An aerosol size spectrometer based on a round-jet impactor of continuously variable geometry is tested using an iris diaphragm as the nozzle. The impactor is run by fixing the nozzle-to-plate distance L, the aerosol mass flow rate m＇, and the volumetric pumping capacity Q, which results in a nearly fixed downstream pressure at variable nozzle diameter d(n). Tests are carried near 30 torr, with oil drops 0.078-0.49 mu m in diameter, which get captured at nozzle apertures at or below 1.3 and 2.5 mm. The size spectrometer has an excellent resolution when the jet Reynolds number and nozzle-to-plate distance are kept within the ranges 175 less than or equal to Re less than or equal to 700; 0.75 less than or equal to L/d(m) less than or equal to 3.33. The instrument may also be run at atmospheric pressure, with an estimated lower size range of 1.7 mu m.