  A multipass laser cavity is presented which can be used to illuminate an
elongated volume from a transverse direction. The illuminated volume can also
have a very large transverse cross section. Convenient access to the
illuminated volume is granted. The multipass cavity is very robust against
misalignment, and no active stabilization is needed. The scheme is suitable for
example in beam experiments, where the beam path must not be blocked by a laser
mirror, or if the illuminated volume must be very large. This cavity was used
for the muonic-hydrogen experiment in which 6 $\mu$m laser light illuminated a
volume of 7 x 25 x 176 mm^3, using mirrors that are only 12 mm in height. We
present our measurement of the intensity distribution inside the multipass
cavity and show that this is in good agreement with our simulation.
