"I have seen them," said the other gravely. "I saw them a few moments after you entered the hall. You see, Mr. Smith," he went on, "we do not employ anything so vulgar as bells to alarm us. When the entrance door opens, a red light shows above my bed. Unfortunately, the moment you came in I happened to be in an adjoining room at work. I had to go into my bedroom to get a paper, when I saw the light. So, though I am perhaps inaccurate in saying that I have been keeping you under observation from the moment you arrived, there was little you did which was not witnessed. I will show you, if you will be good enough to accompany me to my room." "I shall be delighted," said T. B. He was curious to learn anything that the house or its custodian could teach him. Dr. Fall's room was on the first floor, immediately over the entrance hall, a plain office with a door leading to a cosily, though comparatively expensively furnished bedroom. By the side of the doctor's bed was a round pillar, which looked for all the world like one of those conventional and useless articles of furniture which the suburban housewife employs to balance a palm upon. "Look down into that," said the doctor. T. B. obeyed. It was quite hollow, and a little way down was what appeared to be a square sheet of silver paper. It was unlike any other silver paper because it appeared to be alive. He could see figures standing against it, two figures that he had no difficulty in recognizing as Ela and Johnson. "It is a preparation of my own," said the doctor. "I thought of taking out a patent for it. An adjustment of mirrors throws the image upon a luminous screen which is so sensitive to light that it can record an impression of your two friends even in the semi-darkness of the hall." "Thank you," said T. B. There was nothing to do but to accept his defeat as graciously as possible. For baffled he was, caught at every turn, and puzzled, moreover, by his extraordinary experience. "You will find some difficulty in opening the door," said the pleasant Doctor Fall. "In that I think you are mistaken," smiled T. B. The doctor stopped to switch on the light, and the two discomforted detectives watched the scene curiously. "We have left the door ajar." "Still I think you will find a difficulty in getting out," insisted the other. "Open the door." Ela pulled at it, but it was impossible to move the heavy oaken panel. "Electrically controlled," said the doctor; "and you can neither move it one way nor the other. It is an ingenious idea of mine, for which I may also apply for a patent one of these days." He took a key from his pocket and inserted it in an almost invisible hole in the oak panelling of the hall; instantly the door opened slowly. "I wish you a very good night," said Doctor Fall, as they stood on the steps. "I hope we shall meet again." "You may be sure," said T. B. Smith, grimly, "that we shall." CHAPTER XIII