58 TROUBLES WITH THE MAGIC LANTERN [Cn. I 



(5) when the lens next the radiant is of such a focus that the 

 lamp must be put very close to it. 



95. Unequal heating. Breakage often occurs from unequal 

 heating of the lens. This is perhaps as common with large flame 

 sources such as the kerosene flame, the al co-radiant or Welsbach 

 mantle gas flame as with the electric arc. With the electric arc, 

 if the crater is too close to the lens the thick central part of the lens 

 expands rapidly before the edge is heated enough to expand with 

 the middle part. Separating the lamp and condenser somewhat, 

 for a few minutes after starting the lamp would give the condenser 

 a chance to expand uniformly. 



96. Mounting of the lenses. This may not give the lenses 

 sufficient freedom of expansion. In all forms of condensers as now 

 constructed there is almost invariably provision for this expansion, 

 and for free circulation of air between the lenses. The lens next 

 the radiant is usually held by a few obliquely extending springs, 

 (fig. 36 B), thus giving the greatest freedom. To prevent break- 

 age some operators avoid all direct contact of the condenser with 

 the metal mounting by the use of asbestos paper. Others think 

 that a heavy metal ring around the edge of the condenser will 

 lessen breakage by preventing the too rapid cooling. 



The final solution of condenser breakage will come when the 

 glass makers produce heat-resisting, optical glass. 



97. Breakage due to reversing the ends of the condenser. 



That is, the condenser lens which should be next the projection 

 objective is put next the lamp. The lens which should be next the 

 lamp is specially mounted for expansion ( 96). Furthermore, the 

 condenser is not designed optically in most cases so that it will give 

 equally good results if reversed. In the magic lantern the lens next 

 the objective has frequently a longer focus than the one next the 

 radiant, so that a reversal injures the optical effect as well as 

 endangers the condenser. 



If the makers of projection apparatus would so construct their 

 condenser mountings that they could not be reversed, the)- would 

 be doing a friendly service to many. 



