Abstract: An X-ray diffraction apparatus of the kind comprising first and second diaphragm tubes in coaxial alignment, displaceable with respect to each other and defining an adjustable circular diaphragm slit between them, a cylindrical film cassette enclosing the diaphragm slit and provided with a slot for admitting the X-rays, a rotatable specimen spindle enclosed by the first diaphragm tube and an elongated carrier bearing supports for each of the diaphragm tubes at its ends. At least one of the supports is laterally arranged with respect to the associated diaphragm tube and connected with the same by a bridge member having a height slightly smaller than the width of the cassette slot, so that the film cassette may be removed from the apparatus by sliding the cassette slot over the bridge member. In a preferred embodiment, the film cassette performs a reciprocal axial movement and is exclusively supported, during this movement, by the diaphragm tubes.