 We can immediately see in the figure of the isodensity contours of gas through aperture that the gas density decreases by about two orders of magnitude over the length of a few aperture radii.This is a quantitatively vivid demonstration of a first principle that enables the separation of the high-pressure specimen chamber from the low pressure and vacuum regions above.By such means, the gas flow fields have been studied in a variety of instrument situations, in which subsequently the electron beam transfer has been quantified.