From a great distance a voice spoke out of the immense void that now seemed to surround him. But it did not reach him through his ears: it sounded softly in the echoing labyrinths of his brain.

'Calibration starting. From time to time you will be asked questions – you can answer mentally, but it may help to vocalize. Do you understand?'

'Yes,' Poole replied, wondering if his lips were indeed moving. There was no way that he could tell.

Something was appearing in the void – a grid of thin lines, like a huge sheet of graph paper. It extended up and down, right and left, to the limits of his vision. He tried to move his head, but the image refused to change.

Numbers started to flicker across the grid, too fast for him to read – but presumably some circuit was recording them. Poole could not help smiling (did his cheeks move?) at the familiarity of it all. This was just like the computer-driven eye examination that any oculist of his age would give a client.

The grid vanished, to be replaced by smooth sheets of colour filling his entire field of view. In a few seconds, they flashed from one end of the spectrum to the other. 'Could have told you that,' Poole muttered silently. 'My colour vision's perfect. Next for hearing, I suppose.'

He was quite correct. A faint, drumming sound accelerated until it became the lowest of audible Cs, then raced up the musical scale until it disappeared beyond the range of human hearing, into bat and dolphin territory.

That was the last of the simple, straightforward tests. He was briefly assailed by scents and flavours, most of them pleasant but some quite the reverse. Then he became, or so it seemed, a puppet on an invisible strig.

He presumed that his neuromuscular control was being tested, and hoped that there were no external manifestations, if there were, he would probably look like someone in the terminal stages of St Vitus's Dance. And for one moment he even had a violent erection, but was unable to give it a reality check before he fell into a dreamless sleep.

Or did he only dream that he slept? He had no idea how much time had elapsed before he awoke. The helmet had already gone, together with the Brainman and his equipment.

'Everything went fine,' beamed Matron. 'It will take a few hours to check that there are no anomalies. If your reading's KO – I mean OK – you'll have your Braincap tomorrow.'

Poole appreciated the efforts of his entourage to learn archaic English, but he could not help wishing that Matron had not made that unfortunate slip-of-the-tongue.

When the time came for the final filling, Poole felt almost like a boy again, about to unwrap some wonderful new toy under the Christmas free.

'You won't have to go through all that setting-up again,' the Brainman assured him. 'Download will start immediately. I'll give you a five-minute demo. Just relax and enjoy.'

Gentle, soothing music washed over him; though it was something very familiar, from his own time, he could not identify it. There was a mist before his eyes, which parted as he walked towards it...

Yes, he was walking! The illusion was utterly convincing; he could feel the impact of his feet on the ground, and now that the music had stopped he could hear a gentle wind blowing through the great trees that appeared to surround him. He recognized them as Californian redwoods, and hoped that they still existed in reality, somewhere on Earth.

He was moving at a brisk pace – too fast for comfort, as if time was slightly accelerated so he could cover as much ground as possible. Yet he was not conscious of any effort; he felt he was a guest in someone else's body. The sensation was enhanced by the fact that he had no control over his movements. When he attempted to stop, or to change direction, nothing happened. He was going along for the ride.

It did not matter; he was enjoying the novel experience – and could appreciate how addictive it could become. The 'dream machines' that many scientists of his own century had anticipated – often with alarm – were now part of everyday life. Poole wondered how Mankind had managed to survive: he had been told that much of it had not. Millions had been brain-burned, and had dropped out of life.

Of course, he would be immune to such temptations! He would use this marvellous tool to learn more about the world of the Fourth Millennium, and to acquire in minutes new skills that would otherwise take years to master. Well – he might, just occasionally, use the Braincap purely for fun...

He had come to the edge of the forest, and was looking out across a wide river. Without hesitation, he walked into it, and felt no alarm as the water rose over his head. It did seem a little strange that he could continue breathing naturally, but he thought it much more remarkable that he could see perfectly in a medium where the unaided human eye could not focus. He could count every scale on the magnificent trout that went swimming past, apparently oblivious to this strange intruder...

Then, a mermaid– Well he had always wanted to meet one, but he had assumed that they were marine creatures. Perhaps they occasionally came upstream – like salmon, to have their babies? She was gone before he could question her, to confirm or deny this revolutionary theory.

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.

'What's its capacity?' he asked. 'In my time, we were up to a terabyte in something this size. I'm sure you've done a lot better.'

'Not as much as you might imagine – there's a limit, of course, set by the structure of matter. By the way, what was a terabyte? Afraid I've forgotten.'

'Shame on you! Kilo, mega, giga, tera... that's ten to the twelfth bytes. Then the petabyte – ten to the fifteenth – that's as far as I ever got.'

'That's about where we start. It's enough to record everything any person can experience during one lifetime.'

It was an astonishing thought, yet it should not have been so surprising. The kilogram of jelly inside the human skull was not much larger than the tablet Poole was holding in his hand, and it could not possibly be as efficient a storage device – it had so many other duties to deal with.