Spin-O-Vision

Stare at the black dot in the centre of the swirl for 20 seconds. Then, look quickly at your keyboard.

Whoa! See how your keyboard bulges out?

Here, you see the impossible: the keyboard seems to grow, but you also see that its size doesn't change. Your brain is getting two different stories from your eyes, and for a moment, accepts them both!

This happens because your eye and brain have balanced pairs of receptors for sensing movement - right/left, up/down, etc.

Looking at movement in just one direction for a long time tires out one side of the pair. The imbalance causes you to see movement in the opposite direction, in what's known as the "waterfall effect" (watch some moving water for a while and you'll know why). The effect has been studied since the time of Aristotle.

Did you know?

If the movement covers your entire field of view, the illusion weakens. Your vision is best at detecting relative movement against a background.

Look at the spiral up really close, through one eye only. Is the illusion as strong?

Reverse the direction of the spiral. What happens now?