330 



Dr. W. von Bezold on Binocular Vision. 



ropter surface with the horizontal plane. B and B' have been 

 made by my own, V and V by Volkumnn's*, and S and S' by 

 Solger's measurements. C F is half the distance of the fixed 

 point from the middle of the straight line joining the two 

 optical centres. The entire figure is one-sixth of the natural 

 size. Fig. 2 represents the bisection of the two surfaces with 

 the vertical medial plane, one-twelfth of the natural size. These 

 figures show that even with a very moderate distance of the 

 fixed point, the space in which alone double images are perceived 

 is extremely limited. It is, moreover, so placed that under ordi- 

 nary circumstances objects will very seldom be found in it, 

 since, as a matter of fact, we involuntarily endeavour to avoid it. 



If we invert this principle, we arrive at another, which is 

 moreover easily intelligible, that under ordinary circumstances 

 the images of all points fall upon almost identical places. In 

 this the distance of the images 

 corresponding on the double 

 retina to one and the same 

 object will in general be 

 greater in a horizontal than 

 in a vertical direction. As, 

 further, the occurrence of 

 crossed and homonymous 

 images is equally probable, 

 it follows further that the 

 images corresponding to one and the same object fall, on the 

 average, on corresponding points. 



Is not, then, such a continuous simultaneous use of such places 

 for the same object sufficient to explain that peculiar connexion 

 between them the expression of which is the principle of iden- 

 tity ? The correctness of this view is favoured by the circum- 

 stance that the sensibility to double images is far less in a 

 horizontal than in a vertical direction, and that by continuous 

 occupation with experiments on double images it increases with 

 great rapidity. This view is moreover supported by a series of 

 analogies, which are found in the sphere of the sensorial as 

 well as of the motorial activity, I will mention, for instance, 

 the experiment with a marble (which, between crossed fingers, 

 is perceived to be double), the phenomena which are to be ob- 

 served with beginners in pianoforte-playing when they have to 

 play with crossed hands, to the so-called relative accommodation 

 (Donders), and so forth. Perhaps also those cases of inveterate 

 strabismus which have hitherto been regarded as consequences 

 of a so-called incongruence of the retinas may find a natural ex- 

 planation from the above point of view. 



* Given in Volkrnann's paper cited above. 



