An efficient source of monochromatic light is essential to any precise determination of the optical constants of crystals and is even a necessity in many routine identifications of crystals having strong dispersions of principal optical elements,1 or of crystals, which must be immersed in highly dispersive liquids. The simpler types of monochromatic illuminators usually suffer from one or more of several disadvantages; the light is not sufficiently intense, except when mechanically agitated, as by tapping; the heat given off is excessive, due to a large volume of flame; or the source of light is too narrow. In order to be of any use in obtaining the interference figures so essential to accurate work, the source must be sufficiently broad to fill the upper focal plane of the objective with light. Of course, a narrow source may be made to serve, with the aid of a lens, but the adjustment of such an optical system constitutes a continual source of annoyance.

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