A sudden whistle like that from Muller's lips was something that made the Imperial Police Force sit up and take notice, for it meant that things were happening, and that the happenings were likely to become exciting. It was a habit he could control only by the severest effort of the will, an effort which he kept for occasions when it was absolutely necessary. Here, alone with the harmless old man, he was not so much on his guard, and the sudden vibrating of every nerve at the word "Marburg," found vent in the whistle which surprised old Franz. One young police commissioner with a fancy for metaphor had likened this sudden involuntary whistle of Muller's to the bay of the hound when he strikes the trail; which was about what it was.

"Yes, I am merry sometimes," he said with a laugh. "It's a habit I have. Something occurred to me just then, something I had forgotten. Hope you don't mind.""Oh, no, there's no one here now, whistle all you like."But Muller's whistle was not a continuous performance, and he had now completely mastered the excitation of his nerves which had called it forth. He threw another sharp look at the picture of the man who lived in Marburg, and then asked: "And now where is the button?""By the window there, beside the desk." Franz led the way with his candle.

"Why, how funny! What are those mirrors there for?" asked the electrician in a tone of surprise, pointing to two small mirrors hanging in the window niche. They were placed at a height and at such a peculiar angle that no one could possibly see his face in them.

"Something the master is experimenting with, I guess. He's always making queer experiments; he knows a lot about scientific things."Muller shook his head as if in wonderment, and bent to investigate the button which was fastened into the wall beneath the window sill.

His quick ear heard a carriage stopping in front of the house, and heard the closing of the front door a moment later. To facilitate his examination of the button, the detective had seated himself in the armchair which stood beside the desk. He half raised himself now to let the light of the candle fall more clearly on the wiring - then he started up altogether and threw a hasty glance at the mirrors above his head. A ray of light had suddenly flashed down upon him - a ray of red light, and it came reflected from the mirrors. Muller bit his lips to keep back the betraying whistle.

"What's the matter?" asked the butler. "Did you drop anything?""Yes, the wooden rim of the button," replied Muller, telling the truth this time. For he had held the little wooden circlet in his hands at the moment when the red light, reflected down from the mirrors, struck full upon his eyes. He had dropped it in his surprise and excitement. Franz found the little ring in the centre of the room where it had rolled, and the supposed electrician replaced it and rose to his feet, saying: "There, I've finished now."Franz did not recognise the double meaning in the words. "Yes, it's all right! I've finished here now," Muller repeated to himself.

For now he knew beyond a doubt that the red light was a signal - and he knew also for whom this signal was intended. It was a signal for Herbert Thorne! - Herbert Thorne, whom no single thought or suspicion of Muller's had yet connected with the murder of Leopold Winkler.

The detective was very much surprised and greatly excited. But Franz did not notice it, and indeed a far keener observer than the slow-witted old butler might have failed to see the sudden gleam which shot up in the grey eyes behind the heavy spectacles, might have failed to notice the tightening of the lips beneath the blond moustache, or the tenseness of the slight frame under the assumed embonpoint. Muller's every nerve was tingling, but he had himself completely in hand.

"What do we owe you?" asked Franz.

"They'll send you a bill from the office. It won't amount to much.

I must be getting on now."

Muller hastened out of the door and down the street to the nearest cab stand. There were not very many cab stands in this vicinity, and the detective reasoned that Mrs. Bernauer would naturally have taken her cab from the nearest station. He had heard her return in her carriage, presumably the same in which she had started out.

There was but one cab at the stand. Muller walked to it and laid his hand on the door.

"Oh, Jimmy! must I go out again?" asked the driver hoarsely.

"Can't you see the poor beast is all wet from the last ride? We've just come in." He pointed with his whip to the tired-looking animal under his blanket.

"Why, he does look warm. You must have been making a tour out into the country," said the blond gentleman in a friendly tone.

"No, sir, not quite so far as that. I've just taken a woman to the main telegraph office in the city and back again. But she was in a hurry and he's not a young horse, sir.""Well, never mind, then; I can get another cab across the bridge,"replied the stout blond man, turning away and strolling off leisurely in the direction of the bridge. It was now quite dark, and a few steps further on Muller could safely turn and take the road to his own lodging. No one saw him go in, and in a few moments the real Muller, slight, smooth-shaven, sat down at his desk, looking at the papers that lay before him. They were three letters and an empty envelope.

He took up the last, and compared it carefully with the envelope of one of the letters found in Winkler's room - the unsigned letter postmarked Hietzing, September 24th. The two envelopes were exactly alike. They were of the same size and shape, made of the same cream-tinted, heavy, glossy paper, and the address was written by the same hand. This any keen observer, who need not necessarily be an expert, could see. The same hand which had addressed the envelope to Mrs. Adele Bernauer on the letter which was postmarked "Venice," about thirty-six hours previous - this hand had, in an awkward and childish attempt at disguise, written Winkler's address on the envelope which bore the date of September 24th.