BINOCULAR VISION. 



361 



ment of solidity may vary with our interpretation of mathe- 

 matical perspective and shading when these factors are arranged 



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Fig. 157. Zollner's lines. 



to give more than one choice. If the figure is looked at steadily 

 it may assume several different appearances; two are especially 



Fig. 158. Optical illusion in projection. (Franklin.') 



prominent. We may see two cubes resting upon a third one, 

 each with the black side undermost, or we may see one cube rest- 

 ing on two under ones, each with its black side uppermost. Our 



