Following the development of methods for growing single crystals of materials having good optical properties, the search has been intensified for new substances. One such is caesium bromide, and this has been grown by the Kyropoulos-Pohl method of drawing from the melt; a modification of the standard process is described.In the search for new materials, it is useful to have some way of predicting the useful long-wavelength transmission limit, and a rule is stated which forms a useful guide