  We perform spectroscopic measurements of electromagnetically induced
transparency (EIT) in a strongly interacting Rydberg gas, and observe a
significant spectral shift of the transparency from the single-atom EIT
resonance as well as a spectral dephasing of the same order. We characterize
the shift and dephasing as a function of atomic density, probe Rabi frequency,
and principal quantum number of Rydberg states, and demonstrate that the
observed spectral shift and dephasing are reduced if the size of a Gaussian
atomic cloud is increased. We simulate our experiment with a semi-analytical
model, which gives results in good agreement with our experimental data.
