Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

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144 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI hinges more and more until at noon it will be nearly horizontal (fig. 76). In the afternoon, as the sun moves toward the west, the mirror must be rotated to follow it. At the same time it must be turned more and more on its hinges until late in the afternoon, it will be at the same angle as in the morning, and rotated as far toward the west as it was toward the east in the earlier part of the day (fig. 76). (B) For an eastern exposure — In this position, the axis of the entire instrument is preferably east and west with the objective pointing westward. The earlier the time the more nearly hori North FIG. 76. DIAGRAM SHOWING THE POSITION OF THE MIRROR NECESSARY TO RELFECT THE SUNLIGHT DIRECTLY NORTH AT THREE DIFFERENT PERIODS OF THE DAY — (6A.M.; 12 M.; 6 p. M.). The diagram is for the latitude of Ithaca and at the season of the equinox when the sun seems to rise directly in the east and set directly in the west. In the morning thu mirror is turned toward the east at an angle of 45° and inclined about 10° toward the south. In the evening it is turned similarly toward the west and south. At noon the mirror is raised on its hinges about 28° above the horizontal. At all intermediate points the mirror must be set accordingly: that is, so that it will reflect the sun directly north. The diagram also shows the apparent course of the sun from sunrise to sunset.