1—The detailed determination of the structure of crystals requires an accurate knowledge of the relative intensities of reflexion of X-rays from a considerable number of selected planes in a crystal. The ionization spectrometer has long been used for this purpose and recently accurate photometric photographic methods have been developed for the same work. With previously existing apparatus the labour entailed in using either method has been very great and it was thought that an automatic machine would be a great improvement, for certain investigations have been impeded and others made practically impossible by the labour of intensity measurement. There are two kinds of operation performed by the ionization spectrometer which will be treated quite separately, namely the setting of the crystal in the position so that the X-rays will be reflected from a specified plane, and the recording of the ionization current produced in the ionization chamber by the reflected X-rays. The second process has already been extensively studied, but there are two new features in the method used here. The first problem of setting the crystal automatically has been solved for the first time as far as the authors are aware. The Cycle of Operations 2—The use of a crystal plate cut perpendicular to the whole set of planes under investigation (which may number a hundred or more) was introduced by Bragg and West, and has since been extensively used. X-rays are passes through a thin plate which must be set so that the particular plane under investigation is ( a ) vertical (for the crystal and plate rotate about a vertical axis), ( b ) inclined to the X-ray beam at an angle which satisfies Bragg's law.