Talliaferro paused in something like triumph. This would be irrefutable.

Wendell Urth blinked at him and wiggled the thumbs of his clasped hands so that they slapped against his ample shirt front. He said, "And the significance of all that?"

"The significance is that the window was thrown open and the film placed in open air. Now Ryger has lived for ten years on Ceres, Kaunas on Mercury, I on the Moon- barring short leaves and not many of them. We commented to one another several times yesterday on the difficulty of growing acclimated to Earth. , "Our work-worlds are each airless objects. We never go out in the open without a suit. To expose ourselves to unenclosed space is unthinkable. None of us could have opened the window without a severe inner struggle. Dr. Mandel, however, has lived on Earth exclusively. Opening a window to him is only a matter of a bit of muscular exertion. He could do it. We couldn't. Ergo, he did it."

Talliaferro sat back and smiled a bit.

"Space, that's it!" cried Ryger, with enthusiasm.

"That's not it at all," roared Mandel, half rising as though tempted to throw himself at Talliaferro. "I deny the whole miserable fabrication. What about the record I have of Villiers' phone call? He used the word 'classmate.' The entire tape makes it obvious-"

"He was a dying man," said Talliaferro. "Much of what he said you admitted was incomprehensible. I ask you, Dr. Mandel, without having heard the tape, if it isn't true that Villiers' voice is distorted past recognition."

"Well-" said Mandel in confusion.

"I'm sure it is. There is no reason to suppose, then, that you might not have rigged up the tape in advance, complete with the damning word 'classmate.' "

Mandel said, "Good Lord, how would I know there were classmates at the Convention? How would I know they knew about the mass-transference?"

"Villiers might have told you. I presume he did."

"Now, look," said Mandel, "you three saw Villiers alive at eleven. The medical examiner, seeing Villiers' body shortly after 3 A.M. declared he had been dead at least two hours. That was certain. The time of death, therefore, was between 11 P.M. and 1 A.M. I was at a late conference last night. I can prove my whereabouts, miles from the hotel, between 10:00 and 2:00 by a dozen witnesses no one of whom anyone can possibly question. Is that enough for you?"

Talliaferro paused a moment. Then he went on stubbornly, "Even so. Suppose you got back to the hotel by 2:30. You went to Villiers' room to discuss his talk. You found the door open, or you had a duplicate key. Anyway, you found him dead. You seized the opportunity to scan the paper-"

"And if he were already dead, and couldn't make phone calls, why should I hide the film?"

"To remove suspicion. You may have a second copy of the film safe in your possession. For that matter, we have only your own word that the paper itself was destroyed."

"Enough. Enough," cried Urth. "It is an interesting hypothesis, Dr. Talliaferro, but it falls to the ground of its own weight."

Talliaferro frowned. "That's your opinion, perhaps-"

"It would be anyone's opinion. Anyone, that is, with the power of human thought. Don't you see that Hubert Mandel did too much to be the criminal?"

"No," said Talliaferro.

Wendell Urth smiled benignly. "As a scientist, Dr. Talliaferro, you undoubtedly know better than to fall in love with your own theories to the exclusion of facts or reasoning. Do me the pleasure of behaving similarly as a detective.

"Consider that if Dr. Mandel had brought about the death of Villiers and faked an alibi, or if he had found Villiers dead and taken advantage of that, how little he would really have had to do! Why scan the paper or even pretend that anyone had done so? He could simply have taken the paper. Who else knew of its existence? Nobody, really. There is no reason to think Villiers told anyone else about it. Villiers was pathologically secretive. There would have been every reason to think that he told no one.

"No one knew Villiers was giving a talk, except Dr. Mandel. It wasn't announced. No abstract was published. Dr. Mandel could have walked off with the paper in perfect confidence.

"Even if he had discovered that Villiers had talked to his classmates about the matter, what of it? What evidence would his classmates have except the word of one whom they are themselves half willing to consider a madman?

"By announcing instead that Villiers' paper had been destroyed, by declaring his death to be not entirely natural, by searching for a scanned copy of the film-in short by everything Dr. Mandel has done-he has aroused a suspicion that only he could possibly have aroused when he need only have remained quiet to have committed a perfect crime. If he were the criminal, he would be more stupid, more colossally obtuse than anyone I have ever known. And Dr. Mandel, after all, is none of that."

Talliaferro thought hard but found nothing to say.

Ryger said, "Then who did do it?"

"One of you three. That's obvious."

"But which?"

"Oh, that's obvious, too. I knew which of you was guilty the moment Dr. Mandel had completed his description of events."

Talliaferro stared at the plump extraterrologist with distaste. The bluff did not frighten him, but it was affecting the other two. Ryger's lips were thrust out and Kaunas's lower jaw had relaxed moronically. They looked like fish, both of them.

He said, "Which one, then? Tell us."

Urth blinked. "First, I want to make it perfectly plain that the important thing is mass-transference. It can still be recovered."

Mandel, scowling still, said querulously, "What the devil are you talking about, Urth?"

"The man who scanned the paper probably looked at what he was scanning. I doubt that he had the time or presence of mind to read it, and if he did, I doubt if he could remember it-consciously. However, there is the Psychic Probe. If he even glanced at the paper, what impinged on his retina could be Probed."

There was an uneasy stir.

Urth said at once, "No need to be afraid of the Probe. Proper handling is safe, particularly if a man offers himself voluntarily. When damage is done, it is usually because of unnecessary resistance, a kind of mental tearing, you know. So if the guilty man will voluntarily confess, place himself in my hands-"

Talliaferro laughed. The sudden noise rang out sharply in the dim quiet of the room. The psychology was so transparent and artless.

Wendell Urth looked almost bewildered at the reaction and stared earnestly at Talliaferro over his glasses. He said, "I have enough influence with the police to keep the Probing entirely confidential."

Ryger said savagely, "I didn't do it."

Kaunas shook his head.

Talliaferro disdained any answer.

Urth sighed. "Then I will have to point out the guilty man. It will be traumatic. It will make things harder." He tightened the grip on his belly and his fingers twitched. "Dr. Talliaferro pointed out that the film was hidden on the outer window sill so that it might remain safe from discovery and from harm. I agree with him."

"Thank you," said Talliaferro dryly.

"However, why should anyone think that an outer window sill is a particularly safe hiding place? The police would certainly look there. Even in the absence of the police it was discovered. Who would tend to consider anything outside a building as particularly safe? Obviously, some person who has lived a long time on an airless world and has it drilled into him that no one goes outside an enclosed place without detailed precautions.

'To someone on the Moon, for instance, anything hidden outside a Lunar Dome would be comparatively safe. Men venture out only rarely and then only on specific business. So he would overcome the hardship of opening a window and exposing himself to what he would subconsciously consider a vacuum for the sake of a safe hiding place. The reflex thought, 'Outside an inhabited structure is safe," would do the trick."

Talliaferro said between clenched teeth, "Why do you mention the Moon, Dr. Urth?",

Urth said blandly, "Only as an example. What I've said so far applies to all three of you. But now comes the crucial point, the matter of the dying night."

Talliaferro frowned. "You mean the night Villiers died?"

"I mean any night. See here, even granted that an outer window sill was a safe hiding place, which of you would be mad enough to consider it a safe hiding place for a piece of unexposed film? Scanner film isn't very sensitive, to be sure, and is made to be developed under all sorts of hit-and-miss conditions. Diffuse night-time illumination wouldn't seriously affect it, but diffuse daylight would ruin it in a few minutes, and direct sunlight would ruin it at once. Everyone knows that."

Mandel said, "Go ahead, Urth. What is this leading to?"

"You're trying to rush me," said Urth, with a massive pout. "I want you to see this clearly. The criminal wanted, above all, to keep the film safe. It was his only record of something of supreme value to himself and to the world. Why would he put it where it would inevitably be ruined by the morning sun?-Only because he did not expect the morning sun ever to come. He thought the night, so to speak, was immortal.

"But nights aren't immortal. On Earth, they die and give way to daytime. Even the six-month polar night is a dying night eventually. The nights on Ceres last only two hours; the nights on the Moon last two weeks. They are dying nights, too, and Dr. Talliaferro and Ryger know that day must always come."

Kaunas was on his feet. "But wait-"

Wendell Urth faced him full. "No longer any need to wait, Dr. Kaunas. Mercury is the only sizable object in the Solar System that turns only one face to the sun. Even taking libration into account, fully three-eighths of its surface is true dark-side and never sees the sun. The Polar Observatory is at the rim of that dark-side. For ten years, you have grown used to the fact that nights are immortal, that a surface in darkness remains eternally in darkness, and so you entrusted unexposed film to Earth's night, forgetting in your excitement that nights must die-"

Kaunas stumbled forward. "Wait-"

Urth was inexorable. "I am told that when Mandel adjusted the polarizer in Villiers' room, you screamed at the sunlight. Was that your ingrained fear of Mercurian sun, or your sudden realization of what sunlight meant to your plans? You rushed forward. Was that to adjust the polarizer or to stare at the ruined film?"