Pendleton stared in amazement at the simplicity of the thing.

“I see,” said he.  “While Edyth stood listening on the stairs someone opened this door!”

“Yes; someone unacquainted with the place.  Otherwise he would have known of the bell.”

“But how did you know that Edyth heard a door close?”

“Whoever rang the bell closed the door after him.  It has a spring lock like the street door; and was locked when Miss Vale tried it a few moments later.”

“You say that the ringing of the bell shows the person who rang the bell to have been unacquainted with the place.  I think you must be wrong here.  Spatola is acquainted with the place; he was here at the time.  This is proven by the scream of the frightened cockatoo which followed the ringing of the bell.”

“It was not a cockatoo that made the sound,” said Ashton-Kirk.  “Give me a moment and I think I can convince you of that.”

The gas in the hall was lighted; the investigator stopped at the foot of the stairs leading to the fourth floor.

“Persons,” he continued, “who secretly enter buildings, as a rule never trust to the lighting apparatus of the buildings.  One reason for this is that it is not under their control—­another that they cannot carry their light about with them.”

He pointed to the lowermost step of the flight; there, as before, were the stump of candle, the burnt matches, the traces of tallow upon the wood.

“There were two or more men concerned in this crime,” proceeded Ashton-Kirk, “and that is the method of lighting that they chose—­a candle.”

“Two men!  How do you know that?” asked Pendleton.

“You shall see in a moment,” replied the investigator.  Then he continued:  “And the candle was used not only for illumination—­it served another purpose, and so supplied me with the first definite information that my searching had given me up to that time.”

Pendleton looked at the discouraged little candle end, with its long black wick, the two charred splinters of pine wood and the eccentric trail of tallow droppings.  Then he shook his head.

“How you could get enlightenment from those things is beyond me,” he said.  “But tell me what they indicated.”

“The candle and the match-sticks count for little,” said Ashton-Kirk.  “It is the tracings of melted tallow that possess the secret.  Look closely at them.  At first glance they may seem the random drippings of a carelessly held light.  But a little study will show you a clearly defined system contained in them.”

“Well, you might say there were three lines of it,” said Pendleton, after a moment’s inspection.

“Right,” said Ashton-Kirk.  “Three lines there are, and each follows a row of tack heads.  These latter were, apparently, once driven in to hold down a step-protector of some sort which has since become worn out and been removed.”

The speaker took a pad of paper and a pencil from his pocket.  Across the pad he drew three lines one under the other.  Then with another glance at the candle droppings upon the step, he made a copy of them that looked like this: