“The presence of a man or woman in that room would be evident to a person listening miles away,” he went on. “A high-frequency current is constantly passing through that storeroom. That is what causes that normal buzzing.”
It was verging on midnight when Kennedy suddenly cried: “Here, Walter, take this receiver. You remember how the buzzing sounded. Listen. Tell me if you, too, can detect the change.”
I clapped the receiver quickly to my ear. Indeed I could tell the difference. In place of the load buzzing there was only a mild sound. It was slower and lower.
“That means,” he said excitedly, “that some one has entered that pitch-dark storeroom by the broken window. Let me take the receiver back again. Ah, the buzzing is coming back. He is leaving the room. I suppose he has found the electric light cane and the pistol where he left them. Now, Walter, since you have become accustomed to this thing take it and tell me what you hear.”
Craig had already seized the other apparatus connected with the art-gallery and had the wireless receiver over his head. He was listening with rapt attention, talking while he waited.
“This is an apparatus,” he was saying, “that was devised by Dr. Fournier d’Albe, lecturer on physics at Birmingham University, to aid the blind. It is known as the optophone. What I am literally doing now is to HEAR light. The optophone translates light into sound by means of that wonderful little element, selenium, which in darkness is a poor conductor of electricity, but in light is a good conductor. This property is used in the optophone in transmitting an electric current which is interrupted by a special clockwork interrupter. It makes light and darkness audible in the telephone. This thing over my head 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.”