IN many optical experiments, e.g. in examining the dispersion of optic axes in crystals, a homogeneous or monochromatic light is required. A fairly homogeneous red light, nearly corresponding to the Fraunhofer line B, can be obtained by a properly-selected piece of red glass placed in front of a good Argand burner or paraffin lamp. For yellow light, nothing can be better than the flame of a Bunsen's burner in which a bead of sodium carbonate is held in a loop of platinum wire. For blue rays, the light transmitted by a solution of cuprammonium sulphate is generally recommended, since the ordinary blue glass coloured with cobalt invariably transmits red rays as well as blue. But the use of a glass cell containing a strong ammoniacal solution is often inconvenient and unpleasant.
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