An environmental scanning electron microscope with field emission gun providing a spatial resolution of 2 nm which is comparable to the resolution of high vacuum field emission scanning microscope even though the specimen (66) is contained in the gaseous environment of the specimen chamber (68). The object lens assembly includes four differentially pumped vacuum zones (80, 82, 84, 86) to provide a pressure difference of up to approximately 10 Torr between the field emission gun and the specimen. The differentially pumped aperture system further includes at least four pressure limiting apertures (70, 72, 74, 76) defining the differentially pumped vacuum zones. One of the pressure limiting apertures is formed of a pressure limiting aperture assembly (103) which includes a plurality of stacked annular apertures (108, 110, 112, 114). Further, an electron detection assembly is provided for detecting signals from the surface of the specimen. The electron detection assembly includes a printed circuit board (132) having a signal ring electrode (136) biased to collect secondary electrons from the surface of said specimen and a final pressure limiting aperture (144) integrally formed therewith which extends through said signal ring electrode.