"I am afraid they are not artificial. The doubling is chiefly observed at the vernal equinox, our month of May, and is perhaps due to spring floods, or vegetation in valleys of the like trend, as we find in Siberia. The massing of clouds or mists will account for the peculiar whiteness at the edge of the limb, and an occasional veiling of the landscape." While he spoke, my attention was suddenly arrested by a vivid point of light which appeared on the dark side of the terminator, and south of the equator. "Hallo!" I exclaimed, involuntarily. "There's a light!" "Really!" responded Gazen, in a tone of surprise, not unmingled with doubt. "Are you sure?" "Quite. There is a distinct light on one of the continents." "Let me see it, will you?" he rejoined, hastily; and I yielded up my place to him. "Why, so there is," he declared, after a pause. "I suspect it has been hidden under a cloud till now." We turned and looked at each other in silence. "It can't be the light Javelle saw," ejaculated Gazen at length. "That was on Hellas Land." "Should the Martians be signalling they would probably use a system of lights. I daresay they possess an electric telegraph to work it." The professor put his eye to the glass again, and I awaited the result of his observation with eager interest. "It's as steady as possible," said he. "The steadiness puzzles me," I replied. "If it would only flash I should call it a signal." "Not necessarily to us," said Gazen, with mock gravity. "You see, it might be a lighthouse flashing on the Kaiser Sea, or a night message in the autumn manoeuvres of the Martians, who are, no doubt, very warlike; or even the advertisement of a new soap." "Seriously, what do you think of it?" I asked. "I confess it's a mystery to me," he answered, pondering deeply; and then, as if struck by a sudden thought, he added: "I wonder if it's any good trying the spectroscope on it?" So saying, he attached to the telescope a magnificent spectroscope, which he employed in his researches on the nebulæ, and renewed his observation. "Well, that's the most remarkable thing in all my professional experience," he exclaimed, resigning his place at the instrument to me. "What is?" I demanded, looking into the spectroscope, where I could distinguish several faint streaks of coloured light on a darker background. "You know that we can tell the nature of a substance that is burning by splitting up the light which comes from it in the prism of a spectroscope. Well, these bright lines of different colours are the spectrum of a luminous gas." "Indeed! Have you any idea as to the origin of the blaze?" "It may be electrical--for instance, an aurora. It may be a volcanic eruption, or a lake of fire such as the crater of Kilauea. Really, I can't say. Let me see if I can identify the bright lines of the spectrum." I yielded the spectroscope to him, and scarcely had he looked into it ere he cried out--