(fun / off topic) Just an illusion

If you haven't come across this before, it's really quite surprising.

Stare at the crosshairs in the centre of this picture. After a short while you'll notice that there is a green dot revolving around, replacing each purple dot in turn. Then if you keep staring the purple dots disappear, and all that remains is a circling green dot.

But it's all an illusion - there's no green dot and the purple dots never actually disappear. So what the devil is going on?

I'm indebted to Jeremy L Hinton and Michael Bach for a fairly detailed explanation of why this happens. Apparently it's sort of an in-built screen saver that switches off bits of your visual field when the same hue is burning in the same part of your retina for more than a few seconds. Pretty cool.

Anyhow, what I do know is:

a) It's kind of weird and fun at the same time. I never knew I had a built-in screen saver.

b) I'd like to see someone produce a version in XAML....

Addendum: I'm hoping you're actually able to see the gif file in this post. I'm hosting it on msn spaces (spaces.msn.com) and it has a really whacky URL as a result - right-click on it and select properties to see what I'm talking about. If something goes amiss and you can't see it then the link to Hinton & Bach's website shows another version.

This posting is provided "AS IS" with no warranties, and confers no rights.