Xii.—developments in the Technique of Particle‐size Analysis by Microscopical Examination

SYNOPSIS Developments are described in the technique of particle‐size analysis by microscopical examination. The original method, which consisted of matching the particles against a series of calibrated circles on a graticule was applied to the bench microscope. This method is now applied, with considerable reduction in eyestrain, to the projection microscope using a graticule of improved design incorporated in a matte gelatine projection screen. The actual counting operation is carried out on a statistical basis so that the accuracy of the counting can be predetermined and controlled at the same level throughout the size range of any given sample. The question of errors due to the shape of irregular particles is considered and it is concluded that except for extremes of particle shape the shape factor is generally unimportant for particles below 76μ. Preparation of samples for size analysis is discussed, and the use of phase‐contrast illumination and the extension of the size‐analysis technique to the size range covered by the electron microscope is also dealt with.