An energy filter for biological electron microscopy

SUMMARY A prism‐mirror‐prism electron energy filter has been inserted into a fixed beam transmission electron microscope. The insertion of the filter has not degraded the spatial resolution of the microscope, while providing an energy resolution of 2.5 eV. The filter greatly reduces the chromatic aberration of the images, increasing resolution in dark field and contrast in bright field. Images of bacteriophage T4 and of polyoma virus indicate that thick unstained and unshadowed biological specimens can now be observed at high resolution in dark field. In addition, the increased contrast and definition observed in bright‐field images of conventional liver sections suggest that it should be possible to obtain sufficient contrast from stains containing atoms of lower atomic numbers than those employed at present.