Definition of Aplanatism

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Aplanatism Definition from Language, Idioms & Slang Dictionaries & Glossaries
Webster's Revised Unabridged Dictionary (1913)
(n.)
Freedom from spherical aberration.
  
Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter. About
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aplanatism
\a*plan"a*tism\ (&?;), n. freedom from spherical aberration.