  I have found experimentally that in vacuum (refractive index $n$ = 1.) the
shift of interference fringe in Michelson interferometer is absent because of
the absence of particles on the light path. The shift of interference fringe
appears only at a certain concentration of particles in the luminiferous
medium. The increase of the remnant air pressure in the optical tubes of the
interferometer (of the length 6 m in each arm) from 1 mm Hg to 1500 mm Hg
reveals first indications of the interference fringe shift (about 0.015 of its
width) only at the air pressure 300 mm Hg ($n$ = 1.00014...). The increase of
the air pressure in optical tubes up to 760 mm Hg ($n$ = 1.0003...) adds twice
the amplitude of the harmonic shift of the interference fringe giving ~0.03,
and at the pressure 1500 mm Hg the harmonic fringe shift ~0.06 became
conspicuous. The S-shaped with the change of sign dependence of the fringe
shift on the index of refraction $n$ was found. Thus the estimation of the
horizontal projection of the velocity of the Earth relative to aether is
obtained. At the latitude of Obninsk this velocity varies depending on the time
of day and night within 140 - 480 km/s. The forth version of the report is
complemented with the account of the harmful effect on registering the shift
$\Delta X_m$ of the interference fringe of the improperly installed in
turning-points of the zigzag light path glass mirrors. The nullifying of the
shift in air interferometers may occur because of opposite signs of
contributions of the air and glass optical media through the discovered by me
law $\Delta X_m \sim l\Delta\epsilon(1-\Delta\epsilon)$. This may be the cause
of obtaining negative results in majority of Michelson-type experiments known
to date.
