374 



THE SPECIAL SENSES. 



If a long stick is held horizontally in front of the eyes the end 

 near the face will be doubled when the eyes are directed to the 

 far end and vice versa. Moreover, by a simple experiment it 

 may be shown that objects nearer the eyes than the point looked at 

 are doubled heteronymously, that is, the right-hand image be- 

 longs to the left eye and the left-hand one to the right eye. This is 

 easily demonstrated by closing the eyes alternately and noting 

 which of the images disappears. The reason for the cross-projec- 

 tion of the images is made apparent by the construction in Fig. 159, 

 7, 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 6' 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. 159, //, 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 



Fig. 159. Diagrams to show homonymous and heteronymous diplopia: In / the eyes 

 are focused on A; the images of B fall on non-corresponding points, that is, to different 

 sides of the fovese, and are seen double, being projected to the plane of A, giving heter- 

 onymous diplopia. In II the eyes are focused on the nearer point, A, and the farther point, 

 B, forms images on non-corresponding points and is seen double, homonymous diplopia, 

 the images being projected to the focal plane A. 



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 



