"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?"