BINOCULAR VISION. 353 



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- 

 sons suffering from "squint" (strabismus). In this condition the 

 individual, for one reason or another, is unable to adjust the contrac- 

 tions of his eye muscles so as to unite his lines of sight upon the 

 object looked at. The image of the object falls upon non-corre- 

 sponding points and should give double vision, diplopia. This 

 would undoubtedly be the case if the condition came on suddenly ; 

 just as double vision results when we dislocate one eyeball by 

 pressing slightly upon it. But in cases of long standing one of the 

 images, that from the abnormal eye, is usually suppressed. The 

 act of suppression seems to be a case of a stronger stimulus prevail- 

 ing over a weaker one in consciousness, just as a painful sensation 

 from stimulation of one part of the skin may be suppressed by a 

 stronger pain from some other region. 



Struggle of the Visual Fields. When the images of two dis- 

 similar objects are thrown, one on each retina, the mind is presented, 



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