It has been known since the Renaissance that if you look at the inside of a hollow casting of a human face, you still get the impression you are looking at the front of the mask. But it is even more impressive if you reverse the mask and then move around the room as it remains fixed. In such a case, the "face" appears to turn by itself so as to always face toward you as you move. We can't demonstrate that here, but in the animation at right above, notice how the orientation of the face in reverse "compensates" for the rotation of the mask itself.

Below these words is a combination of this and another, more familiar, illusion that will really amaze you, I think. Concentrate on the animated sketch below. Just by concentration you can make it into the following: (a)The innermost corner of a room, or interior of a cube, with a small cube right at the corner. (b) A cube with a smaller cube-shaped hole at the outermost corner. (c) The most amazing, and the illusion related to the hollow face: A cube with another cube attached loosely to it at the outermost corner! Notice that in this case, the smaller cube is seen to pivot in the opposite sense to the larger cube. With practice you can effortlessly get the image to flip back and forth among the three distinctly different possibilities!

Now that you know what to look for, can you "force" the three interpretations looking only at this static image of the cut cube?

Click on the image to go to Prof. Michael Bach''s recent (January 2006) version of this "corner cube" illusion.