CHAP, in.] SIGHT. 1291 



crosses over from its nucleus before it leaves the brain. Yet 

 there appears to be nothing special about the behaviour of the 

 superior oblique to account for this feature. 



The Horopter. 



794. When we look at any object we direct to it the visual 

 axes, so that when the retinal image of the object is small, the 



FIG. 157. DIAGRAM ILLUSTRATING A SIMPLE HOROPTER. 



When the visual axes converge at C, the images a a of any point A on the circle 

 drawn through C and the nodal points k h, will fall on corresponding points. 



' corresponding ' parts of the two 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 other images, besides that of the 

 object to which we are specially directing our attention, should fall 

 on ' corresponding ' parts in the two eyes. Were it not so, while 

 our vision of the particular object would be single, our vision of all 

 its surroundings would be double ; and this, at least in certain 

 cases, would be confusing. For, even when we are concentrating 

 our attention on a particular object, we are still conscious of its 

 surroundings, and besides, our appreciation of any image falling 

 on the fovea is influenced by impressions which we are at the 

 same time receiving from other parts of the retina. 



Now for any given position of the eyes there exists in the field 

 of sight 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, in any 



