"They may capture one of ours by a fluke. Then we'd all be bear-meat."
"Hardly possible, sir."
"Then accept it as hypothetical, Carrington. Take off from there and answer my question."
"That I cannot do, sir. Frankly, I do not know."
"Then listen. I have an idea; I want you to pass on its value."
"I shall try, sir."
"Carrington, is it possible to establish a celestial globe that is capable of giving a negative action? No, wait, I'll explain. Our present celestial globe is positive; it operates by three-dimensional fluorescence in the sphere, glowing when a positive radiation comes in from a spaceship. What I want is a negative indication: one that will glow in any location from which there comes absolutely zero radiation. Is that possible?"
"Hm-m-m," mused Carrington. "Our present level of detection is based upon the maximum level of celestial radiation, which is fairly constant in all directions save Solward. Your supposed sphere would operate on the celestial radiation—with the normal globe the entire sphere would glow—and be dark everywhere except in a place where all radiation were absorbed. It would be devilishly ticklish, sir."
"You follow my reasoning?"
"Oh certainly. Your idea is to prepare a sphere that glows with no signal. That can be done with a local signal, which is cut when no-radiation enters. Hard to say in words, isn't it?"