The main features of a two-crystal Weissenberg goniometer of the Wooster-Martin type are briefly described. The applications to two types of mineralogical problem are considered. (a) The determination of the intensities of reflection of a series of orders of a given plane or of the reflections from all planes having a common zone axis. Special attention is given to the measurement of absolute as well as relative intensities of reflection. (b) The determination of the X-ray absorption coefficients for certain iron-containing series of minerals with a view to a physical measurement of the percentage composition of iron in the mineral.