that the moon rock is of some composition sensitive to the action of
moon rays; somewhat as the metal selenium is to sun rays. The little
circles over the top are, without doubt, its operating agency. When
the light strikes them they release the mechanism that opens the slab,
just as you can open doors with sun or electric light by an ingenious
arrangement of selenium-cells. Apparently it takes the strength of the
full moon both to do this and to summon the Dweller in the Pool. We
will first try a concentration of the rays of the waning moon upon
these circles to see whether that will open the rock. If it does we
will be able to investigate the Pool without interruption
from--from--what emanates.

"Look, here on the chart are their locations. I have made this in
duplicate for you in the event--of something happening--to me. And if
I lose--you'll come after us, Goodwin, with help--won't you?"

And again I promised.

A little later he complained of increasing sleepiness.

"But it's just weariness," he said. "Not at all like that other
drowsiness. It's an hour till moonrise still," he yawned at last.
"Wake me up a good fifteen minutes before."

He lay upon the berth. I sat thinking. I came to myself with a
guilty start. I had completely lost myself in my deep preoccupation.
What time was it? I looked at my watch and jumped to the port-hole. It
was full moonlight; the orb had been up for fully half an hour. I
strode over to Throckmartin and shook him by the shoulder.