(((I'm sure many of you have seen this, but for those who have not....)))
Stare at the center of the image above for at least 30 seconds. Then look at a blank white wall, or a peice of paper. What do you see? On this page you'll see that the after image of the black and white image above is much stronger than the one in color below. Take your time, sometimes it takes an additional 20 seconds before your eyes can really focus on the after image. 
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