“They are a good deal like the old electroscopes, only infinitely more sensitive. You see that gold leaf,” she pointed to two tiny ribbons of gold that hung limply together, “when a wave from a radio-active source, such as radium, comes along, those ribbons fly apart. All our reflectoscopes are discharged now, but they’ll be charged later.” As we spoke, Tom joined us. “I’ve sent Jones down-stairs for the radium in the safe, Dorothy,” he said, and we three stood looking silently at the instruments before us. Through the open windows a fresh breeze fluttered in, and the soft night gave back but the slightest hum, a minimum of that sound that never ceases in the quietest hours of the great city. A church tower rang out--One, Two, Three, Four. Tom glanced at the chronometer. “Just right,” he said, and looked back. A strange hush filled the air. Again a terrific force seemed to be pulling me towards Dorothy, but my eyes never turned from the reflectoscopes. Suddenly, as I gazed, the golden ribbons sprang to life, parted and stood stiffly separate. “Good heavens!” cried Tom. “What did that? They were perfectly insulated. What did that, Dorothy? It must be Jones bringing the radium.” Dorothy’s eyes glowed with excited interest. “I don’t think it was Jones,” she said eagerly. “I believe I know what it was, but anyway, let’s go first and see where Jones is. There’s absolutely nothing else in the laboratory that could have charged them, insulated as they were.” Down the stairs, flight after flight, four in all, we trooped, and found Jones in an office on the first floor, seated in a chair before the safe, and looking disconsolately at its closed door. At Tom’s voice, he rose. “Professor, I’ve forgotten the combination again. I was sitting here trying to bring it to mind.” “Then you haven’t taken the radium from the safe at all?” shouted Tom, in wild excitement. “No,” answered Jones, staring in amazement. “Then how in blazes did those reflectoscopes get charged?” Jones showed a sudden interest, “Have they got charged again?” “Yes, have they been charged before?” “Twice before, and I meant to speak to you about it, but it slipped my mind.” “When did it happen?” Dorothy broke in. “I’ve got full particulars noted down, up-stairs,” said Jones. “But how about the combination?” “Never mind that,” cried Tom. “Let me see your data.” Rapidly we ascended, the slower Jones following some way behind. In the laboratory the assistant turned to a littered desk and fumbled among a mass of papers. I could see that Dorothy was burning with impatience which I could not understand. Jones fumbled on, picking up paper after paper, peering at them blindly through his black-rimmed spectacles. Tom seized my arm and walked me down the room impatiently.