“There’s a natural explanation, of course,” he said. “My theory is 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.