is like a wireless telephone receiver, capable of detecting a current of even a quarter of a microampere."
We were all waiting expectantly for Craig to speak. Evidently the intruder was now mounting the stairs to the art-gallery.
"Actually I can hear the light of the stars shining in through that wonderful plate glass skylight of yours, Mr. Spencer," he went on. "A few moments ago when the moon shone through I could hear it, like the rumble of a passing cart. I knew it was the moon both because I could see that it must be shining in and because I recognised the sound. The sun would thunder like a passing express-train if it were daytime now. I can distinguish a shadow passing between the optophone and the light. A hand moved across in front of it would give a purring sound, and a glimpse out of a window in daylight would sound like a cinematograph reeling off a film.
"Ah, there he is." Craig was listening with intense excitement now. "Our intruder has entered the art-gallery. He is flashing his electric light cane about at various objects, reconnoitring. No doubt if I were expert enough and had had time to study it, I could tell you by the sound just what he is looking at."
"Craig," I interrupted, this time very excited myself, "the buzzing from the high-frequency current is getting lower and lower."
"By George, then, there is another of them," he replied. "I'm not surprised. Keep a sharp watch. Tell me the moment the buzzing increases again."
Spencer could scarcely control his impatience. It
