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 loud 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
