SUMMARYScanning electron microscope images are subject to an appreciable amount of non‐linear distortion which must be allowed for when quantitative analyses are made. A method is described for measuring the distortion by the use of moiré patterns. These greatly magnify any non‐linearities and it is possible to measure distortions as small as 0ṁ1%. In addition, the use of moiré patterns permits a separation of the distortion produced in the cathode‐ray display tube from that occurring in the microsco