350 THE SPECIAL SENSES. 



deviation outward; hyperphoria, deviation up or down. The 

 condition may be obviated by prismatic glasses so placed as to 

 aid the weaker muscle. When the lack of balance between the 

 opposing muscles is so great that the visual axes can not be brought 

 to bear upon the same points we have the condition of squint or 

 strabismus. Such a condition may result from a deficiency in 

 strength or in actual paralysis of one or more of the muscles, or from 

 an overaction in some of the muscles as contrasted 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. 151). 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 

 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 twc 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 

 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, 

 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 



