In real life, the Folies-Bergère did not have bars with mirrors that reflected the audience. Manet has ignored this irritating fact, so that his barmaid appears to be looking out over the crowd. Logically, we ought to see some evidence of ourselves in the mirror. The fact that we cannot means either that we are transparent, or that Manet has chosen a further reflective distortion, or - most unsettling - that we do not exist at all. The expression in the barmaid's eyes does rather suggest this last.