584 STUDIES IN ADVANCED PHYSIOLOGY. 



for the blind spot of one eye are very sensitive points of the 

 other. In a similar way smaller pathological defects in the 

 retina of one eye are more or less fully counteracted by the 

 other. 



Second. We are enabled to look at the same object from 

 two different points of view, and so can see more of the ob- 

 ject than with one eye alone. This vision from two differ- 

 ent points gives rise to the perception of solidity. One may 

 easily satisfy himself that notions of the solidity of objects 

 are very materially dependent upon one's seeing them with 

 both eyes. The stereoscope is based upon this principle. 

 Two views are taken of the same scene from points some 

 distance apart. These two figures are then mounted in 

 such a way that one eye sees one photograph, the other eye 

 the photograph from the other corresponding position. In 

 this way there arises the sensation of solidity to such an ex- 

 tent that we actually seem to see certain portions of the pic- 

 ture projecting in front of the others. Of course the two 

 views taken must not be from remotely removed points. In 

 that case the mind is unable to blend the two pictures into 

 one, and recognizes them as two distinct pictures. 



When two entirely distinct scenes or objects are brought 

 one to one eye and one to the other, the two eyes seem to 

 strive for the mastery, and sometimes we see one picture, 

 then suddenly the other. This contest of the two pictures 

 is not really a process of the eye ; possibly each eye sends 

 the image of the picture before it to the brain, but the brain, 

 unable to look at two different things at the same time, 

 takes cognizance of one, then of the other, and so on. 

 Such a result naturally occurs in persons who are cross- 

 eyed. . In such individuals the corresponding points of the 

 retina are placed entirely awry, and each eye produces its 

 own picture. Only one of these pictures rises into full con- 

 sciousness, or, as we say, cross-eyed persons select one eye 

 to look with. This frequently remains the same. Some- 

 times the attention is shifted to the other eye, which then 

 becomes the seeing one. 



