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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CH. I] CENTERING THE MAGIC LANTERN 39 the other. In general it will be found that if the red parts are at the right then the red glass must be over the right eye and similarly for the green. Presumably if one used the wrong color then there should be a pseudoscopic effect, convex objects looking concave, etc.; but this effect is difficult to obtain. It is seen that in all these methods the observer must be supplied with some means by which only one of the projected images is seen by one eye, the other by the other eye. Stereoscopic projection is necessarily, therefore, expensive. For most people any good lantern slide shows perspective and relief sufficiently. CENTERING THE VARIOUS PARTS OF THE LANTERN AND SEPARATING THEM THE PROPER DISTANCE § 51. Centering. — By this is meant the arrangement of the source of light, the condenser and the projection objective so that the source of light, and the principal optic axis of the condenser and of the objective shall be in one straight line, and each lens be perpendicular to that straight line (fig. 1-4). When the different elements are once centered along one straight line the objective and the condenser should be fixed in position so that they cannot be raised or lowered or turned sidewise. If the source of light gets slightly out of center by the burning of the carbons, it may be recentered by bringing the carbons nearer together or by regulating the position by the fine adjustments of the lamp. In the right-angled arc lamp the upper carbon, which furnishes the light, is constantly in the optic axis. With oblique carbons (fig. 39) the source of light constantly shifts with the burning away of the carbons ; and with the direct current lamp the source of light gradually rises above the axis. With the alternating current and V-arranged carbons one source shifts above and one below the axis, or one to the right and one to the left depending upon the arrangement of the V. In centering the lamp one should start with the carbons in contact and take the point of contact to center from.