A convergent Soller type slit, adaptable to a camera of arbitrary radius, serves to select those pencils of x-rays which emanate from a broad focal spot and impinge upon any selected point on the equator of the cylindrical camera. The diffracting powder is mounted on the camera wall in the form of a thin sheet which intercepts the wedge of x-radiation. In this manner all x-rays diffracted at the same Bragg angle are brought to a focus at the equator of the camera.