Fringes of equal chromatic order were formed by the film between a silvered cleavage face and a partially silvered optical flat. Crystal cleavage steps were determined from measurements of fringe steps. The b dimension of the unit cell was determined by a method of successive approximations to be 8.81±40.07A which agrees with the x-ray value within the experimental error.
W F. KOEHLER and A. EBERSTEIN, "Multiple-Beam Fringes of Equal Chromatic Order. Part III. The Cleavage of Topaz," J. Opt. Soc. Am. 43, 747-749 (1953)