What are four clues to the mystery hidden in the story Wilson tells Holmes and Watson?
One important clue which Sherlock Holmes immediately picks up on while listening to Jabez Wilson's story is that Wilson's new clerk got hired because he was willing to come for half wages. According to the pawnbroker:
"I used to be able to keep two assistants, but now I only keep one; and I would have a job to pay him but that he is willing to come for half wages so as to learn the business.”
Holmes immediately suspects that the new assistant must have some ulterior reason for wanting to work in that particular shop.
Another important clue is that the assistant is supposedly an amateur photographer.
“Oh, he has his faults, too,” said Mr. Wilson. “Never was such a fellow for photography. Snapping away with a camera when he ought to be improving his mind, and then diving down into the cellar like a rabbit into its hole to develop his pictures. That is his main fault, but on the whole he's a good worker. There's no vice in him.”
The facts that Jabez Wilson had spent years at sea while working as a ship's carpenter and that he had spent some time in China would make it plausible that he might not have heard anything about the foundation of the Red-Headed League by the American millionaire, although his assistant tells him that it was talked about all over London. Holmes sees Wilson's long absence from England as a strong clue, although Wilson himself thinks nothing of it. The assistant, whose real name is John Clay, picked a dull-witted man whose shop was in a suitable location for his criminal plan, and who would fall for a story about a totally fictitious institution called The Red-Headed League because he had been away for years and rarely ever left his shop when he was in London.
“‘Have you never heard of the League of the Red-headed Men?’ he asked with his eyes open.”
“‘Why, I wonder at that, for you are eligible yourself for one of the vacancies.’”
Wilson's description of his new assistant provides Holmes with some very special clues, because Holmes thinks he knows who this assistant really is.
“Small, stout-built, very quick in his ways, no hair on his face, though he's not short of thirty. Has a white splash of acid upon his forehead.”
Holmes sat up in his chair in considerable excitement. “I thought as much,” said he. “Have you ever observed that his ears are pierced for ear-rings?”
“Yes, sir. He told me that a gipsy had done it for him when he was a lad.”
Jabez Wilson tells Holmes and Watson that when he reported for work that morning he found a card attached to the office-door stating that the Red-Headed League had been dissolved. Holmes already feels sure that Wilson was only given the job to keep him out of the way while John Clay and his accomplice were digging their tunnel to the bank. Time was an important factor because there was always the chance that the French gold they were after might be moved to another location. Holmes deduces that the two crooks must be ready to go after the gold that very weekend, which was why they dissolved the Red-Headed League, having no further need to keep Wilson away from his shop during the day.
Holmes reads a lot more into Wilson's very long backstory than Wilson can see himself. The same is true for Watson. He hears the whole story but does not see the clues. Holmes does not bother to explain them to his friend after Wilson leaves because the detective wants to concentrate on what he has heard from the red-headed visitor. After the case is all wrapped up and Clay has been taken off to jail, Holmes reveals some of his deductions, including the following.
“You see, Watson,” he explained in the early hours of the morning as we sat over a glass of whisky and soda in Baker Street, “it was perfectly obvious from the first that the only possible object of this rather fantastic business of the advertisement of the League, and the copying of the Encyclopaedia, must be to get this not over-bright pawnbroker out of the way for a number of hours every day. It was a curious way of managing it, but, really, it would be difficult to suggest a better."