A thousand feet of open and empty field separated them from the space ship. With straining eyes they could see tiny human figures scurrying about its base in the moonlight, forming a protective circle. Then floodlights went on all over the field and left not a shadow anywhere. The Germans knew, or feared, that the invisible attackers had slipped inside their citadel. The rain of steel on the gateway had stopped; instead came dull thudding concussions, and a creeping haze obscured the entrance. Gas. They had prepared for that. But now a more formidable threat made itself known; from near the buildings came a frenzied barking of dogs. "We've got to get across the open," snapped Kane. "Better stick together and run for it. If we can get among that gang around the ship--neither dogs nor instruments can tell the difference between visible and invisible men!" They rose from their cover and pelted grimly across the endless-seeming field. To the right, parties of men with dogs were fanning out, too slowly to intercept the raiders. But they were only halfway to the ship when the lights suddenly snapped off--for a moment they stumbled, blinded, in darkness, and the lights flashed dazzlingly on again. A couple of seconds later the puzzling action was repeated-- And from the cordon about the ship, so near now, a voice screamed hysterically, "_Da lauft einer!_" On its heels came a thunder of shooting, and bullets snapped past the hurrying Americans. They flung themselves flat on the scorched ground. The lights flashed again and again as if an insane hand were at the master switch. "What's happened?" gasped Manning. "Did they see us?" "They've learned or guessed one of our weaknesses," said Kane in his ear. "When the whole field of vision is suddenly illuminated, the brain may register an invisible object, especially if it's moving, for a moment before it melts into the background." Something whimpered in the air and burst with an ear-splitting bang only a few yards away, showering the raiders with earth. "They've got our position. Get going--_now_!" as the lights flashed on. [Illustration: _They rose and charged toward the space ship._] Kane's fingers had been busy fusing a bomb, and as he rose erect he threw it straight into the cordon. The crash of the explosion was followed by shrieks and then, as the lights flashed on again, by a prolonged volley of shots. Larrabie sprung around in midstride and rolled on the ground. The long-legged Clark flung out his arm and pitched forward. "Get the--" His voice choked off. * * * * * The survivors charged at the gap where the bomb had wrought bloody havoc. The Germans were closing it from the sides. Manning caught a glimpse of sweating faces, staring eyes glazed with fear of an enemy they could not see; and he saw also the vast loom of the space ship above him. He fumbled woodenly with a detonator cartridge.