Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

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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 breakage 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.