692 SPECIAL SENSES 



Corresponding Points. While it is evident, after the statements just 

 made, that images must be formed on the fovea of each eye in order 

 to produce the effect of a single object, it becomes important to ascer- 

 tain how far it is necessary that the correspondence of points should 

 be carried out in the retina. It is almost certain that for absolutely 

 perfect single vision with the two eyes, the impressions must be made 

 on exactly corresponding points, even to the ultimate sensitive elements 

 of the retina. It may be assumed, indeed, that each rod and each 

 cone of one eye has its corresponding rod and cone in the other, situated 

 at exactly the same distance and in corresponding directions from the 

 visual axis. When the two images of an object are formed on these 

 corresponding points, they appear as one ; but when the images do not 

 correspond, the impression is as though the images were formed on 

 different points in one retina, and of necessity they appear double. 



The effect of a slight deviation from the corresponding points may 

 be illustrated by the following experiment : If a small object, like a 

 lead-pencil, held at a distance of a few inches, is fixed with the eyes, it is 

 seen distinctly as a single object. Holding another small object in the 

 same line, a few inches farther removed, when the first is seen distinctly, 

 the second appears double. If the second object is fixed with the eyes, 

 the first appears double. It is evident here, that when the axes of the 

 eyes bear on one of these objects, the images of the other must be 

 formed at a certain distance from the corresponding retinal points. 



The Horopter. The above-mentioned experiment affords an expla- 

 nation of the horopter. If both eyes are fixed on a point directly in 

 front and are kept in this position, an object moved to one side or the 

 other, within a certain area, may be seen without any change in the 

 direction of the axis of vision ; but the distance from the eye at which 

 there is single vision of this object is fixed, and at any other distance 

 the object appears double. The explanation of this is that at a certain 

 distance from the eye, the images are formed on corresponding points 

 in the retina ; but at a shorter or longer distance, this can not occur. 

 This illustrates the fact that there are corresponding points in a large 

 part of the sensitive layer of the retina as well as in the fovea centralis. 

 By these experiments the following facts have been ascertained : With 

 both eyes fixed on an object, another object moved to one side or the 

 other can be seen distinctly only when it is carried in a certain curved 

 line. On either side of this line, the object appears double. This line, 

 or area for the line may have any direction is called the horopter. 

 It was supposed at one time to be a regular curve, or a portion of a 

 circle drawn through the fixed point and the points of intersection of 

 the rays of light in each eye. Although it has been ascertained that 



