How does Conan Doyle present London as a mysterious and sinister setting for a mystery in The Sign of Four?

Conan Doyle presents London as shrouded in fog and mystery in The Sign of Four. As Holmes and Watson drive to the Lyceum Theatre in Chapter 3 with Mary Morstan, they are surrounded by fog:

"The day had been a dreary one, and a dense drizzly fog lay low upon the great city. Mud-coloured clouds drooped sadly over the muddy streets. Down the Strand the lamps were but misty splotches of diffused light which threw a feeble circular glimmer upon the slimy pavement."

London is dreary, and the light in this type of weather is refracted in strange ways. Watson says about people in this type of light: "they flitted from the gloom into the light and so back into the gloom once more," and he says this movement from the gloom back to the light is "like all mankind."

This is a fitting setting for a mystery in which characters are cast into the darkness of doubt and then removed into the light of innocence, and vice versa. The setting in London, alternating between gloom and light, is also a metaphor for Holmes's moods, as he alternates between gloomy depression, when he takes cocaine and morphine, and periods of great energy, when he is solving mysteries.