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A modified Michelson interferometer with a stable He-Ne laser source has been used to study fluctuations in the mean refractive index over a long path through the turbulent atmosphere. Distinct interference fringes were obtained at mirror separations up to 177 m, corresponding to an optical path difference of 354 m. The spatial stability of the interference fringes was found to decrease with increasing optical path length, indicating an increasing contribution from the atmosphere. Details of the interferometer and the experimental procedure are given as well as a discussion of the fluctuations in the mean refractive index.
Herrick et al. (1966) studied this question.