Medhurst nodded.  “Why, certainly,” he said; and, for the first time, I detected a lingering trace of American accent.  “It’s my business to know about him.”

“Well, I want you to catch him,” Charles went on.

Medhurst drew a long breath.  “Isn’t that rather a large order?” he murmured, surprised.

Charles explained to him exactly the sort of services he required.  Medhurst promised to comply.  “If the man comes near you, I’ll spot him,” he said, after a moment’s pause.  “I can promise you that much.  I’ll pierce any disguise.  I should know in a minute whether he’s got up or not.  I’m death on wigs, false moustaches, artificial complexions.  I’ll engage to bring the rogue to book if I see him.  You may set your mind at rest, that, while I’m about you, Colonel Clay can do nothing without my instantly spotting him.”

“He’ll do it,” Marvillier put in.  “He’ll do it, if he says it.  He’s my very best hand.  Never knew any man like him for unravelling and unmasking the cleverest disguises.”

“Then he’ll suit me,” Charles answered, “for I never knew any man like Colonel Clay for assuming and maintaining them.”

It was arranged accordingly that Medhurst should take up his residence in the house for the present, and should be described to the servants as assistant secretary.  He came that very day, with a marvellously small portmanteau.  But from the moment he arrived, we noticed that Césarine took a violent dislike to him.

Medhurst was a most efficient detective.  Charles and I told him all we knew about the various shapes in which Colonel Clay had “materialised,” and he gave us in turn many valuable criticisms and suggestions.  Why, when we began to suspect the Honourable David Granton, had we not, as if by accident, tried to knock his red wig off?  Why, when the Reverend Richard Peploe Brabazon first discussed the question of the paste diamonds, had we not looked to see if any of Amelia’s unique gems were missing?  Why, when Professor Schleiermacher made his bow to assembled science at Lancaster Gate, had we not strictly inquired how far he was personally known beforehand to Sir Adolphus Cordery and the other mineralogists?  He supplied us also with several good hints about false hair and make-up; such as that Schleiermacher was probably much shorter than he looked, but by imitating a stoop with padding at his back he had produced the illusion of a tall bent man, though in reality no bigger than the little curate or the Graf von Lebenstein.  High heels did the rest; while the scientific keenness we noted in his face was doubtless brought about by a trifle of wax at the end of the nose, giving a peculiar tilt that is extremely effective.  In short, I must frankly admit, Medhurst made us feel ashamed of ourselves.  Sharp as Charles is, we realised at once he was nowhere in observation beside the trained and experienced senses of this professional detective.