CH. I] 



TROUBLES WITH THE MAGIC LANTERN 



53 



SHADOWS AND RESTRICTION IN THE Disc OF LIGHT ON THE SCREEN 

 83. The radiant, i. e., the crater of the upper carbon (fig. 27) 

 may be outside the main axis (above, below, to the right or to the 

 left of it). If sufficiently outside the center there will be only an 

 elliptical light area present. On the side toward which the crater 

 is displaced there will be a blue crescent or spot, and on the oppo- 

 site side a dark crescent, bordered, in extreme cases, by red. 

 Remedy: get the crater back in the axis. 



84. The condenser may be out of center. This will give the 

 same defective light on the screen as when the light source is off 



FIG. 26 (A). DIAGRAM OF A MAGIC LANTERN AND A SCREEN IMAGE WHEN 

 ALL THE PARTS ARE IN CORRECT PROPORTION AND ON ONE Axis. 



Axis The common axis passing from the radiant along the principal axis 

 of the condenser and the objective to the screen. 



C Condenser of three lenses, the first clement (L,) composed of a meniscus 

 and a plano-convex; the second element (LJ, is a plano-convex. The con- 

 vex surfaces face each other as usual. 



F Principal focal distance of the condenser. 



Projection objective. 



R Radiant giving the light. 



S The screen fully and perfectly lighted. 



FIG. 27 (B). DIAGRAM SHOWING THE EFFECT OF HAVING THE RADIANT 

 BELOW THE Axis. 



There appears a blue shadow on the lower part of the screen (S). 



Whenever the radiant is off the axis the dark blue shadow will be on the 

 corresponding side of the screen. In this case the radiant would have to be 

 raised to get rid of the shadow. If the shadow were on the left it would be 

 necessary to move the radiant to the right and so on. 



