From AMS Glossary
An absorption cell devised by John White in 1942 in which light traverses a small volume a large and arbitrarily variable number of times.
Such a cell, which is composed of three spherical concave mirrors with the same radius of curvature, is used to increase the optical pathlength for observing weak absorption spectra, such as those of atmospheric trace gases.
White, J. U. 1942. Long optical paths of large aperture. J. Opt. Soc. Amer.. 32. 285–288.