When Watson goes to Baker Street to rejoin Holmes that night, he finds that his friend has two visitors.
On entering his room, I found Holmes in animated conversation with two men, one of whom I recognised as Peter Jones, the official police agent...
Jones is a detective from Scotland Yard. As a private detective, Holmes would not have the authority to arrest John Clay, whom he expected to break into the bank vault through the flooring. Jones is one of the many official police detectives used by the author to provide Holmes with an aegis; they do not solve the cases but give the great detective the authorization to investigate them and participate actively in the apprehension of criminals. Having a police official as an aegis is one of the conventions established by Edgar Allan Poe when he invented the mystery story with "The Murders in the Rue Morgue" and "The Purloined Letter." In "The Murders in the Rue Morgue" Auguste Dupin tells his friend the anonymous narrator:
We will go and see the premises with our own eyes. I know G—, the Prefect of Police, and shall have no difficulty in obtaining the necessary permission.”
In "The Purloined Letter" the same G--. the Prefect of Police, actually comes to Dupin for help in retrieving the important purloined document.
Peter Jones serves the same function as G--. He and other detectives in the Sherlock Holmes stories customarily exist to give official sanction to Holmes' investigations. Jones makes it convenient for Holmes to wrap up the case. He places John Clay under arrest and takes him away. When Holmes and Watson are driving to the bank that night, Holmes gives Watson some further information about the Scotland Yard detective:
I thought it as well to have Jones with us also. He is not a bad fellow, though an absolute imbecile in his profession. He has one positive virtue. He is as brave as a bulldog and as tenacious as a lobster if he gets his claws upon anyone.