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CH. I] CENTERING THE MAGIC LANTERN 45
first lens of the objective. The light should strike the middle of the first lens and leave through the middle of the last lens of the objective (fig. i).
If it is not centered the cone of light will strike at one side of the center and leave at one side. If it is greatly out of center the cone of light may fall wholly outside the objective; this frequently occurs in micro-projection.
To center the objective it should be moved up or down, to the right or to the left, until the cone of light strikes it exactly in the center and leaves the center. No change of the lamp or the condenser should be made, for that would spoil the centering of those two elements. After the objective is centered, it should be fixed firmly in position. Any slight variation from the center by the irregular burning of the carbons, can be corrected by the fine adjusting screws of the lamp (fig. 3, L. A.; V. A.).
CENTERING THE OBJECTIVE IN A VERTICAL POSITION
§ 59. When the objective must be made vertical in projecting horizontal objects, the radiant and the condenser should first be centered as described above (§ 55). Then the second element of the condenser should be removed and placed in a horizontal position with the convex face downward, and the flat face upward toward the objective. A plane mirror at 45 degrees is placed in the path of the beam of light from the first element of the condenser. The light will be directed vertically upward. The horizontal condenser lens must be moved until it receives this vertical cylinder of light and continues the central or axial ray in a vertical direction. One can tell when this is the condition by pulling the arc lamp back from the condenser until a small circle of light appears on the horizontal condenser lens (fig. 2oA, B.). If it is centered the spot of light will be in the middle. If it is not in the middle move the upper lens until it is, but do not change the position of the lamp. When the horizontal lens is centered, move the arc lamp up toward the condenser until the horizontal lens is filled with light (§ 55).