BINOCULAR VISION. 803 



object. When our vision is perfectly normal, the sensation of the situation of any single 

 object is referred to one and the same point ; and we cannot receive the impression of a 

 double image unless the conditions of vision be abnormal. 



Corresponding Points. While it requires no argument, after the statements we have 

 just made, to show that an image must be formed upon the fovea of each eye in order 

 to produce the effect of a single object, it becomes important to ascertain how far it is 

 necessary that the correspondence of points be carried out in the retina. This leads to 

 considerations of very great interest and importance. It is almost certain that, for abso- 

 lutely perfect, single vision with the two eyes, the impressions must be made upon ex- 

 actly corresponding points, even to the ultimate sensitive elements of the retina. We 

 may suppose, 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 in corresponding directions 

 from the visual axis. When the two images of an object are formed upon these correspond- 

 ing points, they appear as one ; but, when the images do not correspond, the impression 

 is as though the images were formed upon different points in one retina, and, of neces- 

 sity, they appear double. 



The effect of a slight deviation from the corresponding points may be illustrated by 

 the following experiment : We fix a small object, like a lead-pencil, held at a distance of 

 a few inches, with the eyes, and see it distinctly as a single object ; we hold in the same 

 line, a few inches farther removed, another small object ; when the first is seen distinctly, 

 the second appears double ; we fix the second with the eyes, and the first appears double. 

 It is evident here, that, when the axes of the eyes bear upon 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 enables us to understand the situa- 

 tion of the horopter. If we fix both eyes upon any object directly in front and keep 

 them in this position, a similar 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 dis- 

 tance from the eye at which we have single vision of this second object is fixed, and, at 

 any other distance, the object appears double. The explanation of this is, that, at a cer- 

 tain distance from the eye, the images are formed upon corresponding points in the retina; 

 but, at a shorter or longer distance, this cannot occur. This illustrates the fact that there 

 are corresponding points throughout 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 upon an object, another object moved to one side or the other can be 

 distinctly seen 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, 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 the line varies somewhat from a regular 

 curve, and also varies in different meridians, this is due to differences in refraction, etc., 

 and the principle is not altered. 



It is undoubtedly true that education and habit have a great deal to do with the cor- 

 rection of visual impressions and the just appreciation of the size, form, and distance of 

 objects. If we may credit the account of the remarkable case of Caspar Ilaiiscr. who is 

 said to have been kept in total darkness and seclusion, from the iviv of live months until 

 he was nearly seventeen years old, the appreciation of size, form, and distance is acquired 

 by correcting and supplementing the sense of sight by experience, even in binocular ril 

 This boy at first had no idea of the form of objects, or of distance, until he had teamed 

 by touch, by walking, etc., that certain objects were round, others square, and had actually 



