Microspheres. Coloured Scanning Electron Micro- graph (SEM) of clusters of microspheres. These spheres or beads are synthetically made to a variety of very exact sizes. They are used to calibrate microscopes; either examined directly as samples or added to an unknown sample and used as an internal standard of size. Microspheres are used to calibrate both optical and electron microscopes. Magnification: x330 at 6x7cm size. x530 at 4x5"~LANDSCAPE"