"Of course. You mean the background has to rest in the past experiences of the user."

"You're growing brilliant," he scoffed. "Yes. The device will show ten hours of what would have happened if — condensed, of course, as in a movie, to half an hour's actual time."

"Say, that sounds interesting!"

"You'd like to see it? Is there anything you'd like to find out? Any choice you'd alter?"

"I'll say — a thousand of 'em. I'd like to know what would have happened if I'd sold out my stocks in 2009 instead of '10. I was a millionaire in my own right then, but I was a little — well, a little late in liquidating."

"As usual," remarked van Manderpootz. "Let's go over to the laboratory then."

The professor's quarters were but a block from the campus. He ushered me into the Physics Building, and thence into his own research laboratory, much like the one I had visited during my courses under him. The device — he called it his "subjunctivisor," since it operated in hypothetical worlds — occupied the entire center table. Most of it was merely a Horsten psychomat, but glittering crystalline and glassy was the prism of Iceland spar, the polarizing agent that was the heart of the instrument.

Van Manderpootz pointed to the headpiece. "Put it on," he said, and I sat staring at the screen of the psychomat. I suppose everyone is familiar with the Horsten psychomat; it was as much a fad a few years ago as the ouija board a century back. Yet it isn't just a toy; sometimes, much as the ouija board, it's a real aid to memory.