An oversize tube of transparent plastic is put into a slotted steel supporting tube by cooling in liquid air. Such a device using Bakelite BT 43 001 has proven reliable at 400 atmospheres, and calculations indicate 1000 atmospheres as the upper limit of safety for tubes of this general type and material. The most important feature is the absolute reliability. This is due to the fact that in operation no primary tensile stresses exist in the transparent material.