The Black Spectacles (published in the US as The Problem of the Green Capsule,[1] with the subtitle "Being the psychologist's murder case"), first published in 1939, is a detective story by John Dickson Carr featuring his series detective Gideon Fell.

In the small English village of Sodbury Cross, pretty Marjorie Wills is suspected of having poisoned some chocolates in the local tobacco-and-sweet shop, using a method pioneered by historical poisoner Christiana Edmunds.

Her uncle, wealthy Marcus Chesney, believes that eyewitnesses are unreliable.

Marcus Chesney takes a principal role in the staged events and, during them, is fed a large green capsule containing poison by a masked and disguised figure wearing black spectacles.

Amazingly, the three witnesses cannot agree upon the answers to any of the questions and no one can identify the murderer.