"How on earth could I have obtained that photograph of her in the darkness?" he demanded. "N-rays," said the Tracer coolly. "It has been done in France." "Yes, from living people, but " "What the N-ray is in living organisms, we must call, for lack of a better term, the subaura in the phantom." They bent over the photograph together. Presently the Tracer said: "She is very, very beautiful?"