"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: "Whatever their origin, they [the auroras] show the existence of causes in virtue of whose energy the upper strata of our atmosphere become self-luminous, sometimes in a high degree; for, in northern regions, our travelers have _read_ by their brilliance. But the Aurora is not the only phenomenon which indicates the existence of a power in the matter of our globe to emit light. One fact, that must have been often noticed, forcibly impresses me with the conviction, that here, through what seems common, truths of much import will yet be reached. In the dead of night, when the sky is clear, and one is admiring the brilliancy of the stars, hanging over a perfectly obscured earth, a cloud, well known to observing astronomers, will at times begin to form, and it then spreads with astonishing rapidity over the whole heavens. The light of the stars being thus utterly shut out, one might suppose that surrounding objects would, if possible, become more indistinct: but no! what was formerly invisible can now be clearly seen; not because of lights from the earth being reflected back from the cloud--for very often there are none--but in virtue of _the light of the cloud itself_, which, however faint, is yet a similitude of the dazzling shell of the Sun." After mentioning the phosphorescence of the dark hemisphere of Venus and the belief that something similar has been seen on the unillumined surface of our satellite, he continues thus: