STEREOSCOPIC VISION. JUDGMENT OF SOLIDITY. 



875 



of the pyramid; and as the axes of the eyes would have to converge 

 more and more in doing this, the apex of the pyramid appears to project ; 

 for all points appear closer when the eyes must be converged in order to 

 see them. In this way, all corresponding parts of both figures would be 

 brought successively on identical points by the movements of the eyes 

 toward each other. 



To this the objection has been urged that the duration of the electric 

 spark is sufficient for stereoscopic vision ; a length of time that is entirely 

 too short for the ocular movements. Although this is true for many 

 figures, this movement of the visual axes is not precluded for the correct 

 combination of complex or unusual figures, and it is in fact of con- 

 siderable advantage, especially for certain individuals. 



It appears that not merely the movements that actually take place, 

 but rather the sense of innervation of the muscles necessary to the move- 

 ment is sufficient to produce the impression of solidity. Consequently, 



FIG. 306. Wheatstone's Prism-pseudoscope. 



FIG. 307. Ewald's Mirror Pseudoscope. 



stereoscopic vision may depend in part on a muscle-sensation : the feeling 

 that a greater convergence of the visual axes is necessary for the super- 

 position of two points in the stereoscopic figures produces the impression 

 that the points are nearer; conversely, the feeling that to secure the 

 congruence of two points a greater divergence of the visual axes is 

 necessary produces the impression of greater distance. 



If, now, in the momentary superposition of two figures making a solid image, 

 a movement of the eyes does not take place, many points in the stereoscopic 

 figures are apparently united that, strictly speaking, do not fall on identical 

 retinal points. The latter therefore cannot be designated with mathematical 

 accuracy as corresponding points of the retinas, but, from a physiological view- 

 point, all points must be designated as identical, the simultaneous stimulation 

 of which produces as a rule a single image. In this coalescence the mind plays 

 a part: there is a certain psychical tendency to fuse the double impressions of 

 both retinas into one, as experience has taught that they belong to one single 



