Let's post awesome optical illusions

[jahcs]

Hi,
Your eyes have three different types of receptor cells for different colors, BV (blue-violet, ~460 nanometers), GY (green-yellow, ~530 nm), and YR (yellow-red, ~570 nm). Your perception of colors is made out of these three components (it's a bit more complicated in reality, but it suffices to explain this optical illusion).

Now if you stare at the illusion for some time, the BV receptor cells get "overloaded" and exhausted because the color is especially chosen to stimulate them (and only them), and so that color component slowly fades away.

If you move your head/your eyes, the image of the violet dots "move" to other BV receptor cells in the eye which are still fresh, so the dots "reappear".

Another well-known optical trick is to stare at a red shape on a white background for some time, then look at a white background only. Your eyes will then "see" the same shape in the complementary color for some time because the YR cells are exhausted.

-Kylearan
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We are taught the same principles in the Army on sentry duty and when "point shooting." If you look directly at something, especially at night, it can have a tendency to disapear. It is also an effective tactic when driving.

EDIT: Oh and does anyone notice that once you get below a certain level of illumination you only see in black and white? City dwellers may have more trouble with this one ;)