yond it? What did that smooth stone, whose wan deadness
whispered of ages-old corridors of time opening out into alien,
unimaginable vistas, hide? It had cost the world of science
Throckmartin's great brain--as it had cost Throckmartin those he
loved. It had drawn me to it in search of Throckmartin--and its shadow
had fallen upon the soul of Olaf the Norseman; and upon what thousands
upon thousands more I wondered, since the brains that had conceived it
had vanished with their secret knowledge?

What lay beyond it?

I stretched out a shaking hand and touched the surface of the slab. A
faint thrill passed through my hand and arm, oddly unfamiliar and as
oddly unpleasant; as of electric contact holding the very essence of
cold. O'Keefe, watching, imitated my action. As his fingers rested on
the stone his face filled with astonishment.

"It's the door?" he asked. I nodded. There was a low whistle from
him and he pointed up toward the top of the grey stone. I followed the
gesture and saw, above the moon door and on each side of it, two
gently curving bosses of rock, perhaps a foot in diameter.

"The moon door's keys," I said.

"It begins to look so," answered Larry. "If we can find them," he
added.

"There's nothing we can do till moonrise," I replied. "And we've none
too much time to prepare as it is. Come!"

A little later we were beside our boat. We lightered it, set up the
tent, and as it was now but a short hour to sundown I bade them leave
me and make their search. They went off together, and I busied myself
with opening some of the paraphernalia I had brought with me.

First of all I took out the two Becquerel ray-condensers that I had
bought in Sydney. Their lenses would collect and intensify to the
fullest extent any light directed upon them. I had found them most
useful in making spectroscopic analysis of luminous vapours, and I
knew that at Yerkes Observatory splendid results had been obtained
from them in collecting the diffused radiance of the nebulae for the
same purpose.

If my theory of the grey slab's mechanism were correct, it was
practically certain that with the satellite only a few nights past the
full we could concentrate enough light on the bosses to open the rock.
And as the ray streams through the seven globes described by
Throckmartin would be too weak to energize the Pool, we could enter
the chamber free from any fear of encountering its tenant, make our
preliminary observations