286 RECENT PROGRESS OF THE THEORY OF VISION. 



lower part of the stereoscope, which is divided by a 

 partition s. Two slightly prismatic glasses with convex 

 surfaces are fixed at the top of the instrument which 

 show the pictures somewhat further off, somewhat mag- 

 nified, and at the same time overlapping each other, 

 so that both appear to be in the middle of the instru- 

 ment. The section of the double eye-piece shown in 

 Fig. 37 exhibits the position and shape of the right and 

 left prisms. Thus both pictures are apparently brought 

 to the same spot, and each eye sees only the one which 

 belongs to it. 



The illusion produced by the stereoscope is most 

 obvious and striking when other means of recognising 

 the form of an object fail. This is the case with geo- 

 metrical outlines of solid figures, such as diagrams of 

 crystals, and also with representations of irregular objects, 

 especially when they are transparent, so that the shadows 

 do not fall as we are accustomed to see them in opaque 

 objects. Thus glaciers in stereoscopic photographs often 

 appear to the unassisted eye an incomprehensible chaos 

 of black and white, but when seen through a stereoscope 

 the clear transparent ice, with its fissures and polished 

 surfaces, comes out as if it were real. It has often 

 happened that when I have seen for the first time build- 

 ings, cities or landscapes, with which I was familiar 

 from stereoscopic pictures, they seemed familiar to 

 me ; but I never experienced this impression after see- 

 ing any number of ordinary pictures, because these 

 but so imperfectly represent the real effect upon the 

 senses. 



