HELMHOLTZ IN HEIDELBERG 



or to the left. We can also look straight onwards, 

 as when we gaze at the horizon, in which case the 

 visual axes are parallel ; or we can look at a nearer 

 object, converging the visual axes so as to cause 

 them to meet at the object under examination. The 

 effect of these movements is always to bring the 

 images upon corresponding parts of the two retinae, 

 and if they fall upon these, and not upon others, 

 then there is single vision ; but if, from various 

 causes, the images fall on other, or non-correspond- 

 ing points, there will be double vision that is to 

 say, we shall see two objects instead of one. The 

 pairs of points that give rise to single vision were 

 termed by Johannes Miiller corresponding points, 

 and the assumption was that from any such pair of 

 points similar nerve-fibres passed to the brain, and 

 were possibly there so united as to give rise to the 

 consciousness of a single object. 



Further, it has long been known that the retina 

 of each eye is related to both sides of the brain, or, 

 to put it conversely, each side of the brain is related 

 to both eyes. If the optic nerves are traced back- 

 wards, they are found to unite in the well-known 

 optic commissure, and from the latter two great bands 

 of fibres, termed the optic tracts, carry the nervous 

 impulses to the brain. The whole of the nerve-fibres 

 from the retina of each eye do not, however, cross or 

 decussate in the commissure in the human mechan- 

 ism, as was at one time supposed, but they do so in 

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