CH. I] 



CENTERING THE MAGIC LANTERN 



FIG. 1 8. FIGURE SHOWING HOW TO 

 FIND THE CENTER OF A CIRCLE. 



Draw two chords (ch ch) and erect perpendiculars at their middle points. 

 Where these perpendiculars cross is the center of the circle (C). 



As stated above, when once centered, the objective and con- 

 denser should be fixed in position. 



54. Avoidance of obliquity. Not only must all the parts be 

 at the same level and in one straight line, but the lenses must be 

 perpendicular to that straight line and not oblique. Then the 

 straight line or common axis passing from the crater of the upper 

 carbon to the screen will coincide with the principal axis of the 

 condenser and the projection objective, and the arrangement for 

 perfect projection will be realized (fig. 1-4, 26). 



One can usually tell when the parts are in line and not oblique by 

 sighting along them with the eye, or by the use of a straight edge 

 like a T-square. To make sure by measurement one can put the 

 optical bench or base-board (fig. 158, 1 59) , on a level table and next 

 a smooth wall. Then by measuring horizontally the central points 

 can be determined exactly as their height was determined ( 52). 



CORRECT DISTANCE APART OF THE DIFFERENT ELEMENTS 



55. Radiant and condenser. With the three-lens condenser 

 the radiant is at the right distance when it is at the principal focus 

 of the first element of the condenser (fig. 2). This will give a 



