THE detail seen in a highly magnified electron micrograph is to-day limited more by a lack of contrast in the image than by any lack of resolution in the microscope, which is now usually capable of resolving objects at least as small as 15 A., that is, only a few atoms in diameter. Contrast may be increased either by metal-shadowing the surface of the specimen or by treatment with various reagents. By analogy with the use of stains in light microscopy this latter technique has come to be known as ‘electron staining’ (perhaps this is an unfortunate, if convenient, term since the analogy is not very close). However, it is still not quite clear what physical property makes a substance likely to be useful for increasing contrast. Published statements say variously that high density, high atomic number, high atomic weight or the presence of atoms of a metal are required. Even the extreme claim that all substances are almost equally efficient in increasing electron density has been argued.