352 THE SPECIAL SENSES. 



in which we can not trust our senses. The delusion seems to be 

 due to the fact that the two lines, in the position indicated, form 

 a projection on the retina such as would be made by an actual 

 vertical rod placed at the point at which we see one. Fig. 156 

 gives an interesting illustration of the way in which our judg- 

 ment of solidity may vary with our interpretation of mathe- 

 matical perspective and shading when these factors are arranged 



Fig. 156. Figure to illustrate binocular deceptions depending upon different inter- 

 pretations of the mathematical perspective and the lights and shades. On gazing fixedly 

 the image will change from a single cube with black top resting on two others with black 

 tops, to one of two cubes with black bottoms resting upon a single cube with black 

 bottom. Still other figures may appear from time to time. 



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

 it may assume several different appearances ; two are especially 

 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 

 judgment in the matter changes from one interpretation to the 

 other without any apparent cause. 



