We used to do these all the time without the B&W image behind it. They'd do an American flag in green, black & orange, and you'd see it correctly on any white surface for a moment. The B&W gives you better detail, and apparently help in not moving your eyes and eliminating the "fatigue" that makes the image persist.
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Yup
(Anonymous) 2006-06-09 05:56 pm (UTC)(link)