CH. I] TROUBLES WITH THE MAGIC LANTERN 55 



they are re-inverted or erected by the objective. If the condenser 

 and source of light are on one axis and the objective off that axis, 

 it must be recentered; but as stated above ( 54) when the objec- 

 tive and condenser are once centered they should be fixed in posi- 

 tion, then the only element of the lantern to become decentered is 

 the crater of the arc lamp, i. e., the source of light. The fine 

 adjustment screws of the lamp will enable one to center the light. 

 By limiting the changes to one element, viz., the source of light, 

 corrections can be made quickly and accurately. If one tries to 

 get the light centered by changing two or all three of the elements it 

 leads only to chaos. 



86. For the effects of spherical aberration and for a ghost, a 

 white or black spot in the center of the field, see also 828. 



87. The radiant (i. e., crater of the upper carbon) may be too 

 close to the condenser. This will give a restricted field with a blue 

 margin or there may be a blue circle in the center of the disc (fig. 

 29, 30). 



88. The radiant may be too far from the condenser. This 

 will produce a restricted screen disc with the edge bordered with 

 red (fig. 29). It is easily corrected by bringing the radiant and 

 condenser closer together. 



89. The condenser may be of too short focus, so that the 

 light comes to a focus before reaching the objective when the 

 lantern slide is in focus (see 56, fig. 29, 30). Correct the defect 

 by using a lens of longer focus for the second element of the con- 

 denser. It may be less satisfactorily compensated for by putting 

 the radiant nearer the condenser. 



90. The condenser may be of too long a focus (see 56, 

 fig. 28). Correct by using a shorter focus condenser. It may also 

 be compensated for in part by removing the radiant farther from 

 the condenser, but this lessens the available light. 



91. There may be dirt, mist or opacities on some of the glass 

 surfaces. This is easily remedied by cleaning the glass. 



