1126 



VISION, 



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that the amount of difference between ab and cd, which still allowed single 

 vision, varied with the distance between a and b, and was also considerably 

 diminished by practice ; when ab equals 5 - 3 mm., observed at a distance 

 of 16 cm., he found the limit when cd was less than ab to be T84 mm. ; 

 when cd was greater than ab, it was 2 '27 mm. With horizontal lines the 

 amount of disparation consistent with single vision was much less, being 

 under similar conditions, only 42 mm. and 75 mm. Fusion into single 

 vision takes place much more readily with horizontal than with vertical 

 disparation, and consistently with this result he found that a circle fused 

 more easily with a smaller ellipse when the shorter axis of the ellipse 

 was horizontal. From experiments on fusion in different meridians, he 

 found that the tendency for disparate lines to fuse diminished with their 

 deviation from the vertical. 



Wheatstone supposed that it followed, from the occurrence of single 

 vision with disparate points, that double vision could occur with corre- 

 sponding points, and in favour of this described the following experiment. 

 In Fig. 401 the image of the thin line would, in binocular combination, fall 

 in the left eye on points corresponding to those of the thick line in the 

 right eye, but Wheatstone found that the two thick lines fused, giving an 



inclined line, seen in relief 

 crossed by the thin vertical 

 line. This experiment has 

 been used by him and by 

 others as evidence against 

 any necessary physiolog- 

 ical connection between 

 corresponding points. The 

 experiment has been very 

 fully criticised by Hering, 1 

 who has found that if the 

 vertical line presented to the right eye be distinguished by a mark, 

 and care be taken that the two vertical lines are seen with correspond- 

 ing points, and with accurate fixation, the two vertical lines either 

 fuse or can be observed independently of the oblique line. Care has 

 also to be taken to ensure that both eyes are being used. Hering 

 points out that the stereoscopic appearance of the cross exists when it is 

 observed with only one eye, and is especially marked when drawn on 

 glass, there being a natural inclination for two lines crossing each at an 

 acute angle to be seen stereoscopically. Other experiments in favour of 

 double vision with corresponding points, brought forward by Nagel and 

 Wundt, have been similarly shown by Hering to be inconclusive. 



It has been supposed that corresponding points are identical in 

 function, and also that the nerve fibres from corresponding points are 

 connected with the same nerve cell. Single vision with disparate points, 

 and double vision with corresponding points, have both been brought 

 forward as facts inconsistent with identity. If the existence of double 

 vision with corresponding points were proved, the doctrine of identity 

 would have to be given up. Single vision with disparate points, on 

 the other hand, only requires a modification of the doctrine, to the effect 

 that, while single vision is necessary with stimulation of corresponding 

 points, it is possible with that of disparate points. In the latter case the 

 single image is not only in many cases capable of analysis into double 



1 "Beitr. z. Physiologic, " Leipzig, 1862, Heft 2, S. 81. 



FIG. 401. -Wheatstone. 



