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40 CENTERING THE MAGIC LANTERN [Cn. I
Remember that one should never change the position of the condenser or of the objective to compensate for the lack of centering of the source of light.
§ 52. Mechanical method of centering. — This is the method most satisfactory for both manufacturer and user in getting the various parts properly aligned.
Generally some form of track (optical bench) is used on which the various parts are placed and along which they can slide. The straight line or axis to which all parts are to be centered is at a selected, definite position above the base-board or table supporting the track (fig. 3, 40).
The first thing, then, is to decide upon the distance the axis is to be above the base-board or table.
For all work upon centering, the bellows between the condenser and the objective should be removed so that the faces of all parts can be seen.
The position of the common axis may be determined by some part of the apparatus, such as the condenser. Or one can decide upon some convenient level which will give sufficient room for the arc lamp and its carbons, and then adjust all parts to this level. A good way to get all at the proper height is to make a measure or gauge of wood just the height of the axis. If this is a board which just fits between the tracks, and has a peg indicating the middle point between the tracks it will help to get the parts perpendicular to the axis as well as at the right level. If the wooden gauge is carefully made it will enable one to center the parts to within one or two mm. (VIG to V24 inch). Very slight variations from perfect mechanical centering can be compensated for by using the fine adjustment screws of the arc lamp.
§ 53. Getting the center of the lens faces. — This can be done by using a rule in millimeters or Vieth's inch. Or it can be done by pressing some white paper against the lens face and creasing it around the edges with the finger. The center of this circle of paper can then be found as shown in fig. 18. If the center is marked and the paper then put over the lens face one will have a guide to center by.