CH. LV. ] 



VISUAL JUDGMENTS. 



789 



eyes, and viewed with each eye successively while the head is 

 kept perfectly steady, A (fig. 599) will be the picture presented 

 to the right eye, and B that seen by the left eye. Wheat- 

 stone 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 



B 



Fig. 600. Diagram to illustrate visual illusions. 



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. 599) 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. 601. Parallel puzzle. 



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 

 pteudpscope. Viewed with this instrument a bust appears as a 

 hollow mask, and as may readily be imagined the effect is most 

 bewildering. 



The clearness with which an object is perceived irrespective of 

 accommodation, would appear to depend largely on the number 

 of rods and cones which its retinal image covers. Hence the 

 nearer an object is to the eye (within moderate limits) the more 



