The diameters of several frozen-hydrated, spherical viruses were determined using polyoma
virus as either an external or an internal calibration standard. The methods described
provide a reproducible and accurate way to calibrate microscope magnification. The
measured diameters are in excellent agreement with respective measurements previously
reported for aqueous samples at room temperature using X-ray diffraction methods. These
results indicate that the native morphology and dimensions of biological macromolecules …