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.”