in electrophoretic apparatus measuring the velocity of particles in a fluid , a reference beam and a scattered beam scattered by the particles in the fluid at various scattering angles must be heterodyned in order that the doppler frequency shift resulting from the scattering by the moving particles can be determined . the previous need for adjusting the optical path of the reference beam so that is always impinges with the scattered beam , independent of the scattering angle , upon the mixer or heterodyner is avoided by an automatic control of the direction of the reference and scattered beams relative to the mixer so that they both always impinge upon a predetermined location in the mixer independent of the scattering angle . the construction of the measuring cell and ways of mounting same in a housing to allow easy refilling and replacement are also disclosed .