The cruiser vanished back into hyperspace and he was alone in the observation bubble, ten thousand light-years beyond the galaxy’s outermost sun. He looked out the windows at the gigantic sea of emptiness around him and wondered again what the danger had been that had so terrified the men before him.

Of one thing he was already certain; he would find that nothing was waiting outside the bubble to kill him. The first bubble attendant had committed suicide and the second was a mindless maniac on the Earthbound cruiser but it must have been something inside the bubble that had caused it. Or else they had imagined it all.

He went across the small room, his magnetized soles loud on the thin metal floor in the bubble’s silence. He sat down in the single chair, his weight very slight in the feeble artificial gravity, and reviewed the known facts.

The bubble was a project of Earth’s Galactic Observation Bureau, positioned there to gather data from observations that could not be made from within the galaxy. Since metallic mass affected the hypersensitive instruments the bubble had been made as small and light as possible. It was for that reason that it could accommodate only one attendant.

The Bureau had selected Horne as the bubble’s first attendant and the cruiser left him there for his six months’ period of duty. When it made its scheduled return with his replacement he was found dead from a tremendous overdose of sleeping pills. On the table was his daily-report log and his last entry, made three months before:

_I haven’t attended to the instruments for a long time because it hates us and doesn’t want us here. It hates me the most of all and keeps trying to get into the bubble to kill me. I can hear it whenever I stop and listen and I know it won’t be long. I’m afraid of it and I want to be asleep when it comes. But I’ll have to make it soon because I have only twenty sleeping pills left and if--_

The sentence was never finished. According to the temperature recording instruments in the bubble his body ceased radiating heat that same night.

The bubble was cleaned, fumigated, and inspected inside and out. No sign of any inimical entity or force could be found.

Silverman was Horne’s replacement. When the cruiser returned six months later bringing him, Green, to be Silverman’s replacement, Silverman was completely insane. He babbled about something that had been waiting outside the bubble to kill him but his nearest to a rational statement was to say once, when asked for the hundredth time what he had seen:

“Nothing--you can’t really see it. But you feel it watching you and you hear it trying to get in to kill you. One time I bumped the wall and--for God’s sake--take me away from it--take me back to Earth...”

Then he had tried to hide under the captain’s desk and the ship’s doctor had led him away.

The bubble was minutely examined again and the cruiser employed every detector device it possessed to search surrounding space for light-years in all directions. Nothing was found.

When it was time for the new replacement to be transferred to the bubble he reported to Captain McDowell.

“Everything is ready, Green,” McDowell said. “You are the next one.” His shaggy gray eyebrows met in a scowl. “It would be better if they would let me select the replacement instead of them.”

He flushed with a touch of resentment and said, “The Bureau found my intelligence and initiative of thought satisfactory.”

“I know--the characteristics you don’t need. What they ought to have is somebody like one of my engine room roustabouts, too ignorant to get scared and too dumb to go nuts. Then we could get a sane report six months from now instead of the ravings of a maniac.”

“I suggest,” he said stiffly, “that you reserve judgement until that time comes, sir.”

And that was all he knew about the danger, real or imaginary, that had driven two men into insanity. He would have six months in which to find the answer. Six months minus-- He looked at the chronometer and saw that twenty minutes had passed since he left the cruiser. Somehow, it seemed much longer...

He moved to light a cigarette and his metal soles scraped the floor with the same startling loudness he had noticed before. The bubble was as silent as a tomb.

It was not much larger than a tomb; a sphere eighteen feet in diameter, made of thin sheet steel and criss-crossed outside with narrow reinforcing girders to keep the internal air pressure from rupturing it. The floor under him was six feet up from the sphere’s bottom and the space beneath held the air regenerator and waste converter units, the storage batteries and the food cabinets. The compartment in which he sat contained chair, table, a narrow cot, banks of dials, a remote-control panel for operating the instruments mounted outside the hull, a microfilm projector, and a pair of exerciser springs attached to one wall. That was all.

There was no means of communication since a hyperspace communicator would have affected the delicate instruments with its radiations but there was a small microfilm library to go with the projector so that he should be able to pass away the time pleasantly enough.

But it was not the fear of boredom that was behind the apprehension he could already feel touching at his mind. It had not been boredom that had turned Horne into a suicide and Silverman into--

Something cracked sharply behind him, like a gunshot in the stillness, and he leaped to his feet, whirling to face it.

It was only a metal reel of data tape that had dropped out of the spectrum analyzer into the storage tray.

His heart was thumping fast and his attempt to laugh at his nervousness sounded hollow and mirthless. Something inside or outside the bubble had driven two men insane with its threat and now that he was irrevocably exiled in the bubble, himself, he could no longer dismiss their fear as products of their imagination. Both of them had been rational, intelligent men, as carefully selected by the Observation Bureau as he had been.

He set in to search the bubble, overlooking nothing. When he crawled down into the lower compartment he hesitated then opened the longest blade of his knife before searching among the dark recesses down there. He found nothing, not even a speck of dust.

Back in his chair again he began to doubt his first conviction. Perhaps there really had been some kind of an invisible force or entity outside the bubble. Both Horne and Silverman had said that “it” had tried to get in to kill them.

They had been very definite about that part.

There were six windows around the bubble’s walls, set there to enable the attendant to see all the outside-mounted instruments and dials. He went to them to look out, one by one, and from all of them he saw the same vast emptiness that surrounded him. The galaxy--his galaxy--was so far away that its stars were like dust. In the other directions the empty gulf was so wide that galaxies and clusters of galaxies were tiny, feeble specks of light shining across it.

All around him was a void so huge that galaxies were only specks in it...

Who could know what forces or dangers might be waiting out there?

A light blinked, reminding him it was time to attend to his duties. The job required an hour and he was nervous and not yet hungry when he had finished. He went to the exerciser springs on the wall and performed a work-out that left him tired and sweating but which, at least, gave him a small appetite.

The day passed, and the next. He made another search of the bubble’s interior with the same results as before. He felt almost sure, then, that there was nothing in the bubble with him. He established a routine of work, pastime and sleep that made the first week pass fairly comfortably but for the gnawing worry in his mind that something invisible was lurking just outside the windows.

Then one day he accidentally kicked the wall with his metal shoe tip.

It made a sound like that from kicking a tight-stretched section of tin and it seemed to him it gave a little from the impact, as tin would do. He realized for the first time how thin it was--how deadly, dangerously thin.