Robinson ran his fingers through his hair.

“That’s what I’ve been asking myself,” he said.  “I went over that panelling a dozen times myself.”

Bobby and Katherine went closer.  Bobby had been from the first puzzled by Paredes’s easy manner.  He had a quick hope.  He saw the man watch with an amused tolerance while the district attorney bent over, examining the face of the panel.

“An entire section,” Robinson said—­“the thickness of the wall—­has been shifted to one side.  No wonder we didn’t see any joints or get a hollow sound from this panel any more than from the others.  But why didn’t we stumble on the mechanism?  Maybe you’ll tell us that, Paredes.”

The Panamanian blew a wreath of smoke against the ancient wall.

“Gladly, but you will find it humiliating.  I have experienced humility in this hall myself.  The reason you didn’t find any mechanism is that there wasn’t any.  You looked for something most cautiously concealed, not realizing that the best concealment is no concealment at all.  It’s fundamental.  I don’t know how it slipped my own mind.  No grooves show because the door is an entire panel.  There isn’t even a latch.  You merely push hard against its face.  Such arrangements are common enough in colonial houses, and there was more than the nature of the crimes to tell you there was some such thing here.  I mean if you will examine the farther door closer than you have done you will find that it has fewer coats of paint than the one leading to the corridor, that its frame is of newer wood.  In other words, it was cut through after the wing was built.  This panel was the original door, designed, with the private stairway and the hall, for the exclusive use of the master of the house.  Try it.”

Robinson braced himself and shoved against the panel.  It moved in its grooves with a vibrant stirring.

“Rusty,” he said.

Katherine started.

“That’s what I heard each time,” she cried.

Above his heavy black beard the doctor’s cheeks whitened.  Robinson made a gesture of revulsion.

“That gives the nasty game away.”

“Naturally,” Paredes said, “and you must admit the game is as beautifully simple as the panel.  The instrument of death wasn’t inserted through the bedding as you thought inevitable, Doctor.  Suppose you were lying in that bed, asleep, or half asleep, and you were aroused by such a sound as that in the wall behind you?  What would you do?  What would any man do first of all?”

Robinson nodded.

“I see what you mean.  I’d get up on my elbow.  I’d look around as quickly as I could to see what it was.  I’d expose myself to a clean thrust.  I’d drop back on the bed, more thoroughly out of it than though I’d been struck through the heart.”

“Exactly,” Paredes said, with the familiar shrug of his shoulders.

“You’re sensible to give up this way,” Robinson said.  “It’s the best plan for you.  What about Mr. Blackburn?”