He passed inside, while Grayne turned to the policeman in attendance and handed him a scribbled note, to be telegraphed from headquarters.  The man saluted and hurried off; and Grayne, following his friend into the library, found him beside the bookstand in the middle of the room, on which were the empty cups.

“This is where Boyle looked for Budge, or pretended to look for him, according to your account,” he said.

As Fisher spoke he bent down in a half-crouching attitude, to look at the volumes in the low, revolving shelf, for the whole bookstand was not much higher than an ordinary table.  The next moment he sprang up as if he had been stung.

“Oh, my God!” he cried.

Very few people, if any, had ever seen Mr. Horne Fisher behave as he behaved just then.  He flashed a glance at the door, saw that the open window was nearer, went out of it with a flying leap, as if over a hurdle, and went racing across the turf, in the track of the disappearing policeman.  Grayne, who stood staring after him, soon saw his tall, loose figure, returning, restored to all its normal limpness and air of leisure.  He was fanning himself slowly with a piece of paper, the telegram he had so violently intercepted.

“Lucky I stopped that,” he observed.  “We must keep this affair as quiet as death.  Hastings must die of apoplexy or heart disease.”

“What on earth is the trouble?” demanded the other investigator.

“The trouble is,” said Fisher, “that in a few days we should have had a very agreeable alternative—­of hanging an innocent man or knocking the British Empire to hell.”

“Do you mean to say,” asked Grayne, “that this infernal crime is not to be punished?”

Fisher looked at him steadily.

“It is already punished,” he said.

After a moment’s pause he went on.  “You reconstructed the crime with admirable skill, old chap, and nearly all you said was true.  Two men with two coffee cups did go into the library and did put their cups on the bookstand and did go together to the well, and one of them was a murderer and had put poison in the other’s cup.  But it was not done while Boyle was looking at the revolving bookcase.  He did look at it, though, searching for the Budge book with the note in it, but I fancy that Hastings had already moved it to the shelves on the wall.  It was part of that grim game that he should find it first.

“Now, how does a man search a revolving bookcase?  He does not generally hop all round it in a squatting attitude, like a frog.  He simply gives it a touch and makes it revolve.”

He was frowning at the floor as he spoke, and there was a light under his heavy lids that was not often seen there.  The mysticism that was buried deep under all the cynicism of his experience was awake and moving in the depths.  His voice took unexpected turns and inflections, almost as if two men were speaking.