Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) are new techniques that reveal three-dimensional pictures of surfaces with such high resolution that individual atoms can be imaged. The SEM (pictured bottom left) accomplishes this by scanning a conducting probe over the surface while adjusting the height above the sample to maintain a constant tunneling current across a very narrow gap. Since the tunneling current depends exponentially on the gap width, a feedback system can be used to maintain a gap width to within a fraction of one angstrom. The lateral resolution of the SEM also results from this strong dependence of current on gap distance.   There is usually one atom on the probe that is closer to the substrate than any other, and thus the tunneling current flows through this one atom. This highly concentrated current allows the difference in height between the top of the atom on the substrate and the valleys to the adjacent atoms to be distinguished. With appropriate computer control, a 3-D image of the surface can be generated. The AFM determines the contours of a sample surface by using a cantilever spring to sense the force between the outermost atom on the probe and the sample surface. We currently use a Digital Instruments Dimension 3100 atomic force microscope AFM (pictured top right