CHAP, ii.] 



SIGHT. 



547 



right side producing movements of both eyes to the left, of the left 

 side movements to the right ; while stimulation in the middle line 

 behind causes a downward movement of both eyes with convergence 

 of the axes, and in the front an upward movement with return to 

 parallelism, both accompanied by the naturally associated move- 

 ments of the pupil. Stimulation of various parts of the nates causes 

 various movements, depending on the position of the spot stimu- 

 lated. After an incision in the middle line, stimulation of the 

 nervous centre on one side produces movements in the eye of the 

 same side only. 



The Horopter. 



When we look at any object we direct to it the visual axes, so 

 that when the object is small, the ' corresponding ' parts of the two 



FIG. 76. DIAGRAM ILLUSTRATING A SIMPLE HOROPTEK. 



\Yhen the visual axes converge at C, the images a a of any point A on the 

 circle drawn through C and the optical centres k k, will fall on corresponding points. 



retinas, on which the two images of the object fall, lie in their 

 respective fovese centrales. But while we are looking at the 

 particular object the images of other objects surrounding it fall 

 on the retina surrounding the fovea, and thus go to form what 

 is called indirect vision. And it is obviously of advantage 

 that these images also should fall on 'corresponding' parts in 

 the two eyes. Now for any given position of the eyes there exists 

 in the field of vision a certain line or surface of such a kind 

 that the images of the points in it all fall on corresponding points 

 of the retina. A line or surface having this property is called 

 a Horopter. The horopter is in fact the aggregate of all those 

 points in space which are projected on to corresponding points 

 of the retina; hence its determination in any particular case 

 is simply a matter of geometrical calculation. In some instances it 



352 



