I was forced to agree.

"Let us, then, form the provisional theory that every nine days the professor takes some strong drug which has a passing but highly poisonous effect. His naturally violent nature is intensified by it. He learned to take this drug while he was in Prague, and is now supplied with it by a Bohemian intermediary in London. This all hangs together, Watson!"

"But the dog, the face at the window, the creeping man in the passage?"

"Well, well, we have made a beginning. I should not expect any fresh developments until next Tuesday. In the meantime we can only keep in touch with friend Bennett and enjoy the amenities of this charming town."

In the morning Mr. Bennett slipped round to bring us the latest report. As Holmes had imagined, times had not been easy with him. Without exactly accusing him of being responsible for our presence, the professor had been very rough and rude in his speech, and evidently felt some strong grievance. This morning he was quite himself again, however, and had delivered his usual brilliant lecture to a crowded class. "Apart from his queer fits," said Bennett, "he has actually more energy and vitality than I can ever remember, nor was his brain ever clearer. But it's not he – it's never the man whom we have known."

"I don't think you have anything to fear now for a week at least," Holmes answered. "I am a busy man, and Dr. Watson has his patients to attend to. Let us agree that we meet here at this hour next Tuesday, and I shall be surprised if before we leave you again we are not able to explain, even if we cannot perhaps put an end to, your troubles. Meanwhile, keep us posted in what occurs."

I saw nothing of my friend for the next few days, but on the following Monday evening I had a short note asking me to meet him next day at the train. From what he told me as we traveled up to Camford all was well, the peace of the professor's house had been unruffled, and his own conduct perfectly normal. This also was the report which was given us by Mr. Bennett himself when he called upon us that evening at our old quarters in the Chequers. "He heard from his London correspondent today. There was a letter and there was a small packet, each with the cross under the stamp which warned me not to touch them. There has been nothing else."

"That may prove quite enough," said Holmes grimly. "Now, Mr. Bennett, we shall, I think, come to some conclusion tonight. If my deductions are correct we should have an opportunity of bringing matters to a head. In order to do so it is necessary to hold the professor under observation. I would suggest, therefore, that you remain awake and on the lookout. Should you hear him pass your door, do not interrupt him, but follow him as discreetly as you can. Dr. Watson and I will not be far off. By the way, where is the key of that little box of which you spoke?"

"Upon his watch-chain."

"I fancy our researches must lie in that direction. At the worst the lock should not be very formidable. Have you any other able-bodied man on the premises?"

"There is the coachman, Macphail."

"Where does he sleep?"

"Over the stables."

"We might possibly want him. Well, we can do no more until we see how things develop, Good-bye – but I expect that we shall see you before morning."

It was nearly midnight before we took our station among some bushes immediately opposite the hall door of the professor. It was a fine night, but chilly, and we were glad of our warm overcoats. There was a breeze, and clouds were scudding across the sky, obscuring from time to time the half-moon. It would have been a dismal vigil were it not for the expectation and excitement which carried us along, and the assurance of my comrade that we had probably reached the end of the strange sequence of events which had engaged our attention.

"If the cycle of nine days holds good then we shall have the professor at his worst tonight," said Holmes. "The fact that these strange symptoms began after his visit to Prague, that he is in secret correspondence with a Bohemian dealer in London, who presumably represents someone in Prague, and that he received a packet from him this very day, all point in one direction. What he takes and why he takes it are still beyond our ken, but that it emanates in some way from Prague is clear enough. He takes it under definite directions which regulate this ninth-day system, which was the first point which attracted my attention. But his symptoms are most remarkable. Did you observe his knuckles?"

I had to confess that I did not.

"Thick and horny in a way which is quite new in my experience. Always look at the hands first, Watson. Then cuffs, trouser knees, and boots. Very curious knuckles which can only be explained by the mode of progression observed by —" Holmes paused and suddenly clapped his hand to his forehead. "Oh, Watson, Watson, what a fool I have been! It seems incredible, and yet it must be true. All points in one direction. How could I miss seeing the connection of ideas? Those knuckles how could I have passed those knuckles? And the dog! And the ivy! It's surely time that I disappeared into that little farm of my dreams. Look out, Watson! Here he is! We shall have the chance of seeing for ourselves."

The hall door had slowly opened, and against the lamplit background we saw the tall figure of Professor Presbury. He was clad in his dressing gown. As he stood outlined in the doorway he was erect but leaning forward with dangling arms, as when we saw him last.

Now he stepped forward into the drive, and an extraordinary change came over him. He sank down into a crouching position and moved along upon his hands and feet, skipping every now and then as if he were overflowing with energy and vitality. He moved along the face of the house and then round the corner. As he disappeared Bennett slipped through the hall door and softly followed him.

"Come, Watson, come!" cried Holmes, and we stole as softly as we could through the bushes until we had gained a spot whence we could see the other side of the house, which was bathed in the light of the half-moon. The professor was clearly visible crouching at the foot of the ivy-covered wall. As we watched him he suddenly began with incredible agility to ascend it. From branch to branch he sprang, sure of foot and firm of grasp, climbing apparently in mere joy at his own powers, with no definite object in view. With his dressing-gown flapping on each side of him, he looked like some huge bat glued against the side of his own house, a great square dark patch upon the moonlit wall. Presently he tired of this amusement, and, dropping from branch to branch, he squatted down into the old attitude and moved towards the stables, creeping along in the same strange way as before. The wolfhound was out now, barking furiously, and more excited than ever when it actually caught sight of its master. It was straining on its chain and quivering with eagerness and rage. The professor squatted down very deliberately just out of reach of the hound and began to provoke it in every possible way. He took handfuls of pebbles from the drive and threw them in the dog's face, prodded him with a stick which he had picked up, flicked his hands about only a few inches from the gaping mouth, and endeavored in every way to increase the animal's fury, which was already beyond all control. In all our adventures I do not know that I have ever seen a more strange sight than this impassive and still dignified figure crouching frog-like upon the ground and goading to a wilder exhibition of passion the maddened hound, which ramped and raged in front of him, by all manner of ingenious and calculated cruelty.