"'All that we see or seem
Is but a dream within a dream.'
"But come, Bill," he added. "Don't let this a priori stuff bowl you over. In the first place, it isn't dark down there—when, that is, you get down far enough."
"In Heaven's name, how do you know that?"
"Why, for one thing, if this subterranean world was one of unbroken darkness, the angel (and the demon) would be blind, like those poor fishes in the Mammoth Cave. But she is no more blind than you or I. Ergo, if for no other reason, we shall find light down there."
"Of course, they have artificial light, or—"
"I don't mean that. If there had not been some other illumination, this strange race (of whose very existence Science has never even dreamed) would have ceased to exist long ago—if, indeed, it ever could have begun."
"But no gleam of sunlight can ever find its way down to that world."
"It never can, of course. But there are other sources of light—nebulas and comets in the heavens, for example, and auroras, phosphorus and fire-flies here on earth. The phenomena of phosphorescence are by no means so rare as might be imagined. Why, as Nichol showed, though any man who uses his eyes can see it himself, there is light inherent even in clouds."
I have the professor's book before me as I write—J. P. Nichol, LL.D., Professor of Astronomy in the University of Glasgow—and here are his own words: