Most interference experiments demand careful adjustment of the apparatus and special conditions of illumination for the phenomena to be at all observable. By contrast, the interference effects discussed here not only require no such precautions but are, in fact, almost unavoidable in any optical system that contains reflecting surfaces. For all their ubiquity, however, the phenomena are nonetheless elusive to the point that they seem to have become virtually unknown. Modern design of optical instruments may have eliminated much of this interference. In any case, the effects no longer appear to have practical interest and are relegated to obscurity. It was not always thus. Few subjects have attracted such a long line of luminaries in the literature of optics. This paper attempts to recapture some of the by-gone luster. It presents a review of the history and of the theory of the effect, illustrated by some measurements and photographs.