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 center- 

 ing 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. (Vi to y 24 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 Meth'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. 



