When mounted for measurement on the goniometer, the faces of the perovskite crystals described in the preceding paper yield, owing to the complex lamellated structure, a confused mass of images of the signal which cannot be analysed by the ordinary methods (such as using a lens in front of the eyepiece or shading portions of the face with a screen held in the hand), on account of the number and extremely small size of the reflecting surfaces. The apparatus described below was designed to give the means of identifying with certainty the image reflected from any given portion of such a face, and has been found to answer this purpose satisfactorily.