The river ended in a translucent wall; he stepped through it on to the face of a desert, beneath a blazing sun. Its heat burned him uncomfortably – yet he was able to look directly into its noonday fury. He could even see, with unnatural clarity, an archipelago of sunspots near one limb. And – this was surely impossible – there was the tenuous glory of the corona, quite invisible except during total eclipse, reaching out like a swan's wings on either side of the Sun.

Everything faded to black: the haunting music returned, and with it the blissful coolness of his familiar room. He opened his eyes (had they ever been closed?) and found an expectant audience waiting for his reaction.

'Wonderful!' he breathed, almost reverently. 'Some of it seemed – well, realer than real!'

Then his engineer's curiosity, never far from the surface, started nagging him.

'Even that short demo must have contained an enormous amount of information. How's it stored?'

'In these tablets – the same your audio-visual system uses, but with much greater capacity.'

The Brainman handed Poole a small square, apparently made of glass, silvered on one surface; it was almost the same size as the computer diskettes of his youth, but twice the thickness. As Poole tilted it back and forth, trying to see into its transparent interior, there were occasional rainbow-hued flashes, but that was all.

He was holding, he realized, the end product of more than a thousand years of electro-optical technology – as well as other technologies unborn in his era. And it was not surprising that, superficially, it resembled closely the devices he had known. There was a convenient shape and size for most of the common objects of everyday life -knives and forks, books, hand-tools, furniture... and removable memories for computers.