STERESOCOPES. 323 



views of a given pile of wooden blocks seen close at hand with 

 the two eyes respectively ; when these two different views are 

 looked at through a stereoscope they blend together, and the eyes 

 jointly convey to the brain the same sensations as would be produced 

 by viewing the actual solid pile. Similarly, fig. 164 represents the 

 corresponding views of an 

 analogous out-door lands- 

 cape. Fig. 165 represents 

 one of the simplest forms of 

 stereoscope used for produc- 

 ing this effect. 



Wheatstone's reflecting 

 stereoscope is represented by 

 fig. 166; here the two pictures 

 are arranged parallel to one 

 another, and are viewed by 

 reflection from two mirrors 

 placed at right angles to one 

 another between the two pic- 

 tures, so that the reflected 

 rays from each pass forward to 

 the right and left eyes respec- 

 tively of the spectator. In 

 this case the two pictures can- 

 not be simply those that 

 would be obtained by photo- 

 graphing from two spots a 

 little apart ; one must be re- 

 versed in position, as it would 

 be seen in a mirror or in a 

 transparency viewed from the 

 other side. Fig. 167 illus- 

 trates this ; the arrows A and 

 B are obviously arranged in 

 inverted directions, but on re- 

 flection from the two inclined 

 mirrors, abc, each is seen at I 

 by the right and left eyes situ- 

 ated at R and L respectively ; 

 the two superposed slightly 

 different images thus seen 

 simultaneously producing the sensation of perspective. 



Expt. 357. The Pseudoscope. When an observer regards his 

 image in a looking glass, everything appears to be reversed, the 



