328 Dr. W. von Bezold on Binocular Vision. 



surface in two points. It is here presupposed, as is at once 

 evident, that, by the so-called muscular feeling, we are at all times 

 cognizant of the position of our eyes. 



In order now to explain the production of the perception of 

 solidity from this point of view, the assumption was made that 

 in cases in which a small displacement of the images on the 

 retina out of the surface of projection is sufficient to obtain a 

 single image, such a displacement does actually take place ; hence 

 in such cases the lines of projection should be drawn to their bi- 

 secting points. But as single vision can take place notwithstand- 

 ing the use of prisms (which occasion a considerable deviation 

 from the normal position of the eyes) — as, further, stereoscopic 

 images coincide within very wide limits without reference to the 

 position of the eyes (that is, with or without the use of an instru- 

 ment), — in order to explain this case also, the further assumption 

 must be made, that under these circumstances the muscular 

 feeling becomes inactive, is " alienated." 



Thus, as regards simplicity, the projection theory has no ad- 

 vantage over the old theory of identity. For even if, like the 

 latter, it can explain the occurrence of double images, it needs 

 for this the hypothetical surfaces of projection. That such an 

 assumption, however, is totally unjustifiable follows most clearly 

 from the fact that it is never possible to decompose into two an 

 after-image produced by binocular fixation, even if its apparent 

 magnitude proves most decisively that it is placed in thought 

 far before or behind the point where the optic axes intersect. 



In order to explain the perception of solidity, the theory of pro- 

 jection, just like the theory of identity, must abnegate complete 

 precision, and admit exceptional cases. In order to explain 

 stereoscopic phenomena, as well as those which occur with the 

 use of prisms, it again needs an auxiliary hypothesis. Lastly, 

 the whole theory depends on the assumption of a highly deve- 

 loped muscular feeling — an assumption which is by no means 

 supported by the well-known experiments on judging distances in 

 cases in which we have to depend on this muscular feeling alone. 



The theory of projection has substituted for an extremely 

 simple if not fully proven principle a series of hypotheses ; so 

 that it is surprising how its establishment could ever be re- 

 garded as a step in advance. 



These considerations led the author to attempt, by a few 

 slight modifications, to bring the principle of identity into ac- 

 cordance with facts. Panum* and Volkmannf have already 

 investigated how considerable the deviations of the images from 

 identical places may be, and yet allow of single perception. 



* Das Sehen mit zwei Augen. Kiel, 1858. 



f Archiv fur Ophthalmologic, vol. v. part 2. p. 1. 



