Abstract: Apparatus for analysing particle dimensions of particles contained in a fluid with an illuminating device for illuminating the particles to be analysed, incorporates a source for generating light. An optical system is disposed between the source and the fluid with a fluid-side window transparent to the light, which focusses the light on the opposite side of the window in the fluid and displaces the focal point along a path running around a center line of the apparatus. A detector device detects the light scattered by the particles and generates an electric signal dependent on the scattered light. A housing is provided between the fluid and the illuminating device and/or the detector device, with an analysis unit for determining the size and/or the size distribution of the particles.