In a rear projection viewer for showing a programmed viewing web having encoded frames thereon, light beams are directed across the face of a display screen onto photodetectors located about its periphery. A student can indicate a response to a projected answer coded scene, such as a question, by presenting a finger or other object at the face of the display screen at a selected answer point, thereby interrupting a light beam or beams. The photodetectors are connected to correlating circuitry which compares the student's response with information coded on the scene's frame. The next scene presented is program selected in accordance with what response the student registered.
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