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

In this second Sherlock Holmes novel, Arthur Conan Doyle uses various locations in and near central London as a setting for a two-fold mystery: what happened to Captain Morstan, and who has been sending his daughter, Mary, a pearl each year since his disappearance.

In Chapter Three, "In Quest of a Solution," Watson describes the London streets Holmes, Watson, and Miss Morstan pass through:

"The yellow glare from the shop-windows streamed out into the steamy,...

In this second Sherlock Holmes novel, Arthur Conan Doyle uses various locations in and near central London as a setting for a two-fold mystery: what happened to Captain Morstan, and who has been sending his daughter, Mary, a pearl each year since his disappearance.

In Chapter Three, "In Quest of a Solution," Watson describes the London streets Holmes, Watson, and Miss Morstan pass through:

"The yellow glare from the shop-windows streamed out into the steamy, vaporous air, and threw a murky, shifting radiance across the crowded thoroughfare. There was, to my mind, something eerie and ghost-like in the endless procession of faces which flitted across these narrow bars of light,—sad faces and glad, haggard and merry... I am not subject to impressions, but the dull, heavy evening, with the strange business upon which we were engaged, combined to make me nervous and depressed."

Here, Doyle uses the gloom of the city on this September evening to deepen the unease that Watson feels; Holmes has pocketed a small revolver before setting out "to an unknown place, on an unknown errand." The trio are made to switch cabs halfway through their journey and glimpse the "broad, silent water" of the Thames before reaching "a questionable and forbidding neighborhood" where they encounter Thaddeus Sholto in his home "in the howling desert of South London," a newly finished and dimly-lighted home in a block of uninhabited terrace houses.

The South London home of Thaddeus's brother, Bartholomew, is forbidding; Pondicherry Lodge is described as having "a very high stone wall topped with broken glass" and "a single narrow iron-clamped door formed the only means of entrance."

Besides the sense of foreboding accomplished with the mysterious trips through sordid London streets to the homes of the Sholto brothers, in chapter seven,"The Episode of the Barrel," Watson describes his journey to the Pinchin Lane taxidermist, Sherman, as moving through "silent gas-lit streets" to his house with live and stuffed animals.