many applications this presents no problem, but with those ones requiring uninterrupted 100% relative humidity, it has been found that the removal of ambient gas is accompanied by lowering the relative humidity below the 100% level during specimen transfer. This clearly defeats the very purpose of ESEM for this class of applications. However, such a problem does not arise with the original prototype ESEM using an intermediate specimen transfer chamber, so that the main chamber is always maintained at 100% relative humidity without interruption during a study. The specimen transfer chamber (tr-ch) shown in the diagram of ESEM gas pressure stages contains a small water reservoir so that the initial ambient air can be quickly pumped out and practically instantaneously replaced with water vapor without going through a limited conductance tube and valve. The main specimen chamber can be maintained at 100% relative humidity, if the only leak of vapor is through the small PLA1, but not during violent pumping with every specimen change. Once the wet specimen is in equilibrium with 100% relative humidity in the transfer chamber, within seconds, a gate valve opens and the specimen is transferred in the main specimen chamber maintained at the same pressure. An alternative approach involving controlled pumping of the main chamber may not solve the problem entirely either because the 100% relative humidity cannot be approached monotonically without any drying, or the process is very slow; inclusion of a water reservoir inside the main chamber means that one cannot lower the relative humidity until after all of the water is pumped out (i.e. a defective control of the relative humidity). Radiation effects During the interaction of an electron beam with a specimen, changes to the specimen at varying degrees are almost inevitable. These changes, or radiation effects, may or may not become visible both in SEM and ESEM. However, such effects are particularly important in the ESEM claiming the ability to view specimens in their natural state. Elimination of the vacuum is a major success towards this aim, so that any detrimental effects from the electron beam itself require special attention. The best way around this problem is to reduce these effects to an absolute minimum with an optimum ESEM design. Beyond this, the user should be aware of their possible existence during the evaluation of results. Usually, these effects appear on the images in various forms due to different electron