If you want to repeat one of Isaac Newton’s most significant experiments, try holding your index and middle fingers up to the light (don’t do this, as I once did, in a train full of drunken football supporters!). With the tips of the fingers just touching, there will be a small gap lower down through which the light can pass. Look carefully at the middle of the gap. If your fingers are close enough, there will be a black line down it. Where did that come from? Where did the light go?

This phenomenon puzzled Newton mightily, because he thought of light as a stream of “fiery particles” (rather like a stream of bullets from a machine gun) that created the sensation of sight when they hit the eye. Thinking by analogy in this way is often a good starting point in the development of scientific ideas, but in this case it misled Newton. He could only think that his “fiery particles” experienced a hitherto-unknown attractive force when they passed close to an edge, so that their paths were deflected. But what could he make of the fact that sometimes there was not just one, but several dark lines (as you may see for yourself)?

It turned out that quite a different explanation was needed – one that Newton himself might have discovered if he had used a different analogy, based on his own explanation of the extraordinary tide patterns that occur in the Red River, near Hanoi. But that’s a subject for the next Mini Story from Science. Concerned readers may like to know, however, that my experiment on the train had a happy ending. When the large tattooed man in front of me demanded to know why I was raising two fingers to him, I explained. He was fascinated, and showed his friends. When I left the train, it was full of people holding two fingers up to each other.

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