scope:

In fractions of powders which pass 75 micron British Standard test sieve. Sample dispersed, placed on glass slide and particles viewed through microscope by transmitted light. Areas of magnified images of particles compared with areas of reference circles of known sizes inscribed on graticule and simultaneously visible (see BS 3625). Relative numbers of particles in each of series of size classes determined. These numbers, expressed as percentages, constitute size distribution by number. From relative numbers in size classes, size distribution can be calculated by weight if it is assumed that particles of all sizes have same shape and density.