“Yes, there is that,” Bogert seemed concerned about the matter. “The only thing I can advise, boy, is that if you detect the gamma radiation between yourself and the man, you may as well sit where you are.”

The robot was openly relieved. “Thank you, sir. There wouldn’t be any use, would there?”

“Of course not. But if there weren’t any dangerous radiation, that would be a different matter.”

“Naturally, sir. No question of that.”

“You may leave now. The man on the other side of the door will lead you to your stall. Please wait there.”

He turned to Susan Calvin when the robot left. “How did that go, Susan?”

“Very well,” she said, dully.

“Do you think we could catch Nestor 10 by quick questioning on etheric physics?”

“Perhaps, but it’s not sure enough.” Her hands lay loosely in her lap. “Remember, he’s fighting us. He’s on his guard. The only way we can catch him is to outsmart him—and, within his limitations, he can think much more quickly than a human being.”

“Well, just for fun—suppose I ask the robots from now on a few questions on gamma rays. Wave length limits, for instance.”

“No!” Dr. Calvin’s eyes sparked to life. “It would be too easy for him to deny knowledge and then he’d be warned against the test that’s coming up—which is our real chance. Please follow the questions I’ve indicated, Peter, and don’t improvise. It’s just within the bounds of risk to ask them if they’ve ever worked with gamma rays. And try to sound even less interested than you do when you ask it.”

Bogert shrugged, and pressed the buzzer that would allow the entrance of Number Fifteen.

The large Radiation Room was in readiness once more. The robots waited patiently in their wooden cells, all open to the center but closed off from each other.

Major-general Kallner mopped his brow slowly with a large handkerchief while Dr. Calvin checked the last details with Black.

“You’re sure now,” she demanded, “that none of the robots have had a chance to talk with each other after leaving the Orientation Room?”

“Absolutely sure,” insisted Black. “There’s not been a word exchanged.”

“And the robots are put in the proper stalls?”

“Here’s the plan.”

The psychologist looked at it thoughtfully, “Um-m-m.”

The general peered over her shoulder. “What’s the idea of the arrangement, Dr. Calvin?”

“I’ve asked to have those robots that appeared even slightly out of true in the previous tests concentrated on one side of the circle. I’m going to be sitting in the center myself this time, and I wanted to watch those particularly.”

“You’re going to be sitting there—” exclaimed Bogert.

“Why not?” she demanded coldly. “What I expect to see may be something quite momentary. I can’t risk having anyone else as main observer. Peter, you’ll be in the observing booth, and I want you to keep your eye on the opposite side of the circle. General Kallner, I’ve arranged for motion pictures to be taken of each robot, in case visual observation isn’t enough. If these are required, the robots are to remain exactly where they are until the pictures are developed and studied. None must leave, none must change place. Is that clear?”