Abstract: The invention provides a retractable cassette holder for a combined X ray and radiographic examination apparatus. This cassette holder forms a mobile assembly comprising means for forming a recess with variable dimensions in the middle thereof and means for causing the barrel of a luminance amplifier to slide within this recess.

Abstract: In a method for stabilizing the frequency of the stimulated radiation emitted by a laser, the radiation is subjected to a first rotation of its direction of polarization by applying a magnetic field to a material which exhibits the phenomenon of magnetic rotatory polarization and is placed in a two-mirror resonator. Anisotropic losses are introduced in the resonator so as to produce a second rotation of the direction of polarization, part of the radiation contained in the resonator is extracted and its direction of polarization D is measured. The direction D is held in a reference direction D.sub.O by measuring the difference between the two directions, by reducing this difference to zero and by modifying the value of the resonant frequency of the resonator.