Ambiguous Optical Illusions
Ambiguous figures are 'two pictures in one' - looked at one way, you
see one thing (e.g. a vase), but looked at another, you see something
else (e.g. two faces in profile).
They are somewhat similar to unstable figures, in that there are two
ways to interpret each one, but unstable figures don't contain
different subjects; they contain one subject that flips perspective.
Do you only see black shapes on white - or a word?