Answer:
Ryder is the person who stole the blue carbuncle.
“’Precisely so—the head attendant at the Hotel Cosmopolitan.’”
Explanation:
Arthur Conan Doyle's Sherlock Holmes Series "The Adventures of the Blue Carbuncle" revolves around the stolen jewel of the Countess of Morcar’s blue carbuncle. The story would lead Holmes and Watson through the history of a goose's origin and death to help identify the real thief.
In the given passage from the story, we can see that Holmes knows exactly what he was doing. And when the <em>"little rat-faced fellow"</em> hears from Holmes the history of how 'his' goose was passed on from Mrs. Oakshott to the Inn, he knows he must get every detail about it. Up till this part in the story, everyone who had laid hands on the goose has no idea about the diamond, so, the hasty and eager behavior of the <em>"little fellow"</em> makes Holmes realize that he is the only one to know the true importance of the goose. This shows that James Ryder, aka John Robinson, is the person who stole the diamond.
The conclusion that Ryder is the thief is supported by the line <em>"Precisely so—the head attendant at the Hotel Cosmopolitan"</em>. This is because it was at the Hotel Cosmopolitan that the robbery took place and also Ryder was the only witness to have seen and reported the missing diamond.
