, 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