"Thanks," he said, calmly. "Z be it. I thought you would. Reflex hypnotism, my dear Watson, is my forte. If I wish a man to choose Q, B takes hold upon him. If I wish him to choose K, A fills his mind. Have you ever observed how the mind of man repels a suggestion and flees to something else, merely that it may demonstrate its independence of another mind? Now I have been suggesting J to you, and you have chosen Z--" "You misunderstood me," I cried, desperately. "I did not say Z; I said P." "Quite so," said he, with an inward chuckle. "P was the letter I wished you to choose. If you had insisted upon Z, I should really have been embarrassed. See!" he added. He removed the green-ended box that rested in the pigeon-hole marked Z, and, opening it, disclosed an emptiness. "I've never had a Z case. But P," he observed, quietly, "is another thing altogether." Here he took out the box marked P from the pigeon-hole, and, opening it, removed the contents--a single paper which was carefully endorsed, in his own handwriting, "The Mystery of Pinkham's Diamond Stud." "You could not have selected a better case, Watson," he said, as he unfolded the paper and scanned it closely. "One would almost think you had some pre-vision of the fact." "I am not aware," said I, "that you ever told the story of Pinkham's diamond stud. Who was Pinkham, and what kind of a diamond stud was it--first-water or Rhine?" "Pinkham," Holmes rejoined, "was an American millionaire, living during business hours at Allegheny City, Pennsylvania, where he had to wear a brilliant stud to light him on his way through the streets, which are so dark and sooty that an ordinary search-light would not suffice. In his leisure hours, however, he lived at the Hotel Walledup-Hysteria, in New York, where he likewise had to wear the same diamond stud to keep him from being a marked man. Have you ever visited New York, Watson?" "No," said I.