146 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



points. Why we see double all objects nearer and 

 farther off than the point of sight, and differently 

 double in the two cases, is shown in the diagram 

 (Fig. 91). Let A, B, and C be three objects in the 

 median plane, and the eyes Ji and L be fixed on A. The 

 images of A will fall on the central spots and be seen 

 single ; but the images of B will fall on the two nasal 

 halves, b b ', but all points in these are non-correspondent 

 and therefore B will be seen double. Similarly C will 

 be seen double because its images fall on the two tem- 

 poral halves. The kind of doubling in each case may 

 be shown by referring all the external images to the 

 plane of sight, P P It is then seen that the images 

 bb' of B are homonymous, while the images c c' of C are 

 heteronymous. That is, as we before found, objects 

 nearer than the point of sight are doubled heterony- 

 mously while objects farther than the point of sight are 

 doubled homonymously. 



Horopteric Circle. — As already shown, objects be- 

 yond or on this side of the point of sight are seen double. 

 But how is it with points about the same distance, but 

 right or left, or above or below that point ? Take first 

 right and left. Let R and L (Fig. 92) be the two eyes 

 and A the point of sight. Draw a circle through A and 

 through the nodal points n ri. This is the horopteric 

 circle, or circle of single vision, of Muller. For if the 

 eyes be fixed on A, any object at that point will be seen 

 single because its images are on the central spots a a', 

 but at the same time B or any other point in the circle 

 will also be seen single because its images will fall on 

 bb', which are obviously corresponding points. But this 

 is not true of any point B' in the plane P P. 



Horopter. — We have taken points right and left. 

 If there be also points above and below seen single at 

 the same time, then there would be a surface of single 



