Suddenly he jumped up, his face alight.  “Bill, I’ve got it!” he cried.

“What?”

“The shadow on the wall!  I was looking at the shadow on the wall.  Oh, ass, and ten times ass!”

Bill looked uncomprehendingly at him.  Antony took his arm and pointed to the wall of the passage.

“Look at the sunlight on it,” he said.  “That’s because you’ve left the door of that room open.  The sun comes straight in through the windows.  Now, I’m going to shut the door.  Look!  D’you see how the shadow moves across?  That’s what I saw the shadow moving across as the door shut behind him.  Bill, go in and shut the door behind you quite naturally.  Quick!”

Bill went out and Antony knelt, watching eagerly.

“I thought so!” he cried.  “I knew it couldn’t have been that.”

“What happened?” said Bill, coming back.

“Just what you would expect.  The sunlight came, and the shadow moved back again all in one movement.”

“And what happened yesterday?”

“The sunlight stayed there; and then the shadow came very slowly back, and there was no noise of the door being shut.”

Bill looked at him with startled eyes.

“By Jove!  You mean that Cayley closed the door afterwards as an afterthought and very quietly, so that you couldn’t hear?”

Antony nodded.

“Yes.  That explains why I was surprised afterwards when I went into the room to find the door open behind me.  You know how those doors with springs on them close?”

“The sort which old gentlemen have to keep out draughts?”

“Yes.  Just at first they hardly move at all, and then very, very slowly they swing to well, that was the way the shadow moved, and subconsciously I must have associated it with the movement of that sort of door.  By Jove!” He got up, and dusted his knees.  “Now, Bill, just to make sure, go in and close the door like that.  As an afterthought, you know; and very quietly, so that I don’t hear the click of it.”

Bill did as he was told, and then put his head out eagerly to hear what had happened.

“That was it,” said Antony, with absolute conviction.  “That was just what I saw yesterday.”  He came out of the office, and joined Bill in the little room.

“And now,” he said, “let’s try and find out what it was that Mr. Cayley was doing in here, and why he had to be so very careful that his friend Mr. Gillingham didn’t overhear him.”

CHAPTER XIII

The Open Window

Anthony’s first thought was that Cayley had hidden something; something, perhaps, which he had found by the body, and but that was absurd.  In the time at his disposal, he could have done no more than put it away in a drawer, where it would be much more open to discovery by Antony than if he had kept it in his pocket.  In any case he would have removed it by this time, and hidden it in some more secret place.  Besides, why in this case bother about shutting the door?