“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.