"I shall follow this clue," he told her. "The photo was sent for a purpose, and that woman knows why it was sent. I believe you will hear from her again, unless I scared her away." "Mr Smeaton, do tell me what you really think. I am sure there is something curious in your mind," implored the agitated Sheila. But the detective was not to be charmed from his reserve. "I must think over it a lot more yet. Miss Monkton, before I can hazard any opinion," he told her in his grave, deliberate way. "If I were to reveal any half-formed idea that is running through my brain, it is one I should have to dismiss as inapplicable to the circumstances as I see them at present." From that he would not budge. Sheila drove away with a heavy heart. Wingate came round to dinner that night, and they talked about nothing else. The only thing they could arrive at with any certainty was that the mysterious visitor, the young woman dressed in navy blue serge costume, was the sender of the photo. But that did not help them to discover the reason she had sent it. That night Sheila lay awake, very depressed and anxious, still puzzling over this latest mystery. Presently she dozed, and then, after a few moments of fitful sleep, woke with a start. Was it in that brief dream that some chords of memory had been suddenly stirred of a conversation held long ago between her father and a young man named Jack Wendover, a second secretary in the diplomatic service at Madrid? Jack Wendover had told him of an ingenious method of communication invented by a married couple, who were spies in the pay of a foreign Government. She could hear him explaining it to Reginald Monkton, as she sat up in the dark, in that semiconscious state between dreaming and waking. "They were clever. They wouldn't trust to ciphers or anything of that sort, when they were separated; it was much too commonplace. They sent each other photographs. The receiver cut the photograph down, and found between the two thicknesses of cardboard a piece of tissue paper, upon which was written the message that the sender wished to convey." She could hear her father's hearty laughter, as he said: "Truly, a most ingenious method. Has that really been done?" She had not been reminded of that for nothing, she felt sure. Why had this sudden recollection of an old conversation come to her in the dead of the night, if not for some purpose? The photo was still lying upon her desk in the morning-room. The house was quite quiet. Grant slept in the basement and the maids and the footman were at the top of the house. She rose, slipped on a dressing-gown, and lighted a candle. Then noiselessly she descended the stairs and reached the morning-room. She took a small penknife from the drawer of her desk, and carefully split the mount of the photograph. When she had finished, a piece of tissue paper fluttered to the floor, and upon that paper was a message.