Detective Inspector Andrew Elliot is vacationing in Italy when he overhears an English party discussing some poisonings in a town back home. He is struck at first sight by Marjorie Wills. This proves to be unfortunate, because when he returns home, he is assigned the poisoning case and Marjorie is a suspect.
The poisoning case involves someone substituting poisoned chocolates for harmless ones in a local shop. One boy has died. However, this case is soon overshadowed by the murder of Marcus Chesney, Marjorie’s uncle, under bizarre circumstances. Chesney has a hobby horse that people aren’t observant, so he designs a demonstration of his point. During the demonstration, a bizarrely dressed man comes in to the room where Chesney is manipulating objects at a desk and forces a capsule down his throat. Although this is part of the demonstration, it is not part of it for the capsule to be poisoned. Chesney dies and his assistant is found outside bashed over the head. Later, the unconscious assistant is also poisoned.
Present are Chesney’s friend Dr. Ingram, the assistant, Marjorie, and Marjorie’s fianceé, George Harding, whom she met on the trip. Not present is Dr. Joe Chesney, Marcus’s brother, out on a house call.
As Elliot investigates, things keep pointing to Marjorie, but he can’t prove anything. Finally, he asks Gideon Fell for help.
The Black Spectacles is supposedly Carr’s most popular book, even though it doesn’t feature a locked door mystery, his specialty. I enjoyed it a lot, more than the other books I’ve read by Carr, although I immediately picked out the killer and never wavered. Still, I never figured out exactly what was going on during the demonstration.
I received this book from the publisher in exchange for a free and fair review.
What? Me Read? 
I enjoyed this one too and didn’t guess the solution! I’m looking forward to reading more of the Gideon Fell books as this is the only one I’ve read so far.
I think two of the Carrs I read were Gideon Fell books.
I really enjoyed this one too, more than most of the Gideon Fells I’ve read so far, I think because despite the “impossible crime” aspects it actually relied as much on characterisation.
Yes, I think so.