372 



THE SPECIAL SENSES. 



muscles, or from an overaction in some of the muscles as con- 

 trasted with their antagonists. 



The Binocular Field of Vision. When the two eyes are fixed 

 upon a given point, placed, let us say, in front of us in the median 

 plane, each eye has its own visual field that may be charted 

 by means of the perimeter. But the two fields overlap for a 

 portion of their extent, and this overlapping area constitutes 

 the field of binocular vision (see Fig. 158). Every point in the bin- 

 ocular field forms an image upon the two retinas. The most 

 interesting fact about the binocular field is that some of the objects 



061 08! 



Fig. 158. 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. 



contained in it are seen single in spite of the fact that there are two 

 retinal images, while others are seen or may be seen double when 

 one's attention is directed to the fact. Whether any given object 

 is seen single or double depends upon whether its image does or does 

 not fall upon corresponding points in the two retinas. 



Corresponding or Identical Points. By definition corre- 

 sponding or identical points in the two retinas are those which when 

 simultaneously stimulated by the same luminous object give us a 

 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 



