GNU Collaborative International Dictionary of English

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1.       
   Aplanatic , a. [Gr. ἀ priv. +  disposed to wander, wandering,  to wander.] (Opt.) Having two or more parts of different curvatures, so combined as to remove spherical aberration; -- said of a lens.
   1913 Webster
   Aplanatic focus of a lens (Opt.), the point or focus from which rays diverging pass the lens without spherical aberration. In certain forms of lenses there are two such foci; and it is by taking advantage of this fact that the best aplanatic object glasses of microscopes are constructed.
   1913 Webster