Volltext: Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine

[Ch. 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 alco-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 
condenseras 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, they would 
be doing a friendly service to many.


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