If the fourth dimension is really a dimension and not a mathematical abstraction—" he smiled confidentially as he emphasized the if; "can we not build a hyper-stereoscopic instrument which will build up a three dimensional model of a fourth-dimensional object into an image perceptible to the brain in its true four-dimensional form?'

I continued to stare blankly from him to the stereoscope and back again.

"As a matter of fact," he continued; "our three dimensional world is merely a cross-section cut by what we know as space out of the Cosmos that exists in four or more dimensions. Our three-dimensional world bears the same relation to the true status of affairs as do these flat photographs to the models that you photographed.

(Hyperstereoscope from 'The Book of Worlds' by Miles J. Breuer)

"Points on the adjacent leaves of a book are far apart, considered two-dimensionally. But, with the book closed, and to a three-dimensional perception which can see across from one page to another, the two points are very near together. You see?"

I nodded again.

"Now look!"

I saw a dense swamp, among huge trees with broad, rich green leaves. Gigantic saurians stalked about and splashed hugely.

"It is like a story of evolution," I couldn't help remarking.

He nodded in satisfaction and mused on:

"Each of these must be a separate and distinct world. I can go back and forth among them at will. It is not a continuous story. There are steps. Definite jumps. Nothing between. I can see any one of them at any time. Like the leaves of a book!"

I looked again. The professor had not touched the setting and the scene was exactly the same. A huge saurian was devouring some living creature from the water. The water was threshed into a pink foam, and light-red blood was splashed over the green foliage. The professor was talking:

"What we see is worlds or universes arranged side by side in the fourth dimension. Like leaves in a book.

"Heavens! What an encyclopedia!"

"I see," I said slowly, not sure that I really did. "Like serial sections cut in a microtome."