254 RECENT PROGRESS OF THE THEORY OF VISION. 



which are excited by impressions upon corresponding, or, as 

 Miiller calls them, 'identical' points. This supposition was 

 most fully expressed in the anatomical hypothesis that two nerve 

 fibres which arise from corresponding points of the two retina? 

 actually unite so as to form a single fibre, either at the com- 

 missure of the optic nerves or in the brain itself. I may, how- 

 ever, remark that Johannes Miiller did not definitely commit 

 himself to this mechanical explanation, although he suggested 

 its possibility. He wished his law of identical points to be re- 

 garded simply as an expression of facts, and only insisted that 

 the position in the field of vision of the images they receive is 

 always the same. 



But a difficulty arose. The distinction between the double 

 images is comparatively imperfect, whenever it is possible to 

 combine them into a single view ; a striking contrast to the ex- 

 traordinary precision with which, as Dove has shown, we can 

 judge of stereoscopic relief. Yet the Litter power depends upon 

 the same differences, between the two retinal pictures which 

 cause the phenomenon of double images. The slight difference 

 of distance between the objects represented in the right and left 

 half of a stereoscopic photograph, which suffices to produce the 

 most striking effect of solidity, must be increased twenty or 

 thirty-fold before it can be recognised in the production of a 

 double image, even if we suppose the most careful observation 

 by one who is well practised in the experiment. 



Again, there are a number of other circumstances which 

 make the recognition of double images either easy or difficult. 

 The most striking instance of the latter is the effect of relief. 

 The more vivid the impression of solidity, the more difficult are 

 double images to see, so that it is easier to see them in stereo- 

 scopic pictures than in the actual objects they represent. On 

 the other hand, the observation of double images is facilitated 

 by varying the colour and brightness of the lines in the two 

 stereoscopic pictures, or by putting lines in both which exactly 

 correspond, and so will make more evident by contrast the im- 

 perfect coalescence of the other lines. All these circumstances 

 ought to have no influence, if the combination of the two images 



