Explore "The Technique of the Mystery Story" by Carolyn Wells on Textopian. Read with annotations and highlights.
This could rarely be compassed in real life, but it is, of course, easy for the author to provide the tiny clues necessary to his hero's microscope work.
And tiny clues are a favorite device of the detective story writer. There is a fascination about the solving of a big murder mystery by a bit of a broken cuff-link; or the tracing of a professional burglar by a speck of cigarette ash. Of course, the philosophy is that these dues are so small as to be unnoticed by the criminal who so conveniently leaves them behind him. Also they are unnoticed by the amateur or the Central Office sleuth, and so redound to the glory of the Transcendent Detective.