  A popular demonstration experiment in optics uses a round-bottom flask filled
with water to project a circular rainbow on a screen with a hole through which
the flask is illuminated. We show how the vessel's wall shifts the second-order
and first-order bows towards each other and consequentially narrows down
Alexander's dark band. We address the challenge this introduces in producing
Alexander's dark band, and explain the importance of a sufficient distance of
the flask to the screen. The wall-effect also introduces a splitting of the
bows which can easily be misinterpreted.
