is true for the two left halves and the two 

 upper and lower halves. The right half of the retina in one eye is 

 non-corresponding to the left half of the other retina, and vice 

 versa; and the same relation is true of the upper and lower halves, 

 respectively. If we imagine one retina to be lifted without turning 

 and laid over the other so that the fovese and vertical and horizontal 

 meridians coincide, then the corresponding points will be superposed 

 throughout those portions of the retina that represent the binocular 

 field. This statement, however, is theoretical only ; an exact point 

 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 

 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 

 the eyes are fixed on the far finger the near one is seen double 

 and vice versa. In this, as in other experiments in which the eyes 

 are accommodated for one object while the attention is directed 

 to another, some difficulty may be experienced at first in disso- 

 ciating these two acts which normally go together, but a little 

 practice will soon enable one to distinguish clearly the doubling 

 of the point upon which the lines of sight are not converged. 



