  For emitters embedded in media of various refractive indices, different
theoretical models predicted substantially different dependencies of the
spontaneous emission lifetime on refractive index. It has been claimed that
various measurements on $4f\to 4f$ radiative transition of Eu$^{3+}$ in hosts
with variable refractive index appear to favor the real-cavity model [J.
Fluoresc. 13, 201 (2003) and references therein, Phys. Rev. Lett. 91, 203903
(2003)]. We notice that $5d\to 4f$ radiative transition of rare-earth ions,
dominated by allowed electric-dipole transitions with line strengths less
perturbed by the ligands, serves as a better test of different models. We
analyze the lifetimes of $5d\to 4f$ transition of Ce$^{3+}$ in hosts of
refractive indices varying from 1.4 to 2.2. The results favor the macroscopic
virtual-cavity model based on Lorentz local field [J. Fluoresc. 13, 201
(2003)].
