  We demonstrate that a cold, dense sample of 87Rb atoms can exhibit a
micron-scale lensing effect, much like that associated with a
macroscopically-sized lens. The experiment is carried out in the fashion of
traditional z-scan measurements but in much weaker fields and where close
attention is paid to the detuning dependence of the transmitted light. The
results are interpreted using numerical simulations and by modeling the sample
as a thin lens with a spherical focal length.
