  We measure the coherent scattering of light by a cloud of laser-cooled atoms
with a size comparable to the wavelength of light. By interfering a laser beam
tuned near an atomic resonance with the field scattered by the atoms we observe
a resonance with a red-shift, a broadening, and a saturation of the extinction
for increasing atom numbers. We attribute these features to enhanced
light-induced dipole-dipole interactions in a cold, dense atomic ensemble that
result in a failure of standard predictions such as the "cooperative Lamb
shift". The description of the atomic cloud by a mean-field model based on the
Lorentz-Lorenz formula that ignores scattering events where light is scattered
recurrently by the same atom and by a microscopic discrete dipole model that
incorporates these effects lead to progressively closer agreement with the
observations, despite remaining differences.
