406 PHYSIOLOGY CHAP. 



images of a and c is not the transverse plane that passes through 

 the fixation point b, as was formerly supposed ; but we see them 

 at their true distances, i.e. in transverse planes, through the points 

 a and c (Helmholtz, Hering). 



Double vision (physiological diplopia) occurs not only for the 

 points lying along the median plane, farther from or nearer to the 

 fixation point (Fig. 191), but also for all other points lying beyond 

 the median plane which are not along the horopter-line that corre- 

 sponds to the position of the double eye. In the latter case, how- 

 ever, the diplopia is less clear and obvious, because the images fall 

 at more eccentric regions of the retina. Generally speaking, it 

 may be said that the double images of objects in space are 

 sharper when they fall on more central regions, increasingly 

 blurred and indistinct as they fall on more peripheral regions, of 

 the two retinae. 



It is a fact of common observation that with the double eye 

 we usually perceive only single images in the binocular field of 

 vision, and merely become aware of physiological diplopia under 

 quite special conditions, although the points in space correspond- 

 ing to the horopter which form images at identical points of the 

 retina are comparatively tew in comparison with the countless 

 spatial objects which form images at non-identical points. Several 

 facts can be cited in explanation of this apparently paradoxical 

 phenomenon. 



(a) We always see the objects singly which we fixate directly 

 and which form images in the central ioveae ; this is regulated by 

 the three adaptive systems of convergence, accommodation, and 

 pupillary reaction. 



(6) The objects that we see singly excite the corresponding 

 points of the retinae, with double energy, and thus produce a more 

 intense impression, as if they were viewed through a single median 

 cyclopic eye, whereas the images from two disparate points are 

 blurred and separated. 



(c) The images formed at identical points of the retinae have 

 an identical 'local sign' of recognition, with which they are 

 intimately associated in consciousness. 



(d) We fix our attention upon the objects seen single because 

 they form sharp, distinct, and perfect images ; while we neglect 

 the objects seen double because they are blurred and indistinct, 

 and readily disappear on moving the eyes. 



If we focus an object lying in the binocular field of vision, and 

 displace one of the eyes slightly with the finger, so that the two 

 visual axes can no longer converge on it, the visual field of the 

 eye that is compressed is also displaced, and all objects, including 

 that fixated, appear double. Theoretically this binocular diplopia 

 should be permanent in every case of strabismus, but ophthalmo- 

 logical experience shows that this is not so actually. In cases of 



