Artist: Oleg Shuplyak
Title of Work: Unknown
Year Produced: Unknown
Medium: Oil Paint
Source: http://4rtgallery.blogspot.com/2012/09/optical-illusions-by-oleg-shuplyak.html

Artist: Edgar Rubin
Title of Work: Rubin Vase
Year Produced: 1915
Medium: Unknown
Source: http://en.wikipedia.org/wiki/Rubin_vase

The Gestalt Principle of multistability is the idea that an image can actually seem to be two images at once, and can be flipped back and forth between the two. The first example I give of this, is an image of a woman sitting in a chair, but the chair, the woman, and her surroundings can actually be viewed as a mans face as well. The bottom image is a classic example of this principle. If you look at the picture, you should be able to see both a vase (in the positive space) and two faces facing each other (in the negative space).