Disappearing Dot Illusion
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Put your face close to the screen and stare at the black cross in the center. Eventually, you will see a green dot. After that, all the purple dots will disappear. This effect is known as Troxler's fading .
Contributed by: Ed Pegg Jr (September 2007)
Open content licensed under CC BY-NC-SA
 Wikipedia, Troxler's fading.
"Disappearing Dot Illusion"
Wolfram Demonstrations Project
Published: September 28 2007