The paper sets out an analysis, in terms of geometrical optics, of the Buchmann-Meyer, or light, pattern which is formed by reflection from the modulated grooves of a recorded tone-disc. The pattern is considered as lying in the focal plane of the circular groove walls and not on the disc surface, as is usually assumed. A new apparatus for measuring the width of the patterns is described and the errors in disc-surface and focal-plane measurements found when using it are compared. The latter results are shown to possess the higher degree of consistency and their correctness is confirmed by an examination of the pattern formation in terms of physical optics. The origin of two sets of interference fringes which are visible in the patterns is also described. The decrease of luminous intensity which occurs at the edge of the measurement described should give a higher degree of accuracy than hitherto available, even under some practical conditions when the edge of the pattern is not sharply defined.
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