Patent Number: 
Section: claims

1. A transmission electron microscope having:a target body position on the electron optical axis of the microscope,an electrically conductive body off the axis of the microscope,an electron source for producing an axial electron beam which, in use, impinges upon a target body located at the target body position, anda system for simultaneously producing a separate off-axis electron beam which, in use, impinges on the electrically conductive body causing secondary electrons to be emitted therefrom;wherein the electrically conductive body is located such that the emitted secondary electrons impinge on the target body to neutralise positive charge which may build up on the target body, andwherein the system for producing an off-axis electron beam comprises an aperture body positioned between the electron source and the target body position, the aperture body having an axial aperture for transmission of the axial electron beam and further having an off-axis aperture for production of the off-axis electron beam. 2. A transmission electron microscope according to claim 1, wherein the off-axis electron beam is a paraxial electron beam. 3. A transmission electron microscope according to claim 1, wherein the off-axis electrically conductive body is located adjacent the target body position. 4. A transmission electron microscope according to claim 1, wherein the aperture body has a plurality of off-axis apertures for production of respective off-axis electron beams, each off-axis electron beam, in use, impinging on the off-axis electrically conductive body or a respective off-axis electrically conductive body. 5. A transmission electron microscope according to claim 1, further having at least one condenser lens between the electron source and the target body position, the aperture body being positioned between the condenser lens and the target body position, the aperture body limiting the illuminating field of the condenser lens. 6. A transmission electron microscope according to claim 1, wherein the system for producing an off-axis electron beam comprises a further electron source. 7. A transmission electron microscope according to claim 1, wherein:the target body position is a specimen position,the axial electron beam, in use, impinges upon a specimen, andthe emitted secondary electrons impinge on the specimen to neutralise positive charge which may build up on the specimen. 8. A transmission electron microscope according to claim 7, wherein the off-axis electrically conductive body is provided by a specimen support which holds the specimen at the specimen position. 9. A transmission electron microscope according to claim 1, wherein:the target body position is a phase plate position,the axial electron beam, in use, impinges upon a phase plate, andthe emitted secondary electrons impinge on the phase plate to neutralise positive charge which may build up on the phase plate. 10. A transmission electron microscope according to claim 1, wherein:the target body position is an electron biprism position,the axial electron beam, in use, impinges upon an electron biprism, andthe emitted secondary electrons impinge on the electron biprism to neutralise positive charge which may build up on the electron biprism. 11. A transmission electron microscope according to claim 1, having a plurality of target body positions spaced on the electron optical axis of the microscope, a plurality of respective electrically conductive bodies off the axis of the microscope, and a system for simultaneously producing a plurality of separate off-axis electron beams which, in use, respectively impinge on the electrically conductive bodies.