Dark slide, device that holds a sensitized glass plate or sheet of cut film, designed originally to transport daguerreotype or wet-collodion plates, or paper, sensitized in the darkroom to the camera. In the camera a sheath would be removed for exposure, and afterwards replaced and the slide returned to the darkroom. With dry processes, dark slides became the main means of transporting plates and, later, cut film, before processing. Early slides were made of wood and held one plate. Double dark slides (DDS) were introduced from the 1850s with a metal septum to separate the two plates. This became the standard form, with later models from the 1880s sometimes having metal sheaths. From the 1970s plastic was used for both slide and sheath. Single metal slides (SMS) were popular for plates up to 9 × 12 cm (4 × 5 in) from c.1900 to the 1930s.
— Michael Pritchard
