624 HANDBOOK OF PHYSIOLOGY. 



kept perfectly steady, A (Fig. 420) will be the picture presented to the 

 right eye, and B that seen by the left eye. Wheatstone has shown that 

 on this circumstance depends in a great measure our conviction of the 

 solidity of an object, or of its projection in relief. If different perspective 

 drawings of a solid body, one representing the image seen by the right 

 eye, the other that seen by the left (for example, the drawing of a cube, 

 A, B, Fig. 422) 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. 



By transposing two stereoscopic pictures a reverse effect is produced; 

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

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

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

 agined the effect is most bewildering. 



