* * * * * Terror seized him, with the knowledge. He had nerved himself to face desperate men, or a savage animal. But an invisible being, that could creep upon him and strike unseen! It was incredible ... yet he had seen the dog knocked down, and the bleeding wound it had received. His heart paused, then beat very quickly. For the moment he thought only blindly, of escape. He knew only an overpowering desire to hide, to conceal himself from the invisible thing. Had it been possible, he might have tried to leave the flier. Beside him was one of the companionways amidships, giving access to a compartment of the vessel that he had not explored. He turned, leaped down the steps, with the terrified dog at his heels. Below, he found himself in a short hall, dimly lighted. Several metal doors opened from it. He tried one at random. It gave. He sprang through, let the dog follow, closed and locked it. Trying to listen, he leaned weakly against the door. The rushing of his breath, swift and regular. The loud hammer of his thudding heart. The dog's low whines. Then--unmistakable scraping sounds, outside. The scratching of claws, Thad knew. Invisible claws! He stood there, bracing the door with the weight of his body, holding the welding arc ready in his hand. Several times the hinges creaked, and he felt a heavy pressure against the panels. But at last the scratching sounds ceased. He relaxed. The monster had withdrawn, at least for a time. When he had time to think, the invisibility of the thing was not so incredible. The mounted creatures he had seen in the hold were evidence that the flier had visited some unknown planet, where weird life reigned. It was not beyond reason that such a planet should be inhabited by beings invisible to human sight. Human vision, as he knew, utilizes only a tiny fraction of the spectrum. The creature must be largely transparent to visible light, as human flesh is radiolucent to hard X-rays. Quite possibly it could be seen by infra-red or ultra-violet light--evidently it was visible enough to the dog's eyes, with their different range of sensitivity. * * * * * Pushing the subject from his mind, he turned to survey the room into which he had burst. It had apparently been occupied by a woman. A frail blue silk dress and more intimate items of feminine wearing apparel were hanging above the berth. Two pairs of delicate black slippers stood neatly below it. Across from him was a dressing table, with a large mirror above it. Combs, pins, jars of cosmetic cluttered it. And Thad saw upon it a little leather-bound book, locked, stamped on the back "Diary."