A long‐base‐line (90 m) multiple reflection cell has been constructed to carry out spectroscopic measurements on gases for atmospheric sciences purposes. The cell is installed in the Laboratori Nazionali del Gran Sasso at Assergi (L’Aquila). The optical path ranges from about 180 m up to about 1500 m in the UV and up to about 2100 m in the visible region. Tests of the instrument by measuring the Rayleigh cross section of molecular nitrogen have been performed. ©1995 American Institute of Physics.