290 RECENT PROGRESS OF THE THEORY OF VISIOJ^. 



double images is comparatively imperfect, whenever it is 

 possible to combine them into a single view ; a striking 

 contrast to the extraordinary precision with which, as 

 Dove has shown, we can judge of stereoscopic relief. Yet 

 the latter power depends upon ths 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 produc- 

 tion 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 stereoscopic 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 stereo- 

 scopic pictures, or by putting lines in both which exactly 

 correspond, and so will make more evident by contrast 

 the imperfect coalescence of the other lines. All these 

 circumstances ought to have no influence, if the com- 

 bination of the two images in our sensation depended 

 upon any anatomical arrangement of the conducting 

 nerves. 



Again, after the invention of the stereoscope, a 

 fresh difficulty arose in explaining our perceptions of 

 solidity by the differences between the two retinal 

 images. First, Briicke ' called attention to a series 



* Professor of Physiology in the University of Vienna. 



