Abstract
This instrument was designed to isolate narrow spectral bands from discharge lamps and continuous sources more efficiently than is possible by using filters. Two 60° rhombic prisms are used to secure high dispersion for small physical size: the form of the prisms makes for compactness. A brief analysis of the problem of deciding on the prism shape is given, and the properties of a number of prisms are stated. The total deviation produced by the two rhombic prisms is 240°, so that the entrance and exit directions cross near the region of the entrance and exit apertures. These apertures are formed in a rotating turret, by means of which alternative pairs of apertures may quickly be brought into use. The diameter of each pair of apertures is twice the diameter of the next smaller aperture in order to facilitate the repetition of photographic exposures. Methods of making suitable apertures of various sizes down to 50 μ and less are described elsewhere.
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