Reversible Figures- these are illustrations that are drawn in a way that they can be seen by two different perspectives, appearing to be two different things
Example: The illustration below can either look like a vase (the black outline in the center) or two faces looking at each other (the white space on either side of the drawing)
 http://department.monm.edu/chemistry/honors210/fall2005/ncarlson/Topic_Presentation_3.htm