He gave one searching glance at the corpse. In a moment he whipped out a microscope.
"Ha! ha!" he said, as he picked a hair off the lapel of the dead man's coat. "The mystery is now solved."
He held up the hair.
"Listen," he said, "we have only to find the man who lost this hair and the criminal is in our hands."
The inexorable chain of logic was complete.
The detective set himself to the search.
For four days and nights he moved, unobserved, through the streets of New York scanning closely every face he passed, looking for a man who had lost a hair.
On the fifth day he discovered a man, disguised as a tourist, his head enveloped in a steamer cap that reached below his ears. The man was about to go on board the Gloritania.
The detective followed him on board.
"Arrest him!" he said, and then drawing himself to his full height, he brandished aloft the hair.