An aplanatic mirror.    From Wordnet, Princeton University. 

(An aplanatic triplet will be found best for this purpose.).    From Wordnik.com.  [A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public] Reference

It was thought that the use of this Herschel aplanatic combination as an eyepiece, combined with the Wollaston doublet for the objective, came as near perfection as the compound microscope was likely soon to come.    From Wordnik.com.  [A History of Science: in Five Volumes. Volume IV: Modern Development of the Chemical and Biological Sciences] Reference

To compute exactly the extent to which these evils have been developed he has devised a syncretic abacus, in which, on the principle of the spectroscope, the aplanatic foci are arranged in fluorescent nodules each equidistant from the metacentre.    From Wordnik.com.  [Punch or the London Charivari, October 10, 1920] Reference