[f. L. apert-us open + -(O)-METER.] An appliance attached to a microscope for determining the angular aperture of object-glasses.
1880. Nature, XXI. 433/1. Prof. E. Abbe, some remarks on the apertometer.
1881. Athenæum, 26 Nov., 707/3. Mr. Mayall [exhibited] an Abbe apertometer of dense glass for measuring apertures up to 1·50 N.A.
