A particle-sizing instrument is provided, comprising: a sample cell for receiving a sample comprising a plurality of particles; a light source configured to illuminate the sample with a light beam to produce scattered light by the interaction of the light beam with the particles; a first optical system comprising a first and second optical element respectively configured to split a portion of the scattered light into a first and second portion of scattered light: a second optical system configured to receive the first and second portion of scattered light from the first optical system, and to recombine the first and second portion of scattered light to produce an interference signal at a detection location, and a detector configured to detect the interference signal at the detection location.