CHAP. V.] 



THE STEREOSCOPE. 



287 



images, and produce the effect which would be given by the two 

 stereoscopic views A' and A". Now these images, which are super- 

 posed by the effect of the apparatus, are views of a pyramid similar to 



FIG. 215. Direct and inverse stereoscopic vision : relief and hollow. 



the first one, illuminated by the same light, but appearing hollow in- 

 stead of in relief, since in the right eye the left side is enlarged by 

 the perspective, and the contrary effect is felt in the left eye. The 



FIG. 216 Fseudoi-eopic vision : medallion of Moliere. 



effect of the pseudoscope is produced naturally when we are looking 

 at drawings in which the shadows are well denned, as in medallions. 

 An object of this kind will te seen now in relief, now concave. One 

 or other sensation is easily obtained at will, if care is taken to place 

 the drawing in such a position with regard to the light that the shadows 



