A Long Optical Path in a Restricted Space

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Abstract
An instrument is described for attaining a long optical path by repeated traversal between a concave spherical mirror and a totally reflecting prism. Its properties are outlined and compared with those of other devices for similar purposes. The number of traversals is in principle unlimited (we have used ours in practice for several numbers of traversals up to twelve) but the angular spread of the final beam is less than that of the first traversal by an amount that might become serious for a very large number of traversals. The final image, which is almost free of aberration, is as large, and in favorable cases almost as bright, as the source; the prism yields greater brightness than an all-mirror device at short wave-lengths, but less at long wavelengths. The optical adjustments are simple, even for many traversals. The optics of the instrument is considered in detail, with special attention to the influence of the prism angle upon the effective aperture of the emergent beam. A model of the instrument, now in use, is described.

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