B. stole carefully up the stairs, leaving the two men to guard the hall below. At every landing he halted, and listened, but the house was wrapped in silence, and he searched the third floor without mishap.

He recognized the corridor, having taken very careful note of certain peculiarities, and a scratch on the side of the lift door, which he had mentally noted for future reference, showed him he was on the right track.

Unerringly and swiftly he pa.s.sed along the pa.s.sage till he came to the big rosewood doors which opened upon the invalid's bedroom. He turned the handle gently, it yielded, and he stepped noiselessly through the door, and pushed the inner door cautiously. The room was dimly illuminated, evidently by a night light, thought T. B., and he pressed the door farther open that he might secure a better view of the apartment, and then he gasped, for this was not the room he had been in before.

It was a sumptuously arranged bureau, panelled in rosewood, and set about with costly furniture. A man was sitting at the desk, busily writing by the light of a table lamp; his back was toward T. B. The detective pushed the door farther open, and suddenly the man at the desk leapt up, and turning round, confronted the midnight visitor.

T. B. had only time to see that his face was hidden behind a black mask which extended from his forehead to his chin. As soon as he saw T. B.

standing in the doorway, he reached out his hand. Instantly the room was in darkness, and the door, which T. B. was holding ajar, was suddenly forced back as if by an irresistible power, flinging the detective into the corridor, which almost simultaneously was flooded with light. T. B.

turned to meet the smiling face of Dr. Fall.

The big man, with his white, expressionless countenance, was regarding him gravely, and with amused resentment.

Where he had come from T. B. could only conjecture; he had appeared as if by magic and was fully dressed.

"To what do I owe the honour of this visit, Mr. Smith?" he said, in his dry, grim way.

"A spirit of curiosity," said T. B., coolly. "I was anxious to secure another peep at your Mr. Moole."

"And how did he look?" asked the other, with a faint smile.

"Unfortunately," said T. B., "I have mistaken the floor, and instead of seeing our friend, I have unexpectedly and quite unwittingly interrupted a gentleman who, for reasons best known to himself, has hidden his face."

Dr. Fall frowned.

"I do not quite follow you," he said.

"Perhaps if I were to follow you back to the room," said T. B.

good-humouredly, "you might understand better."

He heard a strange wailing sound and a shivering motion beneath his feet, as though a heavy traction engine were pa.s.sing close to the house.

"What is that?" he asked.

"It is one of the unpleasant consequences of building one's house over a disused coal-mine," said the doctor easily; "but as regards your strange hallucination," he went on, "I should rather like to disabuse your mind of your fantastic vision."

He walked slowly back to the room which T. B. had quitted, and the inner door yielded to his touch. It was in darkness. Dr. Fall put his hand inside the room and there was a click of a switch.

"Come in," he said, and T. B. stepped into the room.

It was the room he had left in the earlier part of the day. There was the blue square of carpet and the silver bedstead, and the same yellow face and unwinking eyes of the patient. The walls were panelled in myrtle, the same electrolier hung from the ceiling as he had seen on his previous visit. Smith gasped, and pa.s.sed his hand over his forehead.

"You see," said the secretary, "you have been the victim of a peculiar and unhappy trick of eyesight; in fact, Mr. Smith, may I suggest that you have been dreaming?"

"You may suggest just what you like," said T. B. pleasantly. "I should like to see the room below and the room above."

"With pleasure," said the other; "there is a storeroom up above which you may see if you wish."

He led the way upstairs, unlocked the door of the room immediately over that which they had just left, and entered. The room was bare, and the plain deal floor, the distempered walls, and the high skylight showed it to be just as the doctor had described, a typical storeroom.

"You do not seem to use it," said T. B.

"We are very tidy people," smiled the doctor; "and now you shall see the room below."

As they went down the stairs again they heard the curious wail, and T.

B. experienced a tremulous jar which he had noted before.

"Unpleasant, is it not?" said Dr. Fall. "I was quite alarmed at that at first, but it has no unpleasant consequences."

On the second floor he entered the third room, immediately below that in which the sick Mr. Moole was lying. He unlocked this door and they entered a well-furnished bedroom; on a more elaborate scale than that which T. B. had seen before.

"This is our spare bedroom," said Dr. Fall, easily; "we seldom use it."

T. B. slipped into the apartment and made a quick scrutiny. There was nothing of a suspicious character here.

"I hope you are satisfied now," said Dr. Fall as he led the way out, "and that your two friends below are not growing impatient."

"You have seen them, then," said T. B.

"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."