92 VISION WITH THE COMPOUND M1CKOSCOPE 



tively, these being thrown on the two retina' in the precise positions 

 they would have occupied if formed there direct from the solid 

 object, of which the mental image (if the picture li;i\ been correctly 

 taken) is the precise counterpart. Thus in fig. 69 the upper pair of 

 pictures (A,B) when combined in the stereoscope suggest the idea of 

 & projecting truncated pyramid, with the small square in the centre 

 and the four sides sloping equally away from it ; whilst the combi- 

 nation of the lower pair, C, D (which are identical with the upper. 

 but are transferred to opposite sides), no less vividly brings to tin- 

 mind the visual conception of a receding pyramid, still with the si MM 11 

 square in the centre, but the four sides sloping equally towards it. 



Thus we see that by simply crossing the pictures in the stereo- 

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

 a * con version of relief ' is produced in the resulting solid image, 

 the projecting parts being made to recede and the receding part> 

 brought into relief. In like manner, when several objects are com- 



FIG. 9. 



bined in the same crossed pictures, their apparent relative distances- 

 are reversed, the remoter being brought nearer and the nearer 

 carried backwards ; so that (for example) a stereoscopic photograph 

 representing a man standing in front of a mass of ice shall, by the 

 crossing of the pictures, make the figure appear as if imbedded 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 exterior is made to 

 appear hollow. Hence it is now customary to speak of stereoscopic 

 vision as that in which the conception of the true natural relief of an 

 object is called up in the mind by the normal combination of the 

 two perspective projections formed of it by the right and left eyes 

 respectively ; whilst by pseudoscopic vision we mean that ' conver- 

 sion of relief ' which is produced by the combination of two reversed 



