successive tiers of seats, plus the teacher on his raised platform immediately in front of the large blackboard. In previous years there had been instances of students slipping out after roll call. In spite of everything, it had happened.

Therefore a new system had been inaugurated. Before roll call Martin marched to the back of the room to the only exit and locked it. Pocketing the key, he returned to his podium. It had been going on this way for two years, and was now automatic.

The day watchman, making his rounds, approached this door at precisely two thirty-four. He heard violent pounding. Along with the pounding there was a loud, hoarse voice, gasping, "Lemme out! Lemme out!"

The watchman consulted his clock—the one he used to make a record of his rounds—and determined that it was two thirty-four. He knew that it was Dr. Grant's senior theoretical physics lecture period. He recalled that a couple of years before Dr. Grant had had trouble with students slipping out after roll call. But it occurred to him that it was hardly possible to sneak out, even on Dr. Grant, absent-minded as he was, by pounding on the door and shouting, "Lemme out!" in a terrified tone of voice.

He therefore stopped and knocked on the door, calling, "What's going on in there?"

Whoever was doing the pounding and shouting evidently didn't hear him. Waiting no longer, the day watchman used his master key on the door.

A smallish young man, later identified as Mark Smythe, attempted to run past him into the hall. The watchman blocked Mark's escape and looked toward the podium in an automatic appeal to Dr. Grant.

Dr. Grant was not there. The podium was unoccupied. So were all four hundred seats. There was, in fact, no one in room 304 except the one terrified student.

In due course the police arrived, along with the regents. By five o'clock it had become certain that the greatest mass disappearance of all times had occurred, with Mark Smythe as the sole witness.

He stuck to his story through repeated detailed questionings, and in the end the police were stuck with it.

According to Smythe, class had begun as usual. Dr. Grant had waited until one minute after the bell had sounded, then had marched back and locked the door, and returned to the front. He had rapidly scanned the room to see if there were any absences, quickly called half a dozen names he was uncertain of, and marked the attendance slip. The police found it still resting on the table where he had placed it.

Then he had begun his lecture by remarking that they were behind schedule and would have to catch up. He had been speaking less than five minutes when a student by the name of Marvin Green jumped to his feet in great excitement, waving his hand and shouting, "Dr. Grant! Dr. Grant!"

Dr. Grant had stopped his lecture and frowned darkly, then said, "If you will please take your seat—"

"But Dr. Grant!" Marvin Green had interrupted him excitedly. "I've got it! I've got it!"

What had happened then was impossible for the mind to accept. Marvin Green had simply ceased to be.

There had been a stunned silence. And in that silence, it went on. Student after student popping out of existence in what seemed to be a chain reaction.

He wasn't aware when Dr. Grant vanished. All he knew was that when at last he was alone he looked toward the podium and the professor was also gone.

He kept waiting to go himself. When he didn't, he lost the fear that had rooted him to the spot, and rushed to the exit where he at first tried to break down the door and make his escape, then subsided into pounding and shouting for help when he realized his physical strength was insufficient for the job.

Questioning didn't bring out any additional fact, nor alter any statement. There had been no sound to the vanishing, no movement of the person that could be considered significant, no flashes of light, no strange odors. Nothing.

Fred Grant got the flash on his hot rod radio on the way home from high school.

At the end of the report Fred wrote down Mark Smythe's address on a scrap of paper, and drove home to be with his mother. It was three days before he could get away.

On the morning of the third day, his aunt Emily arrived to take charge of things, and he was able to slip away. He drove immediately to Mark Smythe's address. It was one of the better class rooming houses near the campus. The land-lady wasn't going to let him in nor announce him until he explained he was the son of the professor who had vanished. She immediately swung to the other extreme and didn't bother to find out if Mark wanted to see him.

"My father was your teacher," Fred said.

"Oh? Come on in."

There were tennis rackets. On the bookshelves there were tennis books. On a table there was a tennis trophy. Otherwise there was just a bed, a rug, and two or three chairs.

"I don't know what I can tell you more than I've already told the police and the reporters," Mark said apologetically. "I guess it's tough, losing your father...."

"Yeah," Fred agreed. "I wanted to ask you something though. Dad gave a lecture on his new theory a few days ago, didn't he?"

Mark looked at him blankly. Then, "Oh! I guess he did. As a matter of fact I didn't pay much attention to it." He grinned. Then he remembered he should be solemn and stopped grinning. "I—I sort of slipped by it. He made the mistake of telling us ahead of time it was off the course and no questions on it would be in the finals, so I more or less rested up during the period for a tennis match afterwards. Why?"

"Didn't you get any of what he said?" Fred persisted.

"Oh, a little," Mark admitted. "It was about some system of arriving at the basic laws of nature by pure logic, only what you arrived at didn't agree with facts. Some kind of intellectual curiosity." He thought a minute. "Oh," he said, "I see what you want. Didn't he leave any notes on it? It would be too bad if his theory was lost to the world now that—" He left the rest unsaid.

"Maybe you can remember something," Fred coaxed. "Anything. Did he talk about his theory again?"

"Next day he gave a lecture on the necessity of unbelief in modern science. It was pretty good. He overemphasized it, though. Some of the kids thought he was making a religion of unbelief."

"What did they say about his theory?" Fred asked quickly.

"Oh, they were quite impressed. Two of them live—lived here in the rooming house. They were up here that evening tossing it back and forth. I was too tired from the tag match. I let them talk."

"What did they think about it?"

Mark frowned in an effort to recall. "It had to do with this universe being basically illogical, or at least seeming to be, because it didn't agree with your father's theory. They started building up fantasies on it. One I remember was a good one."

"What was that?"

"I think it was Jimmy. He said it would be funny if we were here because we believed this universe was the only real one. Something about inherited memory. Our coming from a long line of people who believed this was the only place, because all our ancestors who didn't believe it shot off into some other universe and had their children there. Utterly crazy. You know."

"Yeah, I know," Fred agreed. "You going to be around in case I want to see you again?"

"God! I hope so!" Mark said. "It makes me nervous."

"You're safe enough," Fred said. "Well—thanks. I'll be seeing you."

He smoothed out the crumpled sheet of paper and glanced at it.

"What do you hope to find, Fred," his mother asked.

"I don't know," he said. "Anything, I—maybe this is something. Look."

Together they read, "Either: the universe is not constructed according to logical necessity, Or: the observable universe is not the universe." There were doodlings along the right margin that meant nothing.

"What does it mean?" Mrs. Grant asked.

"Probably just