  We suggest a novel approach for probing of local fluctuations of the
refractive index $n$ in solids by means of single-molecule (SM) spectroscopy.
It is based on the dependence $T_1(n)$ of the effective radiative lifetime
$T_1$ of dye centres in solids on $n$ due to the local field effects. Detection
of SM zero-phonon lines at ultra-low temperatures gives the values of SM
natural spectral linewidth (which is inverse proportional to $T_1$) and makes
it possible to reveal the distribution of the local $n$ values in solids. Here
we demonstrate this possibility on the example of amorphous polyethylene and
polycrystalline naphthalene doped with terrylene.
