In Bristol Zoo there is a Nocturnal House. Here, day has been turned into night, and those animals which normally sleep during the day—bush babies, genets, bats and other creatures of the dusk and dark—are fully active. Two doors prevent the bright daylight outside from disturbing the animals: one must be closed before the other is opened. As you enter the House you are momentarily quite blind. You would not know if a black panther crouched a yard away from you, waiting to spring. Then, gradually, faint shreds of light from behind the glass-fronted cages give you the impression of moving shapes as a pardine genet paces restlessly up and down. In another cage two large button eyes pick up a glint of light and, as you move close, around them you slowly distinguish a fur ball of a body, ears and long curling tail. Searching the gloom, you pick out a second and a third.

By the time you are ready to leave you can see with comparative ease. As fresh visitors come to the House you are amused to see them fumble and turn in the dark. For all they know, a black panther crouches a yard away from them waiting to spring….

When I was your age, I lived in Cornwall. We used to walk to the top of a nearby earn and dare each other to jump across the boarded-up mouth of a disused mineshaft. We scratched our hands, our legs, even our faces on the dense brambles that grew there. We gashed our knees on stones, we twisted our ankles.

Then one day we read in the newspaper that a boy from a nearby village, losing his way on a hillside one dark night, had stumbled on the lip of a shaft and fallen through the rotten boarding. Fortunately, he had landed on a ledge not very far down. After that, we played less risky games!

Imagine that you are regaining consciousness lying on a ledge in the dark, part way down that mineshaft. (Have you ever had an operation, or a tooth extracted by gas? What were your feelings as you came round? What shapes and colours could you see?) As your head clears, reach out your hand. What do you feel? Look around you, look up. What do you see? Dare you look down?