Now something stood in the centre of the hall. It looked, to the arts-educated Patrician, like a big magnifying glass surrounded by rubbish.

‘Technically, my lord, an omniscope can see anywhere,’ said Archchancellor Ridcully, who was technically the head of All Known Wizardry.

‘Really? Remarkable.’

‘Anywhere and any time,’ Ridcully went on, apparently not impressed himself.

‘How extremely useful.’

‘Yes, everyone says that,’ said Ridcully, kicking the floor morosely. ‘The trouble is, because the blasted thing can see everywhere, it's practically impossible to get it to see anywhere. At least, anywhere worth seeing. And you'd be amazed at how many places there are in the universe. And times, too.’

‘Twenty past one, for example,’ said the Patrician.

‘Among others, indeed. Would you care to have a look, my lord?’

Lord Vetinari advanced cautiously and peered into the big round glass. He frowned.

‘All I can see is what's on the other side of it,’ he said.

‘All, that's because it's set to here and now, sir,’ said a young wizard who was still adjusting the device.

‘Oh, I see,’ said the Patrician. ‘We have these at the palace, in fact. We call them win-dows.’

‘Well, if I do this,’ said the wizard, and did something to the rim of the glass, ‘it looks the other way.’ Lord Vetinari looked into his own face.

‘And these we call mir-rors,’ he said, as if explaining to a child.

‘I think not, sir,’ said the wizard. ‘It takes a moment to realise what you're seeing. It helps if you hold up your hand…’