"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 and go forth before the moon had dropped so far that the concentration in the condensers would fall below that necessary to keep the portal from closing. I took out also a small spectroscope, and a few other instruments for the analysis of certain light manifestations and the testing of metal and liquid. Finally, I put aside my emergency medical kit. I had hardly finished examining and adjusting these before O'Keefe and Huldricksson returned. They reported signs of a camp at least ten days old beside the northern wall of the outer court, but beyond that no evidence of others beyond ourselves on Nan-Tauach. We prepared supper, ate and talked a little, but for the most part were silent. Even Larry's high spirits were not in evidence; half a dozen times I saw him take out his automatic and look it over. He was more thoughtful than I had ever seen him. Once he went into the tent, rummaged about a bit and brought out another revolver which, he said, he had got from Da Costa, and a half-dozen clips of cartridges. He passed the gun over to Olaf. At last a glow in the southeast heralded the rising moon. I picked up my instruments and the medical kit; Larry and Olaf shouldered each a short ladder that was part of my equipment, and, with our electric flashes pointing the way, walked up the great stairs, through the enclosures, and straight to the grey stone.