Flow cytometers belong to the standard equipment of clinical diagnostics and bioanalytics by which the concentration of cell populations in a sample is measured. The fundamental structure of cytometers has not been changed for more than forty years. The diluted sample is led through a laser focus in such a way that only one cell at a time is in the focus. The cells, which are specifically marked by a fluorescent colour, reflect a fluorescent flash by which they can be detected. At the same time the laser light scattered by its contact with the cell is measured, too. The scattered laser light permits to draw conclusions con¬cerning the size and the morphology of the cells. The newly developed concept of the spatially modulated fluorescence for the flow cytometry uses a mask of alternating transparent and opaque areas of varying size. This mask is situated in the corridor of the fluorescent light signal between the cell and the detector. Thus the signal of a cell is modulated (on/off) and is later detected by the techniques of correlation. This technique precisely measures the velocity of the cells at the same time. Within the context of this dissertation the velocity of rigid spherical particles in a parabolic fluid pattern of a microfluidic channel (480∙12 〖μm〗^2 ) is measured by the technique of spatially modulated fluorescence. The migration of the particles sideways to the flow direction into their equilibrium position (Segré-Silberberg effect), which result from such fluidic conditions, is measured by the velocity of the particles more than it has been possible before. When comparing the theoretical descriptions of this effect with the experimentally measured results of this dissertation it becomes obvious that the courses of the particle migration can explain the distribution of velocity measured, but it also has to be stated that the courses calculated to reach the equilibrium position are too short. In addition to that the dependence of the equilibrium velocity on the Reynolds number and on the particle diameter is measured in this investigation. The dependence of the equilibrium velocity on the particle diameter can technically be used to measure the size of the particles. Particles of a size from 4.24 μm to 6.42 μm can be distinguished from each other under the fluidic conditions used in this dissertation. The analysis of the particle diameter increases up to 50 nm depending on the increase of the Reynolds number. The size of the cells can also be determined by measuring the equilibrium velocity which is proved in this analysis. A comparison between the classic technique of measuring the size of cells with the help of scatter light and the newly developed technique of measuring the size of cells by equilibrium velocity is made at the end of this thesis.