Abstract: A laser tracking interferometric length measuring instrument has a laser source and a tracking articulating optical lever provided on an optical path of an interferometric optical system. To track a retroreflector, the optical lever reflects a laser beam at the retroreflector, which reflects the beam back to the optical lever. This beam is reflected onto a quadrant photodiode. An attitude control means is used to control the attitude of the optical lever so that the laser beam reflected by the optical lever falls on the neutral point of the photodiode.

Abstract: An airship includes a gas envelope filled with a buoyant gas, a duct equipped with an internal propulsion fan and attached to the aft of the gas bag, a boom projecting aft along an extension of the axis of the propulsion fan and an empennage provided on the distal end of the boom and is constituted to prevent boundary layer separation by causing the propulsion fan to suck air flowing along the hull surface into the duct and jet it aft.

Abstract: N-(2-methoxyethyl)-N-isopropylacrylamide is a novel compound. The polymer obtained by polymerizing this novel compound is a novel hydrophilic-hydrophobic thermally reversible type macromolecular compound having a transition temperature in the range of 12.degree. C. to 14.degree. C.

Abstract: Optical spatial phase modulation is achieved by positioning a liquid crystal layer in the path of a light beam to be modulated and controlling the spatial distribution of refractive index of the liquid crystal layer by controlling the spatial distribution of the electrical field generated across the liquid crystal layer.