ISO 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



look at the farther finger we see that one single ; but 

 now the nearer one is doubled heteronymously. Now 

 observe, further, that we are clearly conscious that it 

 requires more convergence, and therefore more effort, 

 to look at the nearer finger and see it single, and less 

 convergence and less effort of the ocular muscles to 

 look at the farther finger and see that single. In 

 other words, we run the point of sight back and forth 

 from one finger to the other by greater and less con- 

 vergence, and thus acquire a distinct perception of dis- 

 tance between the two. It is literally a process of rapid 

 triangulation, with the interocular distance as the base 

 line. The same is true of all objects in space at differ- 

 ent distances if the distance of the nearer one be not 

 too great. 



Experiment 2. — But single objects also occupy 

 depth of space. Take, therefore, next a rod, say a foot 

 long; hold in the median plane, a little below the hori- 

 zontal line, with the nearer end six to eight inches from 

 the face. Looked at with one eye, say the right, the 

 rod is seen projected thus / ; looked at with the left 



eye, \ . Now, it is evident that these two images can 

 not combine. When we open both eyes and look at the 

 farther end, the nearer end is doubled heteronymously, 

 and we see the rod as an inverted V, with the open end 

 toward us, thus /\ ; when we look at the nearer end, 

 the farther end is doubled homonymously, and we see a 

 V with the point toward us, thus \y ; when we look at 

 the middle, we see the two images cross in the middle to 

 make an X, thus Y . Thus we run the point of sight 

 back and forth from one end to the other, by greater 

 and less convergence uniting each point looked at, and 

 acquire thus a distinct perception of the distance be- 



