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 arrange- 

 ment of the V. In centering the lamp one should start with the 

 carbons in contact and take the point of contact to center from. 



