|(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.|
|(Opt.)||See under Aplanatic.|
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.