“And I say somewhere midway between the two,” said Crewe, with a smile.  “But we will soon see.  Just hold down the end of this measuring tape, one of you.”  He produced a measuring tape as he spoke, and started to unwind it, walking rapidly towards the house as he did so.  “Sixty-two yards!” he said, as he returned.  He made a note of the distance in his pocket-book.  “So much for that,” he said, “but that’s not enough.  I want you to stand under the library window, Rolfe, by that chestnut-tree in front of it, and act as pivot for the measuring tape while I look at that window from various angles.  My idea is to go in a semicircle right round the garden, starting at the garage by the edge of the wood, so as to see the library window and measure the distance at every possible point at which Kemp could have stood.”

“You’re going to a lot of trouble for nothing, if your object is to try and prove that he couldn’t have seen into the window,” grunted Inspector Chippenfield, in a mystified voice.  “Why, I can see plainly into the window from here.”

Crewe smiled, but did not reply.  Followed by Rolfe, he went back to the tree by the library window, where he posted Rolfe with the end of the tape in his hand.  Then he walked slowly back across the garden in the direction of the garage, keeping his eye on the library window on the first floor from which Kemp, according to his evidence, had seen Sir Horace leaning out after Holymead had left the house.  He returned to the tree, noting the measurement in his book as he did so, and then repeated the process, walking backwards with his eye fixed on the window, but this time taking a line more to the left.  Again and again he repeated the process, until finally he had walked backwards from the tree in narrow segments of a big semicircle, finishing up on the boundary of the Italian garden on the other side of the grounds, and almost directly opposite to the garage from which he had started.

“There’s no use going further back than that,” he said, turning to Inspector Chippenfield, who had followed him round, smoking one of Crewe’s cigars, and very much mystified by the whole proceedings, though he would not have admitted it on any account.  “At this point we practically lose sight of the window altogether, except for an oblique glimpse.  Certainly Kemp would not come as far back as this—­he would have no object in doing so.”

“I quite agree with you,” said Inspector Chippenfield.  “He would stand more in the front of the house.  The tree in front of the house doesn’t obstruct the view of the window to any extent.”

The tree to which Inspector Chippenfield referred was a solitary chestnut-tree, which grew close to the house a little distance from the main entrance, and reached to a height of about forty feet.  Its branches were entirely bare of leaves, for the autumn frosts and winds had swept the foliage away.

Rolfe, who had been watching Crewe’s manoeuvres curiously, walked up to them with the tape in his hand.  He glanced at the library window on the first floor as he reached them.