Published in the United States as The Problem of the Green Capsule

A Dr Gideon Fell mystery

"Most people," declared Marcus Chesney, "are absolutely incapable of describing accurately what they see or hear. If they see a street accident, a riot, or fight, their minds are so muddled that every account will be wildly at variance, and of no value to the police." But all his friends disagreed with this. So Marcus Chesney challenges them to a test. He will stage a very brief show for them, with his office as a stage and folding doors as a curtain. They shall sit in another room and watch it, while a powerful light shines on the stage and the whole performance is recorded with a cine-camera. Afterwards the guests must answer accurately a series of questions Chesney has prepared for them.
Thus, three persons saw the murder done, and afterwards not one of them was able to tell what had happened. Who, for instance, was the figure in black spectacles? What was the time by the clock on the mantelpiece? And what was the curious article - described by one person as a pen, by another as a pencil, and by a third as a blowpipe dart - which Chesney picked up in the course of the show?
The murder of Marcus Chesney comes as a conclusion of a series of senseless poisonings which have been terrifying the village of Sodbury Cross. Chesney's niece, Marjorie Wills, is under strong suspicion; but the evidence against her is not strong enough, and, at the murder of her uncle, she, like everybody else, has a sound alibi.
Dr. Fell, taking the waters at Bath, is summoned by Inspector Elliot. And Dr. Fell's explanation of the real black spectacles is perhaps the greatest detective triumph of his career.