and BSE as a formidable obstacle, since he believed that the ionisation did not carry any information about the specimen. However, he later embraced to correct role of gaseous ionisation during image formation. Specimen charging The electron beam impinging on insulating specimens accumulates negative charge, which creates an electrical potential tending to deflect the electron beam from the scanned point in conventional SEM. This appears as charging artifacts on the image, which are eliminated in the SEM by depositing a conductive layer on the specimen surface prior to examination. Instead of this coating, the gas in the ESEM being electrically conductive prevents negative charge accumulation. The good conductivity of the gas is due to the ionization it undergoes by the incident electron beam and the ionizing SE and BSE signals. This principle constitutes yet another fundamental deviation from conventional vacuum electron microscopy, with enormous advantages. Contrast and resolution As a consequence of the way ESEM works, the resolution is preserved relative to the SEM. That is because the resolving power of the instrument is determined by the electron beam diameter which is unaffected by the gas over the useful travel distance before it is completely lost. This has been demonstrated on the commercial ESEMs that provide the finest beam spots by imaging test specimens, i.e. customarily gold particles on a carbon substrate, in both vacuum and gas. However, the contrast decreases accordingly as the electron probe loses current with travel distance and increase of pressure. The loss of current intensity, if necessary, can be compensated by increasing the incident beam current which is accompanied by an increased spot size. Therefore, the practical resolution depends on the original specimen contrast of a given feature, on the design of the instrument that should provide minimal beam and signal losses and on the operator selecting the correct parameters for each application. The aspects of contrast and resolution have been conclusively determined in the referenced work on the foundations of ESEM. Further, in relation to this, we have to consider the radiation effects on the specimen. Specimen transfer The majority of available instruments vent their specimen chamber to the ambient pressure (100 kPa) with every specimen transfer. A large volume of gas has to be pumped out and replaced with the gas of interest, usually water vapor supplied from a water reservoir connected to the chamber via some pressure regulating (e.g. needle) valve. In