THE SENSES. 



233 



Fig. 391), be presented to corresponding parts of the two retinae, as may 

 be readily done by means of the stereoscope, the mind will perceive not 

 merely a single representation of the object, but a body projecting in 

 relief, the exact counterpart of that from which the drawings were made. 



FIG. 391. 



By transposing two stereoscopic pictures a reverse effect is produced: 

 the elevated parts appear to be depressed, and vice versd. An instru- 

 ment contrived with this purpose is termed a pseudoscope. Viewed with 

 this instrument a bust appears as a hollow mask, and as may readily be 

 imagined the effect is most bewildering. 



