CH. I] CENTERING THE MAGIC LANTERN 45 



first lens of the objective. The light should strike the middle of 

 the first lens and leave through the middle of the last lens of the 

 objective (fig. i). 



If it is not centered the cone of light will strike at one side of the 

 center and leave at one side. If it is greatly out of center the cone 

 of light may fall wholly outside the objective; this frequently 

 occurs in micro-projection. 



To center the objective it should be moved up or down, to the 

 right or to the left, until the cone of light strikes it exactly in the 

 center and leaves the center. No change of the lamp or the con- 

 denser should be made, for that would spoil the centering of those 

 two elements. After the objective is centered, it should be fixed 

 firmly in position. Any slight variation from the center by the 

 irregular burning of the carbons, can be corrected by the fine 

 adjusting screws of the lamp (fig. 3, L. A.; V. A.). 



CENTERING THE OBJECTIVE IN A VERTICAL POSITION 



59. When the objective must be made vertical in projecting 

 horizontal objects, the radiant and the condenser should first be 

 centered as described above ( 55). Then the second element of 

 the condenser should be removed and placed in a horizontal posi- 

 tion with the convex face downward, and the flat face upward 

 toward the objective. A plane mirror at 45 degrees is placed in 

 the path of the beam of light from the first element of the con- 

 denser. The light will be directed vertically upward. The hori- 

 zontal condenser lens must be moved until it receives this vertical 

 cylinder of light and continues the central or axial ray in a vertical 

 direction. One can tell when this is the condition by pulling the 

 arc lamp back from the condenser until a small circle of light 

 appears on the horizontal condenser lens (fig. 2oA, B.). If it is 

 centered the spot of light will be in the middle. If it is not in the 

 middle move the upper lens until it is, but do not change the posi- 

 tion of the lamp. When the horizontal lens is centered, move the 

 arc lamp up toward the condenser until the horizontal lens is filled 

 with light ( 55). 



