On a night like this, when the breeze was cool and it was difficult to remember his wife's voice, he could believe that more centuries would pass before ylem yielded any of its secrets — and by then it might be too late. Mankind would have dispersed into and been absorbed by Orbitsville's infinite meadows. A thousand times a thousand rural tribes would be permanently busy re-inventing the steam and internal combustion engines, and would have no use for the blueprints of the ultimate machine. Cavendish gave a low sigh, deciding that he should be in bed. He got to his feet and was stooping to pick up the stool when the surface on which he was stand ing blinked with green radiance. A tine of light, glowing on the full width of the Valley's floor, swept by him, travelled the length of the strip and vanished at its western end in a fraction of a second.

"What the…!" Cavendish stared into the familiar backdrop of night, suddenly feeling unsafe.

He had lived all his life on the Orbitsville shell and knew the material to be totally inert, changeless, more stable by far than any planetary crust. It was not supposed to pulse with green light… because if that could happen other changes might occur… and he could almost fed the ylem dissolving beneath his feet… hurling his unprepared and unprotected body downwards and outwards… into the space between the stars…

Mark Denmark was obviously unhappy. He squared up various stacks of paper on his desk, frowning ail the while, then went to the window of his office and rocked on the balls of his feet while he inspected the view. The Valley and its wide-caved buildings and the surrounding slopes reflected the sun's vertical rays, looking exactly as they always did, embalmed in brilliance which had an ancient Egyptian quality to it. Denmark shook his head as though something outside had failed to pass muster, returned to his chair and began tapping his front teeth with a pen.

"Dan, we've checked every read-out from every instrument," he finally said. "There aren't any spikes. There aren't any blips or dips. In fact, there is simply no trace of any abnormal event."

"That's not surprising — considering we don't have any photometers aimed at the shell." Cavendish spoke in a matter-of-fact voice, concealing his disappointment over the lack of corroborative readings. He had been awake all night, voyaging on mental oceans of surmise, elated by the near-certainty that the science of spherology was about to take that long-awaited step forward. Now it was beginning to look as though he had taken a step backwards, lost ground in the battle over his delayed retirement.

Denmark leaned forward, hunching his shoulders. "Dan, I don't need to tell you that light can't come into being by itself. What you described sounded like an excitation phenomenon, but nothing got excited — except maybe your optic nerves."

"I'm perfectly healthy," Cavendish snapped. "Don't try that stuff on me."

"I'm only trying to stop you making a pig's ass of yourself. If you insist on your report going on log you'll draw a lot of attention, then you'll really find out what it's like to be under pressure. Christ, Dan, if everybody else can accept retirement at eighty, why can't you?"

"Because I'm not ready."

"Ready or not, one of these days… The Commission can insist, you know."

Cavendish glanced around the office in feigned surprise. "Have I come to the wrong place? Are you the chief scientist or the personnel manager?"

Denmark's grey eyes clouded with annoyance and his mouth withered into a thin line. Watching the change of expression. Cavendish wondered if he had gone too far. Denmark was a natural researcher whose temperament had been poisoned by the constant struggle to keep all his projects functioning on dwindling budgets, and in the past months he had become moody and unpredictable. Att it takes is one word from him. Cavendish thought, becoming alarmed. Just one interoffice memo and Fm out in the cold with nothing but…

"Good morning, men!"

The unexpected voice, coming from the open door, provided what Cavendish hoped would be a welcome diversion. He turned towards it and his spirits sank as he saw the stubby, bull-necked form of his old adversary, technical manager Phil Vigus, entering the office. Vigus, who was ultimately responsible for the reliability of all equipment, took every complaint as a personal insult — a trait which had often brought him into conflict with Cavendish. He had long regarded Cavendish as a cantankerous geriatric, and at that particular moment was the last person Cavendish wanted to see.

"Take a seat, Phil," Denmark said. "I'm glad you dropped by."

"Really?" Vigus lowered his bulk into a chair. "What have I got that you want?"

Denmark gave him a cold fleeting smile. "You've got millions of monks' worth of equipment strung out along the Valley, and I've just been informed by Mr. Cavendish here — who else? — that it's a load of crap. I used to think the shell material was inert, didn't you? Well, apparently ylem can become so agitated that it lights up like a traffic sign, and not one of your piss-poor instruments even gives a quiver. What do you think of that?"

"That's not what I said," Cavendish protested, shocked by the sheer crudity of Denmark's attack and deducing from it that he had landed himself in a genuine crisis. This was a tailor-made chance for Vigus to come down on him with both feet, to add his managerial clout to that of Denmark. Both men working in unison could have him issued with his walking papers in a couple of hours.

"I presume, from the scuttlebutt I picked up this morning, that we're talking about Dan's wonderful green flash." Vigus's lips twitched in amusement. "They're already calling it his nocturnal emission."

"That's it," Denmark said gleefully, and having been presented with a joke went on, as humourless people do, to run it ragged. "Some people get hot flushes at night — Dan gets green flashes. What do you say, doc? Is it a serious ailment?"

"I can tell you one thing about it." Vigus stared hard at Cavendish for a second, eyes unreadable, his expression curiously benign.

Executioner's compassion^ Cavendish thought. Here comes the big knife.

"Tell me the worst," Denmark prompted.

"It's infectious," Vigus said. His tone was calm, very neutral, but Cavendish felt a premonition.

Denmark looked disappointed. "What does that mean?"

"I read a lot of reliability reports, from all over. Not just on research equipment, on nearly anything.

Three weeks or so ago a pilot called Jean Antony baled out of an old freighter on the equatorial run, close to 156. The ship apparently was a death-trap, nothing working right, and she got herself ejected downwards. Her capsule actually grazed the shell before it was recovered, so it was a near-miracle she got out of it alive." Vigus paused for an unnecessarily long time, still looking at Cavendish. "Stirring stuff," Denmark said drily, "but I don't…"

"Just before the collision Jean Antony saw a green flash. Just like Dan's. She described it in her accident report, but nobody paid any heed. It was written off as nerves or a stray reflection on the capsule's transparencies."

Denmark nodded. "Td say nerves — you know what women are like."

"This one saw a thin band of green radiance moving fast across the shell from east to west," Vigus asserted. "Something is happening, Mark — something very unusual — and the sooner you report Dan's observations to Commission HQ the more credit you'll be able to grab for yourself."

"I'd like to thank you," Cavendish said to Vigus as they left the admin building together. "If you hadn't chimed in when you did… Mark was all set to dump me, you know."

"He'd only have been forced to take you back on again, after you'd become famous." Vigus grinned in the vertical prism of shadow cast by his coolie-style hat. "You're going to be insufferable enough as it is."