An interferometric survey of the micas (With Plates VII–IX)

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Abstract
In a group of earlier publications details have been given of the application of multiple-beam interference methods to the study of mica. The characteristic structure of this mineral is exhibited by the Fizeau fringes which are produced when one side of a mica slip is silvered and placed against a similarly coated optical flat. The fringes show that the surface is violently contorted and crossed by cleavage steps, the heights of which appear to be integral multiples of the crystal-lattice spacing perpendicular to the basal plane of cleavage. If, however, a mica slip (thickness circa 1/100 mm.) is silvered on both sides then fringes of a totally different appearance are observed. It is frequently found that, despite the distortions on the surface, the Fizeau fringes are large areas of uniform illumination bounded by cleavage ‘lines’.

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