Abstract: An arrangement for storing cryogenic liquid comprises a Dewar-flask-like vessel. The vessel is filled completely with a material able to absorb a cryogenic substance when said substance is in liquid form but only capable of absorbing said substance to a limited extent when said substance is in gas form. Adjacent to the mouth of said vessel and externally thereof there is provided a cavity, which is arranged to cooperate with means by which any gases formed in said cavity can be released to atmosphere when the pressure in said cavity exceeds a given pressure.

Abstract: A method and apparatus for cleaning a surface of an optical element, such as germanium or silicon, of a larger arrangement, such as an infra-red camera. Cleaning liquid is applied to the surface and vibrated at a frequency to facilitate cleaning, such as 20-50 kHz. The invention is readily utilized in situations where the optical element frequently becomes dirtied such as in military equipment. A liquid container for the cleaning liquid has an outlet adjacent the surface to be cleaned. A vibrating device is also provided and may be directly on the surface or on a member adjacent thereto. The liquid is applied to the surface and the vibrator is actuated to vibrate the liquid to facilitate cleaning.