The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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double convex, preferably a crossed lens. It is essential with a lantern - very essential - that as much light as possible should be utilised, and assuming the dotted lines a and B to represent an angle of illumination of 900, it is seen that unless the con-denser is placed very close to the light, and is of short focus, it cannot grasp more than c d; therefore, by placing
Fig. 33-
a small lens nearer the radiant, as in fig. 33, it is evident that more light will be collected, provided the third lens is thin and of long.focus, when the light is brought to within two inches of the small lens, and, passing through it, diverges, and then is rendered parallel by the second lens and condensed by the third lens. Further than that, by using three lenses the spherical
Fig. 34-
aberration is reduced to one-ninth of that ol a single lens. Mr. Traill Taylor suggested many years ago the condenser shown m fig- 34. which consists of three plano-convex lenses, the centre being achromatised. This, however, is expensive. Grubb sug-gested the form shown in fig. 35, in which a is a piece of plain glass to absorb the heat, b a plano-convex lens which acts as a condenser, c a plano-convex, and d an over-corrected combina-tion. From c to d the rays are practically parallel, and thence