Electron phase microscopy at 1 MeV with graphite crystal supports

Thin phase objects can be seen with good contrast with 1 MeV electrons if they are supported on a graphite crystal approximately half an extinction distance thick and viewed near 101 or 112 bend contours. The shape of unstained tobacco mosaic virus is easily seen with crystal phase contrast although it is almost invisible in a normal bright field micrograph taken with 1 MeV electrons. Phase contrast images of both TMV and 109 nm diameter latex spheres are observed to be bright against a grey background on one side of a 101 or 112 bend contour and dark against a grey background on the other side of the bend contour. For strong contrast the crystal diffraction vector g must have a component parallel to the beam. Contours such as 100 and 110 do not give strong phase contrast. The phase contrast images for biological material gradually increase in contrast as the specimens are irradiated by the electron beam. An electron exposure of about 10⁴ electrons nm^?2 is required to obtain maximum contrast.