it be but millionths of an inch, is sufficient. A considerable
improvement over time-traveling in past or future, with its impossible
velocities and ridiculous distances!"

"But--are those--worlds of 'if'--real?"

"Real? What is real? They are real, perhaps, in the sense that two is a
real number as opposed to [sq]-2, which is imaginary. They are the
worlds that would have been _if_-- Do you see?"

I nodded. "Dimly. You could see, for instance, what New York would have
been like if England had won the Revolution instead of the Colonies."

"That's the principle, true enough, but you couldn't see that on the
machine. Part of it, you see, is a Horsten psychomat (stolen from one of
my ideas, by the way) and you, the user, become part of the device. Your
own mind is necessary to furnish the background. For instance, if George
Washington could have used the mechanism after the signing of peace, he
could have seen what you suggest. We can't. You can't even see what
would have happened if I hadn't invented the thing, but _I_ can. Do you
understand?"

"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