1) Stare at the cross in the middle. Bye bye purple dots.
2) Notice the green dot going around the edges? There is no green dot!

Link.

I have been repeatedly asked to explain this in more detail, so here goes:

1. There is something called the “negative retinal afterimage”. It becomes visible when one given hue stays on the same retinal position for several seconds (usually we would move our eyes typically 3 times per second, so this is not disadvantage in normal viewing). The afterimage builds up as that retinal location adapts to this special hue, and when looking at a neutral background the complementary colour is seen.
2. This is a good thing, normally, because it helps “colour constancy”, that is we see colours somewhat independent of the ambient illumination (compare the bluish glacier noon sun with a reddish tint in the evening living room by the fireside).
3. Ok, so the afterimage is “burnt in”, that is that retinal location is adapted. Now the magenta patch is suddenly switched to grey. Because of the adaptation, the complementary colour is now seen, which would be green for magenta, or light grey for a dark grey.
4. The retinal afterimage typically fades away rapidly (over a few seconds under normal conditions). But here this fade-out does not reduce the perception of the afterimage, because a new one is uncovered right after at the next location.
5. In addition, a Gestalt effect, here the “phi phenomenon” comes into play: the afterimage from the successive retinal locations is integrated and perceived as one single moving object, namely the green disk.
6. In summary, the following factors make this illusion rather compelling:
* it is rather easy to steadily fixate on the centre
* most of the time the retinal locations are re-adapted and the afterimage is uncovered only briefly
* a Gestalt effect leads to the perception of a flying green disk.

Link

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