By making an allusion to murderers well-known in the contemporary imagination, the story generates fear and apprehension in the reader. As Sherlock and Watson surveil the Stoke Moran Manor House from the window of their inn, Sherlock makes the following reference:
When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge. Palmer and Pritchard were among the heads of their profession. This man strikes even deeper, but I think, Watson, that we shall be able to strike deeper still.
William Palmer and Edward William Pritchard were doctors in 1800s England who became notorious after committing grisly murders. Specifically, both men used poison to kill: Palmer murdered a friend, and Pritchard, his wife and mother-in-law. Here, Sherlock states that the two murderers were even more lethal as a result of their medical experience: in fact, they excelled in their "profession" of murder. Dr. Grimesby Roylott, Sherlock suggests, is even more dangerous than these well-known murderers. By suggesting that Dr. Roylott is a force to be reckoned with like Palmer and Pritchard, Sherlock's allusion increases the reader's fear and apprehension of Dr. Roylott and deepens the impression that Sherlock and Watson are in grave danger.
