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.
He reported his success crisply.
Burke took off the helmet.