Abstract: A method and apparatus for monitoring the fineness of solids in a first suspension of solids in a liquid medium is described. The apparatus comprises a cyclone for separating the first suspension under the influence of a force field into two second suspensions containing respectively solids generally coarser than and generally finer than a predetermined separation size. A constant head device is provided to cause the first and second suspensions to flow through the apparatus. Density measurement means comprising radiation sources, presentation columns, and detectors measure the density of the first suspension and the second suspensions respectively. A computer determines from the measured densities the fineness of solids in the first suspension.