The Spinning Dancer Illusion

The Spinning Dancer is a kinetic, bistable opt...

I saw this illusion a few years ago.  At first I suspected the animation caused the girl to alternately spin clockwise then stop and spin counterclockwise.  I tested this suspicion with my wife. We both watched at the same time. 

At first I saw the girl spin the way my wife saw it.  Then the girl changed the direction of her spin for me while not for my wife.  Then she changed again.  Sometimes we saw the same thing other times we saw the opposite. This difference in our view of which direction she was spinning proved it was a real optical illusion and not a trick in the animation.
Here she is:

Image via Wikipedia

Motion Parallax (Part Two)

In my last post I mentioned how when looking out the window of a moving vehicle the passing trees seemed to be rotating on their own turntables. 

Yesterday I payed more attention to the scenery and noticed that when looking out the right side of the car the trees seemed to rotate counterclockwise.  Looking out the left side of the car the trees seemed to rotate clockwise.

Let me know how you see this effect when you travel in a moving vehicle.


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