“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 nebulae, 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—­

“By all that’s wonderful, the spectrum has changed.  Eureka!  It’s thallium now.  I should know that splendid green line amongst a thousand.”

“Thallium!” I exclaimed, astonished in my turn.

“Yes,” responded Gazen, hurriedly.  “Make a note of the observation, and also of the time.  You will find a book for the purpose lying on the desk.”

I did as directed, and awaited further orders.  The silence was so great that I could plainly hear the ticking of my watch laid on the desk before me.  At the end of several minutes the professor cried—­