BINOCULAR VISION. 381 



judgment in the matter changes from one interpretation to the 

 other without any apparent cause. That the act of accommoda- 

 tion plays a part in the changes is shown by the fact that if one 



Fig. 166. — 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. 



focuses for the point a, this point may be held in the foreground 

 and the second of the above appearances be seen. While if the 

 eyes are accommodated strongly for point b, it will be brought 

 forward and the first of the two appearances described is brought 

 into view 



