that Watson had got wet lately and had "a most clumsy and careless servant girl". When Watson asks how Holmes knows this, the detective answers: In the first Holmes story, A Study in Scarlet, Dr. Watson compares Holmes to C. Auguste Dupin, Edgar Allan Poe's fictional detective, who employed a similar methodology. Alluding to an episode in "The Murders in the Rue Morgue", where Dupin determines what his friend is thinking despite their having walked together in silence for a quarter of an hour, Holmes remarks: "That trick of his breaking in on his friend's thoughts with an apropos remark... is really very showy and superficial". Nevertheless, Holmes later performs the same 'trick' on Watson in "The Cardboard Box" and "The Adventure of the Dancing Men". Though the stories always refer to Holmes's intellectual detection method as "deduction", he primarily relies on abduction: inferring an explanation for observed details. "From a drop of water", he writes, "a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other". However, Holmes does employ deductive reasoning as well. The detective's guiding principle, as he says in The Sign of Four, is: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." Despite Holmes's remarkable reasoning abilities, Conan Doyle still paints him as fallible in this regard (this being a central theme of "The Yellow Face"). Forensic science Though Holmes is famed for his reasoning capabilities, his investigative technique relies heavily on the acquisition of hard evidence. Many of the techniques he employs in the stories were at the time in their infancy. The detective is particularly skilled in the analysis of trace evidence and other physical evidence, including latent prints (such as footprints, hoof prints, and shoe and tire impressions) to identify actions at a crime scene; using tobacco ashes and cigarette butts to identify criminals; handwriting analysis and graphology; comparing typewritten letters to expose a fraud; using gunpowder residue to expose two murderers; and analyzing small pieces of human remains to expose two murders. Because of the small scale of much of his evidence, the detective often uses a magnifying glass at the scene and an optical microscope at his Baker Street lodgings. He uses analytical chemistry for blood residue analysis and toxicology to detect poisons; Holmes's home chemistry laboratory is mentioned in "The Naval Treaty". Ballistics