282 HUMAN PHYSIOLOGY 



located in the same direction from the foveae centralis (see 

 Fig. 37)- 



I r 



FIG. 37. IDENTICAL POINTS ON THE RETINAE. 



The right (r) and the left (/) retina are divided into the quadrants I, 2, 3, and 

 4 by the perpendicular and horizontal lines drawn through the foveae c . If the 

 points c and the corresponding dividing lines are placed over each other, every 

 point of one retina will be covered by its identical point of the other retina. 



A luminous point whose image does not fall on identical 

 points of the retinae is seen double. 



If identical points of the retinae are stimulated by different 

 objects, the two objects are not seen simultaneously, but first 

 one and then the other is seen, according to whether the 

 attention is first fixed upon the one or upon the other. This 

 is called the struggle of the two fields of vision [Wettstreit 

 der Sehfelder]. 



For a given position of the eyes the field in which all 

 points are seen as single points is called the horopter. 



To find the horopter, draw lines from a pair of identical points 

 through the nodal points ; the point where these two lines cross is 

 seen as a single point. All the points thus found form the horopter 

 for this given position of the eyes. 



2. Perception of solidity. By binocular vision we can 

 see an object from two different directions. Hence the posi- 

 tion of the object is where the two visual lines cut each 

 other. If a solid object is viewed with both eyes, two dis- 

 tinct images of the object are formed upon the retinae because 

 the two eyes view the object from two different points of 

 view. Hence the images falling upon identical points of the 

 retinae are not the same. Consequently only a part of the 

 points of the observed object appear as single points; the 

 others are seen double. This gives us the impression of a 

 solid body. 



