786 



SIGHT. 



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 optical centres 

 k, k will fall on corresponding points. 



The Horopter. 



686. When we look at any object we direct to it the visual axis, so that 

 when the object is small, the " corresponding " parts of the two retinas, on 

 which the two images of the object fall, lie in their respective fovese cen- 

 trales. But while we are looking at the particular object, the images of other 



objects surrounding it fall on the retina 



FIG. 181. 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 " correspond- 

 ing " parts in the two eyes. Now for any 

 given position of the eyes there exists in 

 the field of vision a certain line or sur- 

 face 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 mat- 

 ter of geometrical calculation. In some 

 instances it becomes a very complicated 

 figure. The case whose features are 

 most easily grasped is a circle drawn 

 in the plane of the two visual axes 

 through the point of the convergence of the axes and the optic centres of the 

 two eyes. It is obvious from geometrical relations that in Fig. 181 the 

 images of any point in the circle will fall on corresponding points of the 

 two retinas. When we stand upright and look at the distant horizon, the 

 horopter is (approximately, for normal emmetropic eyes) a plane drawn 

 through our feet, that is to say, is the ground on which we stand ; the 

 advantage of this is obvious. 



VISUAL JUDGMENTS. 

 



687. Binocular vision is of use to us, inasmuch as the one eye is able 

 to fill up the gaps and imperfections of the other. For example, over and 

 above the monocular filling up of the blind spot, of which we spoke on page 

 779, since the two blind spots of the two eyes, being each on the nasal side, 

 are not " corresponding " parts, the one eye supplies that part of the field 

 of vision which is lacking in the other. And other imperfections are sim- 

 ilarly made good. But the great use of binocular vision is to afford us 

 means of forming visual judgments concerning the form, size, and distance 

 of objects. 



688. Judgment of distance and size. The perceptions which we gain 

 simply and solely by our field of vision concern two dimensions only. We 

 can become aware of the apparent size of any part of the field correspond- 

 ing to any particular object, and of its topographical relations to the rest of 

 the field, but no more. Had we nothing more to depend on, our sight would 

 be almost valueless as far as any exact information of the external world 

 was concerned. By the association of visual sensations with sensations of 

 touch, and with sensations derived from the movements of the eyeballs re- 

 quired to make any such part of the field as corresponds to a particular 



