Theoretical aspects, production methods, testing, and application of high-order interference filters are described which employ a thin sheet of mica in place of the usual evaporated dielectric spacer layer. Such filters have been made with half-widths less than 1 A and transmissions of up to 70%. In particular, details are given of the cleaving process, the double refraction and the absorption of mica. A filter mount is described which makes it possible to construct filters of practical size. Measurements are reported on the uniformity of transmission peaks over the 4 cm diameter area of the filters. Theoretical and experimental values are given for the angular field. Other properties discussed are the effect of the filters on resolving power, the background, the spectral region for which they can be constructed and the stability of mica filters with time and with temperature changes. A narrow band filter is described. Another filter described was designed to transmit the 5896 A and reject the 5890 A line of sodium. A third filter rejects all the lines of the krypton spectrum except the 6056 A line which was recommended as the primary standard of length.
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