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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154 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI parallel with the clock-shaft. When the beam of light from the mirror extends out parallel with these lines, as indicated by the streak of light, the mirror will be in the correct position. § 245. Arranging the second mirror. — For getting the light in a desired direction, a second mirror is used in the path of the beam extending directly northward, from the first mirror, and so arranged that the light is reflected as desired (fig. 81 M2). § 246. Heliostat with the mirror parallel with the clock-shaft.— With the other heliostats described in this chapter, the clock-work rotates the shaft once in 24 hours, but with this form, the rotation is once in 48 hours, i. e., half the rate of rotation of the earth. The clock-shaft is somewhat extended and the mirror is fixed directly to the shaft and parallel with it. The mirror is therefore in a plane which if extended would cut the celestial north pole (fig. 82). Light reflected from this mirror may be made to take any direction in a circle. § 247. Setting up the heliostat with the mirror parallel with the clock-shaft. — The heliostat is placed in a proper position for receiving the sunlight. The support is made level, and the instrument set north and south. The clock-shaft is then elevated from the horizontal until it is at an angle equal to the latitude of the place where it is to be used. As the mirror in this form may be set to reflect the light anywhere in a circle, it is best to loosen the clamp of the clock-shaft and rotate the mirror until it receives the full light of the sun and reflects it in a convenient direction. Then clamp the shaft to the clock-work and the mirror will follow the sun. § 248. Arranging the second mirror. — The second mirror is now placed so that it will receive the beam from the movable mirror, and then it is turned, raised, or lowered on its stand, until the light extends in the desired direction. It should continue to hold the light in one place so long as the sun shines on the movable mirror (fig. 82). One must make sure that the position of the second mirror is such that it will not shade the heliostat mirror as the sun moves toward the west.