e, 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--

"It has changed again: make another note."

"What is it now?"

"Sodium. The yellow bands are unmistakable."

A deep stillness reigned as before.

"There she goes again," exclaimed the professor, much excited. "Now I
can see a couple of blue lines. What can that be? I believe it's
indium."

Another long pause ensued.

"Now they are gone," ejaculated Gazen once more. "A red and a yellow
line have taken their place. That should be lithium. Hey, presto!--and
all was dark."

"What's the matter?"

"It's all over." With these words he removed the spectroscope from the
telescope, and gazed anxiously at the planet "The light is gone," he
continued, after a minute. "Perhaps another cloud is passing over it.
Well, we must wait. In the meantime let us consider the situation. It
seems to me that we have every reason to be satisfied with our night's
work. What do you think?"

There was a glow of triumph on his countenance as he came and stood
before me.

"I believe it's a signal," said I, with an air of conviction.

"But how?"

"Why should it change so regularly? I've timed each spectrum, and found
it to last about five minutes before another took its place."

The professor remained thoughtful and silent.

"Is it not by the light which comes from them that we have gained all
our knowledge of the constitution of the heavenly bodies?" I continued.
"A ray from the remotest star brings in its heart a secret message to
him who can read it. Now, the Martians would naturally resort to the
same medium of communication as the most obvious, simple, and
practicable. By producing a powerful light they might hope to attract