958 THE HOROPTER. [BOOK in. 



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 l 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 given position of the eyes, are projected on to cor- 

 responding points of the retina ; hence its determination in any 

 particular case is simply a matter of geometrical calculation. In 

 some instances it becomes a very complicated figure. The case 

 whose features are most easily grasped is that of a circle drawn 

 in the plane of the two visual axes through the point of the con- 

 vergence of the axes and the nodal points of the two eyes such 

 as is shewn in Fig. 161. It is obvious from geometrical rela- 

 tions that the two images of any point in such circle, the rays 

 from which can enter the two pupils and fall on the two retinas, 

 will fall on corresponding points of the two retinas. When we 

 study the various horopters of the several positions which the 

 two eyes can take up, we find that the characters of the horop- 

 ter are adapted to the needs of our daily life. Thus in the posi- 

 tion assumed by the two eyes when we stand upright and look 

 at the distant horizon the horopter is (approximately, for nor- 

 mal 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. 



Nevertheless, in most positions of the eyes a large number of 

 the images which make up the binocular visual field, do not lie 

 on any horopter, do not fall on corresponding points, and give 

 rise not to one sensation only but to two sensations differing to 

 a certain extent from each other. A great deal of what we see 

 is seen double by us, we receive from many objects two unequal 

 impressions ; but the inequality chiefly serves to give an appear- 

 ance of " solidity " to the objects, to assist in our judgment of 

 solidity. To the consideration of these and other visual judg- 

 ments as well as of some other psychological features of vision 

 we must n >w turn. 



