Abstract
This paper describes an X-ray goniometer of the Weissenberg type in which two crystals may be alternately exposed to an X-ray beam. The photograph obtained from the reflexions on one side of the direct beam contains the spots due to both crystals, one of which is a standard; the photograph obtained from the reflexions on the other side of the direct beam contains the spots due to the standard crystal only. In this way difficulties in obtaining relative intensities of reflexion due to fluctuations of the X-ray output and to variations in the sensitivity and development of the film are overcome. Unusual features of the instrument include (1) the arrangement of both crystals on the axis of the cylindrical camera (2) a reversing switch which has no dead centre position even when it is actuated extremely slowly, (3) a three-phase induction motor to obviate the use of carbon brushes and yet allow of easy reversal.