A single spiral rotating through several cycles has been observed in the fringe pattern obtained from a common type of interferometer when laser illumination is used. Such an interference pattern would indicate the presence of a very unusual discontinuity in the wavefront or in the surface generating the wavefront. It is shown that such a pattern may be generated in several ways and may be explained in a relatively simple way. The implications of this behavior for data reduction from interferograms are commented upon. A simple demonstration of this effect is described.
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