  We experimentally study resonant light scattering by a one-dimensional
randomly filled chain of cold two-level atoms. By a local measurement of the
light scattered along the chain, we observe constructive interferences in
light-induced dipole-dipole interactions between the atoms. They lead to a
shift of the collective resonance despite the average interatomic distance
being larger than the wavelength of the light. This result demonstrates that
strong collective effects can be enhanced by structuring the geometrical
arrangement of the ensemble. We also explore the high intensity regime where
atoms cannot be described classically. We compare our measurement to a
mean-field, nonlinear coupled-dipole model accounting for the saturation of the
response of a single atom.
