The Aluminium Dagger

The lamps have burned all night in a flat between Westminster and Victoria, and the door is bolted on the inside. When the crowbar opens it, Alfred Hartridge lies face down before his fireplace, the clock key on the carpet, a slender hilt standing out of his back. The hilt is aluminium, absurdly light for a blow driven through the edge of a rib; the guard carries one engraved word, the Italian for traitor. The windows are forty feet up.

Inspector Badger fancies the Italian street musicians who loitered outside all week, then arrests Edith Curtis, who called that evening and is left-handed. Dr Thorndyke takes callipers to the dagger and refuses to theorise ahead of his measurements. Fiction's first scientific detective, working as a crime lab still works.