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It's an afterimage in the eye. I know the Exploratorium has a few  
exhibits/activities based on the same thing. Here's what I found after  
a quick search for more explanation:
http://www.exploratorium.edu/snacks/bird_in_cage/index.html
http://www.exploratorium.edu/snacks/afterimage/index.html

It's an effect I know all too well after spending a lot of time with  
anaglyph glasses.

- Paul Koenig

On Jul 6, 2008, at 4:50 PM, [log in to unmask] wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology  
> Centers
> Incorporated, a worldwide network of science museums and related  
> institutions.
> *****************************************************************************
>
> Dear all,
>
>
>
> My son sent me an optical illusion unlike any I've seen before.  I
> thought you would enjoy it.  Does anyone know what is happening in the
> eye/brain to make this work?
>
>
>
> http://www.johnsadowski.com/big_spanish_castle.php
>
>
>
> Dennis
>
> "Work hard to find something that fascinates you. When you find it you
> will know your lifework" -- Richard Feynman
>
> Dennis Schatz
>
> Senior Vice President for Strategic Programs
>
> Pacific Science Center
>
> 200 Second Ave. No.
>
> Seattle, WA 98109
>
> Phone - 206-443-2867
>
> Fax - 206-443-3631
>
> www.dennisschatz.org <http://www.dennisschatz.org>
>
>
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