CHAP. V.J 



THE STEREOSCOPE. 



285 



coincides with that of the eyes, or that their heights are equal above 

 the base-line. The eye-pieces may be shifted, by help of the screws, 

 either laterally or up and down. The movement of these draw-tubes 

 is intended to bring the photographs into focus. 



Monuments, figures, in short every salient object is depicted in the 

 stereoscope with a wonderful fidelity of relief, which causes complete 

 illusion. But, as Helmholtz justly remarks, 1 " the advantage of 

 stereoscopic vision is most strongly felt in examining reproductions of 

 those objects which cannot be successfully represented in ordinary 



FIG. 213. Helmholtz's stereoscope. 



drawing or painting ; such as irregular rocks, blocks of ice, micro- 

 scopic objects, animals, forests, &c. Glaciers especially, with their deep 

 fissures illuminated transparently through the thickness of the ice, 



1 Physiological Optics. 



