Glass, masking paper 8.2 x 8.2 cm,,

A Lantern Slide of "untitled" (three female figures)

The magic lantern or laterna magica was an early type of image projector employing painted pictures or photographs on sheets of glass, a lens and a bright light source. It was mostly developed in the 17th century and commonly used for entertainment purposes. It became a common medium for educational purposes around 1800 until it was succeeded by slide projectors in the 1950s.
Source - wikipedia.org