BINOCULAR VISION. 343 



images is made apparent by the construction in Fig. 149, /, bearing 

 in mind the essential fact that in projecting our retinal images we 

 always project to the plane of the object upon which the eyes are 

 focused. In the figure the eyes are converged on A; the images of 

 point B fall to opposite sides of the line of sight and are seen 

 double and are projected to the plane of A, the image on the 

 right eye being projected to b f on the left of A and that on the left eye 

 to 6 on the right of A. In a similar way it may be shown that ob- 

 jects farther away from the eye than the point looked at are doubled 

 homonymously, that is, the right-hand image belongs to the right 

 eye, and the left-hand one to the left eye. The fact is explained by 

 the construction in Fig. 149, //, in which A is the point converged 

 upon and B the more distant object. In all binocular vision, there- 

 fore, the series of objects between the eye and the point looked at are 

 doubled heteronymously, and those extending beyond the point in 

 the same line are doubled homonymously. Normally we take no 

 conscious notice of this fact, our attention being absorbed by the 

 object upon which the lines of sight are directed. Some physi- 

 ologists, however, have assumed that the knowledge plays an im- 

 portant part subconsciously in giving us an idea of depth or per- 

 spective, an immediate perception, as it were, of the distinction 

 between foreground an background. It is usually assumed that the 

 explanation of corresponding points is to be found in the anatomical 

 arrangement of the optic nerve fibers. Those from the right halves 

 of the two retinas, which are corresponding halves, unite in the 

 right optic tract and are distributed to the right side of the brain, 

 while the fibers from the left halves go to the left side of the brain. 

 The basis of the single sensation from two visual images is to be 

 found probably in the fact that the cerebral terminations through 

 which the final psychical act is mediated lie close together or possibly 

 unite. 



The Horopter. In every fixed position of the eyes there are 

 a certain number of points in the binocular field which fall 

 upon corresponding points in the two retinas and are therefore 

 seen single. The sum of these points is designated as the horopter 

 for that position of the eyes. It may be a straight or curved line, 

 or a plane or curved surface. Helmholtz calls attention to the fact 

 that, when standing with our eyes in the primary position, that 

 is, directed toward the horizon, the horopter is a plane coinciding 

 with the ground, and this fact may possibly be of service to us in 

 walking. 



Suppression of Visual Images. It happens not infrequently 

 that when an image of an object falls upon non-corresponding 

 points in the two retinas the mind ignores or suppresses one of the 

 images. This peculiarity is exhibited especially in the case of per- 



