Many slides were finished with a layer of transparent lacquer, but in a later period cover glasses were also used to protect the painted layer. Usually black paint was used as a background to block superfluous light, so the figures could be projected without distracting borders or frames. Sometimes the painting was done on oiled paper. Initially, figures were rendered with black paint but soon transparent colors were also used. Originally the pictures were hand painted on glass slides. Stereopticons added more powerful light sources to optimize the projection of photographic slides. īiunial lanterns, with two objectives, became common during the 19th century and enabled a smooth and easy change of pictures. Some lanterns, including those of Christiaan Huygens and Jan van Musschenbroek, used three lenses for the objective. The lens adjusted to focus the plane of the slide at the distance of the projection screen, which could be simply a white wall, and it therefore formed an enlarged image of the slide on the screen. The magic lantern used a concave mirror behind a light source to direct the light through a small rectangular sheet of glass-a "lantern slide" that bore the image-and onward into a lens at the front of the apparatus. The depicted lantern is one of the oldest known preserved examples, and is in the collection of Museum Boerhaave, Leiden
A page of Willem 's Gravesande's 1720 book Physices Elementa Mathematica with Jan van Musschenbroek's magic lantern projecting a monster.
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