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An optical device for throwing a large image of a two-dimensional object onto a screen. In an episcope, light is reflected from the surface of an opaque two-dimensional object (such as a diagram or photographic print) and an enlarged image is thrown onto a distant screen by means of a system of mirrors and lenses. The diascope passes light through the two-dimensional object (such as a photographic transparency, slide, or film) and uses a converging projection lens to form an enlarged image on a distant screen. An epidiascope is a device that can be used as both episcope and diascope. An overhead projector is a form of diascope that throws its image on a wall or screen behind and above the operator. In a motion-picture projector (or ciné projector) the film, consisting of a long sequence of transparent pictures, is driven by a motor past the light source in such a way that each picture comes to rest for a brief period in front of the light source. The illusion of motion is created as each image on the screen is replaced by the next; during the picture change the light is interrupted.

Subjects: Physics.

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