BINOCULAR VISION. 



341 



single sensation, while non-corresponding points are those which 

 when so stimulated give us two visual sensations. It is evident, 

 from our experience, that the fovese form corresponding points or 

 areas. When we look at any object we so move our eyes that the 

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

 f ovea ; 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, 



Fig. 148. 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. 



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

 placed to the right side of the fovese 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 foveae 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 



