Meaning of dark slide

dark' slide"

Pronunciation: [key]
— Photog. Photog.
  1. Also calleda black plastic, metal, or fabric sheet that is inserted into a film holder to protect the film from light.
  2. a lightproof holder for sheet film or a photographic plate.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.