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15.2 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI
INSTALLATION AND USE OF TWO-MIRROR HELIOSTATS
§ 243. Heliostat with the mirror at the end of the clock-shaft. —
Place the heliostat in a position either inside a room or outside a window where the full light of the sun can fall upon the movable mirror. The stand supporting the clock-work, etc., must be made level, and set in a north and south direction (fig. 81).
Elevate the clock-shaft above the level to an angle equal to the latitude of the place where it is to be used. One can use a good protractor for this. The clock-shaft will then point toward the north star, and be parallel with the earth's axis (fig. 85).
This form of heliostat often has the clock-shaft in a fixed position for cheapness of construction (fig. 81). If such a heliostat is purchased, the manufacturer must know the latitude of the place where it is to be used, then he will give the proper inclination to the clock-shaft so that when the instrument is arranged in a north and south line the shaft will point toward the north star.
§ 244. Arranging the movable mirror. — The mirror is fixed to the end of the shaft by a collar which permits it to rotate around the shaft. It is also held in a kind of fork, which permits the mirror to be raised and lowered in a way similar to the hinge movement of the porte-lumiere (fig. 75).
For setting this mirror so that the clock-work will cause it to throw a beam of light in one direction continuously, it is necessary first of all to set the mirror for the local time. This is done by the use of a perforated screen admitting a narrow pencil of light from the sun. This screen is so placed that the pencil of light falls upon the mirror. The mirror is then turned by loosening the clamp (fig. 8 1 c) and rotating it on the shaft, and by tipping it in the fork until the pencil of light is reflected back along its path through the hole again.
Then the clamp is tightened and the screen removed. The mirror is now tipped in the fork until the light is reflected from it directly in line with the clock-shaft, i. c., directly toward the north star (fig. 81 N. P.). The easiest way to do this is to take a piece of white cardboard with parallel black lines on it and place it