_Burke sweated. There was a speck with a yellow ring about it. It was a globe, poised and ready to dart in any conceivable direction if an Enemy detection-device ranged it. The globes did not go seeking an Enemy. They placed themselves where they would be sought. They set themselves up as targets. But when a radar-pulse touched them, they flung themselves at its source, their reflex chooser-circuits pouring incredible power into a beam of the same characteristics as the radar-touch. That beam, of course, paralyzed or burned out the Enemy device necessarily tuned to it. And the globes plunged at the thing which had found them. They accelerated at a hundred and sixty gravities and mere high explosive would be wasted if they carried it. Nothing could stand their impact. Nothing!_ _But in drills three drones had dodged them. The counter-tactics men understood the drones, of course, as it was hoped the Enemy did not. But it should not be possible to get to the fortress! If the fortress was vulnerable, so was the Empire. If the Empire was vulnerable, the Enemy would wreck its worlds, blast its cities, exterminate its population and only foulness would remain in the Galaxy._ _On the monitor-board a light flashed. A line of green light darted across the screen. It was the path of a globe hurtling toward something that had touched it with a radar-frequency signal. The acceleration of the globe was breathtaking. It seemed to explode toward its target._ _But this globe hit nothing. It went on and on.... A second globe sprang. It also struck nothing. It went away to illimitable emptiness. Its path exactly crossed that of the first. A third and fourth and fifth.... Each one flung itself ferociously at the source of some trickle of radiation. Their trails crossed at exactly the same spot. But there was nothing there...._ _Burke suddenly flung up a row of switches, inactivating the remaining globes under his control. Five had flung themselves away, darting at something which radiated but did not exist. Something which was not solid. Which was not a drone ship impersonating an Enemy. They'd attacked an illusion...._ _At the control-board. Burke clenched his fist and struck angrily at the flat surface before him. An illusion! Of course!_ _Cunningly, he made adjustments. He had five globes left. He chose one and changed the setting of its reflex chooser-circuit. It would ignore radar frequencies now. It would pick up only stray radiation--induction frequencies from a drone ship with its drive on._ _The globe's light flashed. A train of green fire appeared. A burst of flame. A hit! The drone was destroyed. He swiftly changed the setting of the reflex circuits of the rest. Two! Three! Three drones blasted in twice as many seconds._ _He mopped his forehead. This was only a drill, but when the Enemy came it would be the solution of such problems that would determine the survival of the fortress and the destruction of the Enemy._