BINOCULAR VISION 



557 



Left Eye 



Right Eye 



It is evident that the visual angle can figure only when we are dealing 

 with objects which vary but slightly in size, and which are well known to us. 

 But under these circumstances and especially at great distances where accom- 

 modation and convergence can have no part the 

 visual angle is of very great importance. 



As we have already seen (page 534), accom- 

 modation and convergence are very closely con- 

 nected, and convergence occurs in accommodat- 

 ing for near vision even when one eye is covered. 

 Since accommodation is not necessary for vision 

 with emmetropic eyes at distances in the sagittal 

 direction of more than 5 m., and only becomes 

 of great importance at a much smaller dis- 

 tance, it is evident that these two factors can only figure for relatively slight 

 distances. 



If a thread be placed obliquely in the mid line of the body, so that its near 

 end (Fig. 248, a) is higher than its farther end (Z>), one can tell even with an 

 instantaneous flash of light from an electric spark when the eyes have not 

 time to move the correct position of the thread, and it never appears double 

 (Aubert). And yet, as Fig. 249 shows, the images of the ends of the thread do 

 not fall on corresponding points of the retina, and from what we have already 

 learned we should suppose that the thread would produce an impression of two 

 lines lying in the same plane and crossing each other. By looking very sharply, 



FIG. 249. The position of the two 

 images (Fig. 248) on the retinae. 



FIG. 250. 



in fact, one can get such an impression ; this means that we have here an ability 

 which is not inborn, but is acquired by practice and experience. 



It follows from the experiment that the excitation of two dissimilar 

 points of the retina does not always produce a double image, but under some 

 circumstances gives the idea of a single objective point. This point, how- 

 ever, is not in the plane of the fixed point, but lies either in front of or 

 behind it. 



The accuracy with which we can perceive differences of depth by vision 

 with dissimilar points is exceedingly great. According to Heine, for persons 

 endowed with extraordinary acuteness of vision a displacement of the retinal 

 picture of only 6 seconds of an arc (0.0005 mm.) is perceptible. Thus at a 

 distance of 5 m. a displacement in the sagittal direction of 10 mm. would 

 be perceptible, and at a distance of 100 m. a displacement of 20 m. Indi- 



