STEREOSCOPIC AND PSEUDOSCOPTC VISION. 



20 



cope,* suggest the idea of a. projecting truncated Pyramid, with the 

 small square in the centre, and the four sides sloping equally away 

 from it ; whilst the lower pair, c, D (which are identical with the 

 upper, hut are transferred to opposite sides), no less vividly hring 

 to the mind the visual conception of a receding Pyramid, still with 

 the small square in the centre, but the four sides sloping equally 

 towards it. 



Fig. 16. 



24. Thus we see that by simply crossing the Pictures in the 

 Stereoscope, so as to bring before each eye the picture taken for 

 the other, a ' Conversion of Relief is produced in the resulting solid 

 image ; the projecting parts being made to recede, aud the receding 

 parts brought into relief. In like manner when several objects are 

 combined in the same picture, their apparent relative distances 

 are reversed ; the remoter being brought nearer, and the nearer 

 carried backwards ; so that (for example) a Stereoscopic photo- 

 graph, representing a man standing in front of a mass of ice, shall, 

 by the crossing of the pictures, make the figure appear as if im- 

 bedded in the ice. A like conversion of relief may also be made 

 in the case of actual solid Objects by the use of the Pseudoscope, an 

 instrument devised by Professor Wheatstone, which has the effect 

 of reversing the perspective projections of objects seen through it 

 by the two eyes respectively ; so that the interior of a basin or 

 jelly-mould is made to appear as a projecting solid, whilst the 



• This combination may be made without the Stereoscope, by looking 

 at these figures with the axes of the eyes brought into convergence upon 

 a somewhat nearer point, so that a is made to fall on b, and c on d. 



