Through dramatic irony concerning Sherlock's eccentricities as a detective, the story contributes not only to the humorous tone, but also to the characterization of Sherlock as an oddball genius. "The Adventure of the Speckled Band" is one of eight stories in a collection by Sir Arthur Conan Doyle featuring Sherlock Holmes. This story, then, operates under the assumption that readers are already familiar with Sherlock's quirks. As a result, dramatic irony is present each time a new character is taken aback upon encountering Sherlock for the first time.
There are two instances of this in this story. First, Helen Roylott is so astounded by Sherlock's ability to deduce the fact that she traveled to his office by dog-cart and train that she "gave a violent start and stared in bewilderment at my companion." In this example, the reader is aware of Sherlock's deductive abilities whereas Helen is not, thus producing dramatic irony.
Second, when Dr. Grimesby Roylott charges into Sherlock's office to berate him for meddling in his affairs, he is enraged by Sherlock's "imperturbable" confidence. Here, it is not Sherlock's prowess as a detective but rather his eccentricity that generates dramatic irony. In both instances, the reader's existing knowledge of Sherlock lets them enjoy other characters' bewilderment and consternation from a distance.
