Lantern slide

The magic lantern or laterna magica is 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 was increasingly applied to educational purposes during the 1800s and was in wide use until the mid-20th century, when a smaller-format version for photographs on film, marketed and commonly known as a slide projector, superseded it.

The magic lantern used a concave mirror in back of a light source to direct as much of the light as possible through a small rectangular sheet of glass—a "lantern slide"—on which was the image to be projected, and onward into a lens at the front of the apparatus. The lens was adjusted to optimally 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. Some lanterns, including those of Christiaan Huygens and Jan van Musschenbroek, used 3 lenses.

Originally the pictures were hand painted on glass slides. Initially figures were rendered with black paint but soon transparent colors were also used. Sometimes the painting was done on oiled paper. Usually black paint was used as a background to block superfluous light, so the figures could be projected without distracting borders or frames. 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. Most hand-made slides were mounted in wood frames with a round or square opening for the picture.

After 1820 the manufacturing of hand colored printed slides started, often making use of decalcomania transfers. Many manufactured slides were produced on strips of glass with several pictures on them and rimmed with a strip of glued paper.

The first photographic lantern slides, called "Hyalotypes", were invented by the German-born brothers Ernst Wilhelm (William) and Friedrich (Frederick) Langenheim in 1848 in Philadelphia and patented in 1850.

Apart from sunlight, the only light sources available at the time of invention in the 17th century were candles and oil lamps, which were very inefficient and produced very dim projected images. The invention of the Argand lamp in the 1790s helped to make the images brighter. The invention of limelight in the 1820s made them very much brighter. The invention of the intensely bright electric arc lamp in the 1860s eliminated the need for combustible gases or hazardous chemicals, and eventually the incandescent electric lamp further improved safety and convenience, although not brightness.

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