tell you. _One second!_" He glared at me. "Now all you have to do
is to design such a machine, and then van Manderpootz will admit the
possibility of traveling into the future--for a limited number of
seconds. As for the past, I have just explained that all the energy in
the universe is insufficient for that."

"But," I stammered, "you just said that you--"

"I did _not_ say anything about traveling into either future or past,
which I have just demonstrated to you to be impossible--a practical
impossibility in the one case and an absolute one in the other."

"Then how _do_ you travel in time?"

"Not even van Manderpootz can perform the impossible," said the
professor, now faintly jovial. He tapped a thick pad of typewriter paper
on the table beside him. "See, Dick, this is the world, the universe."
He swept a finger down it. "It is long in time, and"--sweeping his hand
across it--"it is broad in space, but"--now jabbing his finger against
its center--"it is very thin in the fourth dimension. Van Manderpootz
takes always the shortest, the most logical course. I do not travel
along time, into past or future. No. Me, I travel across time,
sideways!"

I gulped. "Sideways into time! What's there?"

"What would naturally be there?" he snorted. "Ahead is the future;
behind is the past. Those are real, the worlds of past and future. What
worlds are neither past nor future, but contemporary and
yet--extemporal--existing, as it were, in time parallel to our time?"

I shook my head.

"Idiot!" he snapped. "The conditional worlds, of course! The worlds of
'if.' Ahead are the worlds to be; behind are the worlds that were; to
either side are the worlds that might have been--the worlds of 'if!'"

"Eh?" I was puzzled. "Do you mean that you can see what will happen if I
do such and such?"

"No!" he snorted. "My machine does not reveal the past nor predict the
future. It will show, as I told you, the conditional worlds. You might
express it, by 'if I had done such and such, so and so would have
happened.' The worlds of the subjunctive mode."

"Now how the devil does it do that?"

"Simple, for van Manderpootz! I use polarized light, polarized not in
the horizontal or vertical planes, but in the direction of the fourth
dimension--an easy matter. One uses Iceland spar under colossal
pressures, that is all. And since the worlds are very thin in the
direction of the fourth dimension, the thickness of a single light wave,
though