He had known such a barely visible, near ultra-violet tenebrosity, on the very edge of night, only once in his life when he had descended further than was altogether wise down the face of a sheer cliff at the outer edge of the Great Barrier Reef. Looking down into hundreds of metres of crystalline emptiness, he had felt such a sense of disorientation that he experienced a brief moment of panic, and had almost triggered his buoyancy unit before regaining control. Needless to say, he had never mentioned the incident to the Space Agency physicians...

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.