Since crystal lattice plane spacings are accurately known from x-ray measurements, they provide a good test of microscope stability and a calibration of magnification in the upper range of magnification of the instrument.
Plane Spacing 0.204 nm, 0.143 nm and 0.102 nm – Oriented Single Crystal Gold Foil
These gold foils are mounted on gold grids, a valuable tool to check resolution, image quality, magnification and instrument stability.