Duh. I meant a polarizer oriented at 45 degrees - obviously the horizontal polarizer will block the light.
I sentence you to go read Dorothy L. Sayers' superb mystery The Documents in the Case.
A mureder mystery involving a racemic mixture of muscarine instead of the expected L-rotatory form.
All wrapped up with a portrait painter, a demented spinster, a newlywed couple, and old school chums.
And published in 1930.
Cheers!