Is the dancer below spinning clockwise or anti-clockwise..
The fact is it doesn't matter… but it looks clockwise from where I'm sitting.
Still it's quite an interesting optical illusion, and the reasons behind the directional ambiguity as explained in an article oin the New York times today…
Dr. Toppino advises staring at one part of the image, such as the foot,
and most of the time it will eventually flip. I tried this several
times, but it never flipped. Dr. Toppino says in people who can’t see
the reversal, it may be that one underlying neural structure is more
dominant, but once someone finally manages to see the flip, it will
start to happen more often.
http://well.blogs.nytimes.com/2008/04/28/the-truth-about-the-spinning-dancer