Abstract: It comprises a monochromatic radiation source; an optical fibre connected by one end to the radiation source and by the other end to an observation probe for transmitting the monochromatic radiation from the source to the observation probe and the photoluminescent radiation from the probe to signal processing means; an observation probe able to move in the cavity and having a mirror inclined with respect to the surface to be observed for directing the radiation transmitted by the optical fibre towards said wall and for reflecting a return radiation to a return optical fibre conducting it to processing means; and a rotary optical connection placed on the optical fibre for permitting the continuity of the optical line and the advance of the probe without any corkscrewing of said optical fibre.

Abstract: In a heat-insulating structure conisting of stacks of metallic modular elements placed in imbricated relation by means of overlapping edge portions, each stack is assembled by means of a single fastening member which is passed right through; and each modular element comprises a leak-tight chamber divided into two equal compartments having the shape of right-angled parallelepipeds, overlapping edge portions formed by the compartments which are relatively displaced along one diagonal, a spacer member fixed between the large bearing faces of the chamber and a filling structure within each compartment.