In one embodiment, an improved multi-pass cell for a long path-length spectrometer is designed to include a perturbing mirror that causes a base pattern of reflections to be repeated multiple times, where each subsequent base pattern of reflections is rotated about the axis at an angle from a prior base pattern, to circulate the base patters about the cell. The base pattern may be a Herriott cell pattern. The improved multi-pass cell may be constructed with a concave front mirror centered along an axis of the cell, and a concave back mirror centered along the axis and facing the front mirror. The perturbing mirror may be centered along the axis, facing the front mirror and located at a perturbing mirror spacing in front of the back mirror or behind the back mirror, depending on the implementation.