Some Experiments Concerning the Limitations and Errors in Size Measurement of Small Objects by Visual Microscopy

SYNOPSIS The systematic errors which arise in the measurement of small, non‐periodic objects by visual microscopy are discussed, together with a method for reducing these errors by calibration with objects of known size. Such a calibration is described for the case where the diameters of circular holes and opaque disks are measured using a filar micrometer eyepiece, an oil‐immersion objective, and full‐cone illumination. Examples are given showing the influence of the numerical apertures of the condenser and objective upon the sizing errors. The results are used to derive size correction factors applicable to the measurement of any thin, high contrast object when the same sizing technique is used. Values are also given for the useful lower limits of measurement. Further experimental evidence emphasizes the difficulty of accurate measurement where the objects are of finite thickness.