Almost as soon as Watson meets Mary Morstan in Chapter 2, he begins to fall in love with her. This causes considerable tension for Watson, as the emotional effect of Mary's presence is a confusing counterpoint to Holmes's relentless, logic-driven detective work. Watson uses a metaphor to describe this tension:
So I sat and mused until such dangerous thoughts came into my head that I hurried away to my desk and plunged furiously into the latest treatise upon pathology. What was I, an army surgeon with a weak leg and a weaker banking account, that I should dare to think of such things? She was a unit, a factor—nothing more. If my future were black, it was better surely to face it like a man than to attempt to brighten it by mere will-o'-the-wisps of the imagination.
As Watson steels himself to "face" the reality of his situation, he dismisses his feelings for Morstan as "will-o'-the-wisps," or misleading, supernatural ghost lights, of the imagination. Will-o'-the-wisps are a common feature of the gothic genre, a genre whose trappings that Doyle employs throughout his novel, and this is a prime example of how the mysterious, otherworldly imagery of gothic fiction pervades all corners of the story.
It is fitting that Watson should lean on such supernatural imagery: if Holmes is the embodiment of scientific reason in the novel, Watson is the embodiment of a certain kind of emotional irrationality—just the sort of person who might believe in a will-o'-the-wisp. As Watson struggles to think of Morstan as "a unit" or "a factor" rather than a person, this passage becomes a key moment in the early pages of the novel in which the reader sees how Watson's emotional side comes into conflict with a colder, harsher value system clearly imposed by Holmes.
Adhering to the conventions of the Victorian Gothic, Doyle describes the city of London as a dark, forbidding place. In Chapter 3, he describes London's urban sprawl in the metaphorical language of some vast, tentacled monster:
We had indeed reached a questionable and forbidding neighbourhood. Long lines of dull brick houses were only relieved by the coarse glare and tawdry brilliancy of public-houses at the corner. Then came rows of two-storeyed villas, each with a fronting of miniature garden, and then again interminable lines of new, staring brick buildings—the monster tentacles which the giant city was throwing out into the country.
One common feature of the gothic genre is the sinister invocation of the supernatural. The Sign of the Four contains no literal supernatural entities—this would be entirely incompatible with the novel's valorization of science and reason through the detective work of Sherlock Holmes. But nonetheless, Doyle is able to suggest the presence of a dark, supernatural influence on the human world through metaphorical language like the "monster tentacles" found in this passage.
By this description, London's suburban outgrowth becomes a subject of no uncertain horror and suspense—and Doyle sets the reader on edge without needing to break the scientific laws of his world. Language like this pervades The Sign of the Four, enhancing its gothic ambience and intensifying its dark mood.
The Sign of the Four is the product of colonialism in more ways than one. The narrative revolves around the theft of a vast treasure trove from India, which was under Britain's colonial rule at the time, and Doyle's descriptions of some of his characters are also marked by racist, colonized attitudes toward people of color. In his descriptions of Tonga, in Chapter 10, Doyle relies on metaphorical language of inhumanity and animal rage:
Never have I seen features so deeply marked with all bestiality and cruelty. Hs small eyes glowed and burned with a sombre light, and his thick lips were writhed back from his teeth, which grinned and chattered at us with half animal fury. [...]
I can see the two of them now as they stood, the white man with his legs far apart, shrieking out curses, and the unhallowed dwarf with his hideous face, and his strong yellow teeth gnashing at us in the light of our lantern.
This racist metaphorical language includes the description of Tonga's lips and teeth as writhing, grinning, chattering, and gnashing with "half animal fury" as well as the characterization of his features as "marked with all bestiality." The color and light imagery of Tonga's yellow teeth and glowing eyes also adds to the animalistic description. Through such language, Doyle reduces Tonga to qualities that he identifies with wild animals—and, contrasting Tonga to Jonathan Small, Doyle also demonstrates the racist belief that Black people are less human than white people. This is a particularly blatant instance of Doyle's colonial and imperialist sympathies.
