CENTERING THE MAGIC LANTERN 



[CH. I 



FIG. 20 A. CONDENSER FACE WITH THE SPOT OF LIGHT IN THE MIDDLE, 

 SHOWING THAT THE LAMP AND CONDENSER ARE ON THE SAME Axis. 



FIG. 20 B. CONDENSER FACE WITH THE SPOT OF LIGHT OUT OF THE 



CENTER. THIS SHOWS THAT THE CONDENSER AND LAMP ARE NOT ON 



ONE Axis. 



To get the appearance here shown the lamp must be pulled back considerably 

 beyond the principal focus of the first element of the condenser. 



will appear in the center (fig. 20) . This can be easily seen by hold- 

 ing a piece of paper against the condenser face. In case the two 

 are not properly aligned, the white spot on the paper will appear 

 outside the center, at the right or left, above or below. On account 

 of the inverting effect of lenses the arc light will be too far from the 

 center in just the opposite direction from the spot of light. For 

 example, in figure 2oB the light spot is too far to the left, 

 consequently the crater of the positive carbon must be too far to 

 the right. One should change it to the left by the adjusting screws 

 until the circle of light appears exactly in the middle (fig. 20 A). 



58. Optical test for centering the condenser and the objec- 

 tive. After the condenser and the radiant are properly centered, 

 and the radiant put at the principal focus of the condenser one can 

 tell whether the objective is on the same axis by looking at both 

 ends of the objective when it is at the proper distance from the 

 condenser (fig. 1-2). 



If the objective is in line with the lam]) and the condenser the 

 spot of light from the condenser can be seen in the middle of the 



