He stood regarding us sardonically. And in that instant while we were stricken with the shock of it, I saw that the figure was fading. It was a solid human form no longer! A silvery cast had come upon it. Another second passed; it was visibly growing tenuous, wraithlike! It was melting while we stared at it, until in that breathless instant I realized that the wall behind it was showing through. A wraith! An apparition! The vision of a ghost standing there, leering at us! The soldiers had retreated back into the corridor behind us. The sergeant gripped me, and his other hand, wavering with fright, clutched a revolver. "But it's--it's going!" Don gasped, "Too late! Sergeant, give me that gun!" "Wait!" I shouted. "Don't shoot at it!" The shimmering glowing white figure was slowly moving downward as though floating through the cell-floor. Its own invisible surface was evidently not here but lower down, and it was beginning to drop. I don't know what frenzied courage--if courage it could be called--was inspiring me. I was wholly confused, but nevertheless I struck Don and the sergeant aside and rushed at the thing. * * * * * It was a sensation most horrible. From the waist up it was still above the floor of the cell. My wildly flailing arms went through the chest! But I felt nothing. It was not even like waving aside a mist. There was nothing. I saw my solid fist plunge through the leering ghostly face. I fought wildly, with a panic upon me, against the glowing phosphorescent nothingness of the apparition. My feet were stamping on its chest and shoulders. Then, as it sank lower, only the grinning face was down there. Panting, and with the cold sweat of horror upon me, I felt Don shoving me aside. "Too late!" And then the sergeant's shot rang out. The bullet clattered against the solid stone floor of the cell. The acrid smoke of the powder rolled over us; and cleared in a moment to show us the apparition several feet below the floor level. It seemed to strike its solidity of ground. I saw it fall the last little distance with a rush; land, and pick itself up. And with a last sardonic grin upward at us, the dim white figure ran. Dwindling smaller, dimmer, until in a moment it was gone into the Unknown. As though a light had struck upon me came the realization. "Don, this is rational, this thing! Some strange science!" All day we had been vaguely realizing it. Intangible, but rational enemies were stealing white girls of Bermuda. Invaders from another planet? We had thought it might be that. Certainly it was nothing supernatural. These was not ghosts. But now came a new realization. "Don! That's another world down there! Another realm! The fourth dimension--that's what it is! These things everybody's calling ghosts--it's the fourth dimension, Don! People of the fourth dimension coming out to attack us!"