He walked to the wall and stared, for the dislodgment of plaster and paper had revealed three round black discs, set flush with the plaster and only separated from the room by the wallpaper, which had been stripped. "Jumping Moses!" said the colonel softly. "Detectaphones!" He took Pinto's knife from his hand and prised one of the discs loose. It was attached to a wire which was embedded in the plaster and this the colonel severed with a stroke of the knife. "This is the business end of a microphone," he said. "The voice!" gasped Pinto, and the colonel nodded. "Of course. I was mad not to guess that," he said. "That's how he heard and that's how he spoke. Now, we're going to get to the bottom of this." With a knife he slashed the plaster and exposed three wires that led straight downward and apparently through the floor. The colonel rested and eyed the debris thoughtfully. "What is under this flat? Lee's office, isn't it? Of course, Lee's!" he said. "I'm the fool!" He handed the knife back to Pinto, took an electric torch from his pocket and led the way from the flat. They passed down the half-darkened stairs to the floor beneath, on which was situated the three sets of offices. The colonel took a bunch of keys and tried them on the door of the surveyor's office. Presently he found one that fitted, and the door opened. He fumbled about for the electric switch, found it and flooded the room with light. It was a very ordinary clerk's office, with a small counter, the flap of which was raised. Inside the flap he saw something white on the floor, and, stooping, picked it up. It was a lady's handkerchief. "L," he read. "That sounds like Lollie. Do you know this, Crewe?" Crewe took the handkerchief and nodded. "That is Lollie's," he said shortly. "I thought so. This is where she was when we were looking for her. Here with Jack o' Judgment, eh? Let's try the inner office." The inner office was locked, but he had no difficulty in gaining admission. Inside this was a private office which was simply furnished and had in one corner what appeared to be a telephone box. He opened the glass door and flashed his lamp inside. There was a little desk, a pair of receivers fastened to a headpiece, and a small vulcanite transmitter. "This is where he sat," said the colonel meditatively, pointing to a stool, "and this----" he lifted up the earpieces--"is how he heard all our very interesting conversations. Go upstairs, Pinto, I want to try this transmitter." He fixed the receiver to his ears and waited, and presently he heard distinctly the sound of Pinto closing the door of the room upstairs. Then he spoke through the receiver. "Do you hear me, Pinto?" "I hear you distinctly," said Pinto's voice. "Speak a little lower. Carry on a conversation with yourself and let me try to hear you."