|Mad and Sad by Humberto Machado|
For some reason, I couldn't find the two faces - the mad face and the sad face - in this drawing. I knew logically where the other face had to be hidden, but it eluded me. Then I finally found it, and now the two faces flip back and forth into my awareness: sad, mad, sad, mad...
This process happens to me when I watch an illusion - at first I cannot see it, then I can see it, and then the illusion seems to have a vibrant internal life of its own, as the illusion continually changes back and forth. Of course, that "change" is also an illusion, because it's not the illusion that's changing at all - it's my perception that's changing.
Found via New Optical Illusions>>
See probably the most famous optical illusion that keeps flipping, the Necker Cube>>