Uncle Otto said, “Don’t show it to me. Just tear it up. In little pieces tear it up and in this beaker the fragments put.”

I tore it into one hundred and twenty-eight pieces.

He considered them thoughtfully and began adjusting knobs on a – well, on a machine. It had a thick opal-glass slab attached to it that looked like a dentist’s tray.

There was a wait. He kept adjusting.

Then he said, “Aha!” and I made a sort of queer sound that doesn’t translate into letters.

About two inches above the glass tray there was what seemed to be a fuzzy piece of paper. It came into focus while I watched and – oh, well, why make a big thing out of it? It was my notes. My handwriting. Perfectly legible. Perfectly legitimate.

“Is it all right to touch it?” I was a little hoarse, partly out of astonishment and partly because of my uncle Otto’s gentle ways of enforcing secrecy.

“You can’t,” he said, and passed his hand through it. The paper remained behind, untouched. He said, “It’s only an image at one focus of a four-dimensional paraboloid. The other focus is at a point in time before you tore it up.”

I put my hand through it, too. I didn’t feel a thing.

“Now watch,” he said. He turned a knob on the machine and the image of the paper vanished. Then he took out a pinch of paper from the pile of scrap, dropped them in an ashtray, and set a match to it. He flushed the ash down the sink. He turned a knob again and the paper appeared, but with a difference. Ragged patches in it were missing.

“The burned pieces?” I asked.

“Exactly. The machine must trace in time along the hypervectors of the molecules on which it is focused. If certain molecules are in the air dispersed – pff-f-ft!”

I had an idea. “Suppose you just had the ash of a document.”

“Only those molecules would be traced back.”