As electrons are scattered, they do not contribute to the final image thus making that region of the specimen to appear dark. Heavy metals can easily be seen in the E.M. because they deflect electrons as they pass through. Chemicals that contain heavy metals like osmium tetroxide, which binds to cell membranes, are used to stain biological specimens for E.M. Immunohistochemical techniques are also used in E.M. microscopy. Antibodies tagged with electron-dense gold particles instead of fluorescent probes are used in immunoelectron microscopy. The resolution of a microscope is its ability to distinguish as separate two objects that are close together.
Resolution is mainly a function of the wavelength of the source with which objects are radiated.
The shorter the wavelength the higher the resolution. The instrument design as well as the contrast properties of the specimen also contribute to set the resolution. The ability of the microscope to produce enlarged images of the specimen, which is a function of its optical system. The level of detail detected after magnification will ultimately be determined by the resolution and not by the increased level of enlargement of the image.