THE PERCEPTION OF SIGHT. 251 



convex surfaces are fixed at the top of the instrument which 

 show the pictures somewhat further off, somewhat magnified, 

 and at the same time overlapping each other, so that both appear 

 to be in the middle of the instrument. 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 geometrical outlines of solid 

 figures, such as diagrams of crystals, and also with representa- 

 tions 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 photo- 

 graphs 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 buildings, cities or landscapes, 

 with which I was familiar from stereoscopic pictures, they 

 seemed familiar to me ; but I never experienced this impression 

 after seeing any number of ordinary pictures, because these 

 so imperfectly represent the real effect upon the senses. 



The accuracy of the stereoscope is no less wonderful. Dove 1 

 has contrived an ingenious illustration of this. Take two pieces 

 of paper printed with the same type, or from the same copper- 

 plate, and hence exactly alike, and put them in the stereoscope 



1 Heinrich VVilheltn Dove, Professor in the University of Berlin, author of 

 Optiache Studien (1859) ; also eminent for his researches in meteorology and 

 electricity. 



His paper, Anwenditng des Stereoskops urn falsches von echtem Papiergcld zn 

 untemcheiden, was published in 1859. TB. 



