342 



THE SPECIAL SENSES. 



to point correspondence has not been determined experimentally. 

 Within the limits of our powers of observation for ordinary objects 

 we may adopt Tscherning's rule, namely, that when the images 

 of an object on the two retinas are projected to the same side of the 

 point of fixation they are seen single, their retinal images in this case 

 falling on the retina to the same side of the lines of sight; when, 

 however, the retinal images fall on opposite sides of the lines of sight 

 and are projected to opposite sides of the point of fixation, they are 

 seen double. The doubling of objects that do not fall on correspond- 

 ing points (physiological diplopia) is most readily demonstrated for 

 objects that lie between the lines of sight, either closer or farther away 

 than the object looked at. If, for instance, one holds the two forefin- 

 gers in front of the face, in the median plane, one hand being at about 

 the near point of distinct vision and the other as far away as possible, 



Fig. 149. 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 fovea?, and are seen double, being projected to the plane of A, giving heter- 

 onymous diplopia. In // 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. 



it will be noticed that when the eyes are fixed on the far finger 

 the near one is seen double and vice versa. In this, as in other 

 experiments in which the eyes are accommodated for one object 

 while the attention is directed to another, some difficulty may be 

 experienced at first in dissociating these two acts which normally 

 go together, but a little practice will soon enable one to distinguish 

 clearly the doubling of the point upon which the lines of sight are not 

 converged. 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 belongs 

 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-projection of the 



