A new double differential refractometer for student laboratories,
based on holographic interferometry in real time with reference hologram
and reference fringes, is presented. By studying the interferograms
one gets a graphical record of the radial, axial, and temporal
distribution of the refraction index in cylindrical tubes. This method permits
the determination of the experimental parameters for cases when
the relationship between these parameters and the refraction index is
known. The paper presents experimental results for gas-discharge parameters.