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68o OPTIC PROJECTION
If any individual should be mentioned in connection with the projection microscope, it is Kepler, for in his Dioptrics, 1611, he showed the advantage of adding an amplifier in projection, and also a second convex lens (ocular), to magnify the real image of the objective, and also at the same time to render it erect. See Opera Omnia, vol. ii, pp. 549-550, 555.
3. Moving Pictures
Moving picture projection is like micro-projection when no ocular is used. The screen distance is usually rather great and the many slightly differing pictures are changed so rapidly that the successive screen images seem to fuse together and thus give the appearance of motion.
The first step in getting moving pictures was an investigation of persistence of vision by momentary glimpses of similar moving objects. The men investigating the matter were all physicists, and the results of their observations were given in scientific papers. See in the bibliography papers by Faraday, Plateau, Homer and Stampfer. The paper on the magic disc by Plateau was dated Jan., 1833, and the paper of Homer on the daedaleum (zoetrope) was dated 1834, as was also the paper of Stampfer on the magic disc. Both the magic disc (fig. 408) and the zoetrope (fig. 409) give the appearance of movement with great satisfaction.
As the instruments were for one or at most for very few observers, the magic lantern was called in to give screen images so that many could see at the same time. The magic lantern was used successfully by Uchatius in 1853. He used several (as many as 12) slightly differing transparencies, each transparency having its own projection objective. The objectives were all directed toward the same point on the screen, hence the images all appeared in the same place. A lime light and condenser were attached to a crank, and moved from picture to picture in rapid succession, and the projected images gave the appearance of movement as perfectly as did the magic disc.
It was also natural that the new art of photography should be called upon to depict the various phases of a moving body for use in place of the drawings which had been previously used; this was suggested by Plateau about 1848. In 1870 Heyl realized this possibility by arranging a series of photographic transparencies of posed motion, and projecting them on the screen. The transparencies were arranged on the edge of a large disc, and by the step by step movement of the disc the successive transparencies were brought in the axis of the magic lantern. To prevent the blur while the pictures were changed, a two wing shutter was used to cut off the view. This method of projecting was very successful and required only one projection objective, consequently the number of pictures was limited only by the practicable size of the rotating disc.
Up to 1872 the pictures used were either drawings or photographic transparencies of posed movements, not photographs of movement in continuous change as at present.