Ăp'la-năt'ic, a.

a. [Gr. ἀ priv. and (πλανᾶν) πλανᾶσθαι, to wander, πλανητικός, disposed to wander, wandering.] (Opt.) Having two or more lenses of different curvatures, so combined that their respective aberrations compensate or neutralize each other, and the resulting compound lens is free from spherical aberration;—said of the object-glass of a telescope or microscope.

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.