  We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant $\lambda (p)$ can
be estimated directly from the slope of the reflectance spectrum. We find that
$\lambda (p)$ decreases smoothly with the hole doping level, from underdoped
samples with $ p=0.103$ ($T_c = 67$ K) where $\lambda (p)= 0.93$ to overdoped
samples with $p=0.226$, ($T_c= 60$ K) where $\lambda(p)= 0.53$. An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point $p=0.16$. The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm$^{-1}$ and with an additional Drude component or
an additional Lorentz component up to 7000 cm$^{-1}$. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.
